嘿，同學，你要的Java內存模型(JMM)來了

{"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、 計算機的硬件內存結構","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2、 Java內存模型的背景和定義","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"3、 Java內存模型3.1 主內存、工作內存的定義3.2 內存的交互操作3.3 JMM緩存不一致問題","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"4、 Java內存模型的實現","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在學習Java內存模型(JMM)前，我們先了解下計算機的硬件內存結構，因爲JMM結構就是基於此演變而來的。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"1、 計算機的硬件內存結構","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在單核計算機中，計算機中的CPU計算速度是非常快的，但是與計算機中的其它硬件(如IO、內存等)同CPU的速度比起來是相差甚遠的，所以協調CPU和各個硬件之間的速度差異是非常重要的，要不然CPU就一直在等待，浪費資源。單核尚且如此，在多核中，這樣的問題會更加的突出。硬件結構如下圖所示：","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":"我們先大概梳理下這個流程：當我們的計算機要執行某個任務或者計算某個數字時，主內存會首先從數據庫中加載計算機計算所需要的數據，因爲內存和CPU的速度相差較大，所以有必要在內存和CPU間引入緩存(根據實際的需要，可以引入多層緩存)，主內存中的數據會先存放在CPU緩存中，當這些數據需要同CPU做交互時會加入到CPU寄存器中，最後被CPU使用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"事實上，在單核情況下，基於緩存的交互可以很好的解決CPU與其它硬件之間的速度匹配，但是在多核情況下，各個處理器都要遵循一定的協議來保障內存中的各個處理器的緩存和主內存中的數據一致性問題，這類協議通常被稱爲緩存一致性協議。","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內存模型的背景和定義","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們在開發時會經常遇到這樣的場景，我們開發完成的代碼在我們自己的運行環境上表現良好，但是當我們把它放在其它硬件平臺上時，就會出現各種各樣的錯誤，這是因爲在不同的硬件生產商和不同的操作系統下，內存的訪問邏輯有一定的差異，結果就是當你的代碼在某個系統環境下運行良好，並且線程安全，但是換了個系統就出現各種問題。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了解決這個問題，Java內存模型(JMM)的概念就被提出來了，它的出現可以屏蔽系統和硬件的差異，讓一套代碼在不同平臺下能到達相同的訪問結果，實現平臺的一致性，使得Java程序能夠","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"一次編寫，到處運行","attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣的描述的好像有點熟悉啊，這不是JVM的概念描述麼，它們兩者有什麼區別啊?","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"JVM與JMM間的區別？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"實際上，JMM是Java虛擬機(JVM)在計算機內存(RAM)中的工作方式，JMM定義了線程和主內存之間的抽象關係：線程之間的共享變量存儲在主內存（Main Memory）中，每個線程都有一個私有的本地內存（Local Memory），本地內存中存儲了該線程以讀/寫共享變量的副本，本地內存是JMM的一個抽象概念，並不真實存在。它涵蓋了緩存、寫緩衝區、寄存器以及其他的硬件和編譯器優化。而JVM則是描述的是Java虛擬機內部及各個結構間的關係。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"小夥伴這時可能會有疑問，既然JMM是定義線程和主內存之間的關係，那麼它的出現是不是解決併發領域的問題啊？沒錯，我們先回顧一下併發領域中的關鍵問題。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"併發領域中的關鍵問題？","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":"線程之間的通信","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在編程中，線程之間的通信機制有兩種，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"共享內存","attrs":{}}],"marks":[{"type":"color","attrs":{"color":"#1394d8","name":"user"}}],"attrs":{}},{"type":"text","text":"和","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"消息傳遞","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":"在共享內存的併發模型裏，線程之間共享程序的公共狀態，線程之間通過寫-讀內存中的公共狀態來隱式進行通信，典型的共享內存通信方式就是通過共享對象進行通信。","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":"消息傳遞的併發模型裏，線程之間沒有公共狀態，線程之間必須通過明確的發送消息來顯式進行通信，在java中典型的消息傳遞方式就是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":"線程間的同步","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":"同步是指程序用於控制不同線程之間操作發生相對順序的機制。在共享內存併發模型裏，同步是顯式進行的。程序員必須顯式指定某個方法或某段代碼需要在線程之間互斥執行。在消息傳遞的併發模型裏，由於消息的發送必須在消息的接收之前，因此同步是隱式進行的。","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"事實上，Java內存模型(JMM)的併發採用的是共享內存模型。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面，我們一起來學習Java內存模型","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"3、 Java內存模型","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們先看一張JMM的控制模型作圖","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/3b/3be664e6b9167e361954a15cd1d8edf3.png","alt":"image-20210201190128153","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":"由此可見，Java內存模型(JMM)同CPU緩存模型結構類似，是基於CPU緩存模型來建立的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們先梳理一下JMM的工作流程，以上圖爲例，我們假設有一臺四核的計算機，cpu1操作線程A，cpu2操作線程B，cpu3操作線程C，當這三個線程都需要對主內存中的共享變量進行操作時，這三條線程分別會將主內存中的共享內存讀入自己的工作內存，自己保存一份共享變量的副本供自己線程本身使用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這時有的小夥伴可能會有以下疑問：","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":"主內存、工作內存的定義是什麼？","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"如何將主內存中的共享變量讀入自己線程本身的工作內存?","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"當其中的某一條線程修改了共享變量後，其餘線程中的共享變量值是否變化，如果變化，線程間是怎麼保持可見性的？","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面，我們針對這兩個疑問一一解答。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#135ce0","name":"user"}}],"text":"3.1 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和double類型的變量是64位，但是在32位JVM中，32位的JVM會將64位數據的讀寫操作分爲2次32位的讀寫操作來進行，這就導致了long、double類型的變量在32位虛擬機中是非原子操作，數據有可能會被破壞，也就意味着多個線程在併發訪問的時候是線程非安全的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於非原子操作的基本類型，可以使用synchronized來保證方法和代碼塊內的操作是原子性的。","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"synchronized (this) {\n a=1; \n 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能的，他可以同時滿足以上三種特性，這其實也是很多人濫用synchronized的原因。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是synchronized是比較影響性能的，雖然編譯器提供了很多鎖優化技術，但是也不建議過度使用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"參考文獻","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[1]https://www.jianshu.com/p/8a58d8335270","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[2]https://blog.csdn.net/javazejian/article/details/72772461","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[3]https://blog.csdn.net/zjcjava/article/details/78406330","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"[4]https://segmentfault.com/a/1190000016085105","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}