嘿，同学，你要的Java内存模型(JMM)来了

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"1、 计算机的硬件内存结构","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2、 Java内存模型的背景和定义","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"3、 Java内存模型3.1 主内存、工作内存的定义3.2 内存的交互操作3.3 JMM缓存不一致问题","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"4、 Java内存模型的实现","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在学习Java内存模型(JMM)前，我们先了解下计算机的硬件内存结构，因为JMM结构就是基于此演变而来的。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"1、 计算机的硬件内存结构","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在单核计算机中，计算机中的CPU计算速度是非常快的，但是与计算机中的其它硬件(如IO、内存等)同CPU的速度比起来是相差甚远的，所以协调CPU和各个硬件之间的速度差异是非常重要的，要不然CPU就一直在等待，浪费资源。单核尚且如此，在多核中，这样的问题会更加的突出。硬件结构如下图所示：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/f4/f486263d6ba6af582f5a5099a6f32ae4.png","alt":"image-20210129225817989","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们先大概梳理下这个流程：当我们的计算机要执行某个任务或者计算某个数字时，主内存会首先从数据库中加载计算机计算所需要的数据，因为内存和CPU的速度相差较大，所以有必要在内存和CPU间引入缓存(根据实际的需要，可以引入多层缓存)，主内存中的数据会先存放在CPU缓存中，当这些数据需要同CPU做交互时会加入到CPU寄存器中，最后被CPU使用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"事实上，在单核情况下，基于缓存的交互可以很好的解决CPU与其它硬件之间的速度匹配，但是在多核情况下，各个处理器都要遵循一定的协议来保障内存中的各个处理器的缓存和主内存中的数据一致性问题，这类协议通常被称为缓存一致性协议。","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/32/328678bebda3e5ee1c9a33893ae3177e.png","alt":"image-20210129232410774","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"2、 Java内存模型的背景和定义","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们在开发时会经常遇到这样的场景，我们开发完成的代码在我们自己的运行环境上表现良好，但是当我们把它放在其它硬件平台上时，就会出现各种各样的错误，这是因为在不同的硬件生产商和不同的操作系统下，内存的访问逻辑有一定的差异，结果就是当你的代码在某个系统环境下运行良好，并且线程安全，但是换了个系统就出现各种问题。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为了解决这个问题，Java内存模型(JMM)的概念就被提出来了，它的出现可以屏蔽系统和硬件的差异，让一套代码在不同平台下能到达相同的访问结果，实现平台的一致性，使得Java程序能够","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"一次编写，到处运行","attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这样的描述的好像有点熟悉啊，这不是JVM的概念描述么，它们两者有什么区别啊?","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"JVM与JMM间的区别？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"实际上，JMM是Java虚拟机(JVM)在计算机内存(RAM)中的工作方式，JMM定义了线程和主内存之间的抽象关系：线程之间的共享变量存储在主内存（Main Memory）中，每个线程都有一个私有的本地内存（Local Memory），本地内存中存储了该线程以读/写共享变量的副本，本地内存是JMM的一个抽象概念，并不真实存在。它涵盖了缓存、写缓冲区、寄存器以及其他的硬件和编译器优化。而JVM则是描述的是Java虚拟机内部及各个结构间的关系。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"小伙伴这时可能会有疑问，既然JMM是定义线程和主内存之间的关系，那么它的出现是不是解决并发领域的问题啊？没错，我们先回顾一下并发领域中的关键问题。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"并发领域中的关键问题？","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"线程之间的通信","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在编程中，线程之间的通信机制有两种，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"共享内存","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"消息传递","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#6a737d","name":"user"}},{"type":"bgcolor","attrs":{"color":"#fff9f9","name":"user"}}],"text":"在共享内存的并发模型里，线程之间共享程序的公共状态，线程之间通过写-读内存中的公共状态来隐式进行通信，典型的共享内存通信方式就是通过共享对象进行通信。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#6a737d","name":"user"}},{"type":"bgcolor","attrs":{"color":"#fff9f9","name":"user"}}],"text":"消息传递的并发模型里，线程之间没有公共状态，线程之间必须通过明确的发送消息来显式进行通信，在java中典型的消息传递方式就是wait()和notify()。","attrs":{}}]}],"attrs":{}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"线程间的同步","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#6a737d","name":"user"}},{"type":"bgcolor","attrs":{"color":"#fff9f9","name":"user"}}],"text":"同步是指程序用于控制不同线程之间操作发生相对顺序的机制。在共享内存并发模型里，同步是显式进行的。程序员必须显式指定某个方法或某段代码需要在线程之间互斥执行。在消息传递的并发模型里，由于消息的发送必须在消息的接收之前，因此同步是隐式进行的。","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"事实上，Java内存模型(JMM)的并发采用的是共享内存模型。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面，我们一起来学习Java内存模型","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"3、 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