sync

1 package com.paddx.test.concurrent;

2

3 public class SynchronizedDemo {

4 public void method() {

5 synchronized (this) {

6 System.out.println("Method 1 start&quot😉;

7 }

8 }

9 }

反編譯結果：

關于這兩條指令的作用，我們直接參考JVM規范中描述：

monitorenter ：

Each object is associated with a monitor. A monitor is locked if and only if it has an owner. The thread that executes monitorenter attempts to gain ownership of the monitor associated with objectref, as follows:

? If the entry count of the monitor associated with objectref is zero, the thread enters the monitor and sets its entry count to one. The thread is then the owner of the monitor.

? If the thread already ow

? If another thread already owns the monitor associated with objectref, the thread blocks until the monitor's entry count is zero, then tries again to gain ownership.

這段話的大概意思為：

每個對象有一個監視器鎖(monitor)。當monitor被占用時就會處于鎖定狀態，線程執行monitorenter指令時嘗試獲取monitor的所有權，過程：

1、如果monitor的進入數為0，則該線程進入monitor，然后將進入數設置為1，該線程即為monitor的所有者。

2、如果線程已經占有該monitor，只是重新進入，則進入monitor的進入數加1.

3.如果其他線程已經占用了monitor，則該線程進入阻塞狀態，直到monitor的進入數為0，再重新嘗試獲取monitor的所有權。

monitorexit：

The thread that executes monitorexit must be the owner of the monitor associated with the instance referenced by objectref.

The thread decrements the entry count of the monitor associated with objectref. If as a result the value of the entry count is zero, the thread exits the monitor and is no longer its owner. Other threads that are blocking to enter the monitor are allowed to attempt to do so.

這段話的大概意思為：

執行monitorexit的線程必須是objectref所對應的monitor的所有者。

指令執行時，monitor的進入數減1，如果減1后進入數為0，那線程退出monitor，不再是這個monitor的所有者。其他被這個monitor阻塞的線程可以嘗試去獲取這個 monitor 的所有權。

通過這兩段描述，我們應該能很清楚的看出Synchronized的實現原理，Synchronized的語義底層是通過一個monitor的對象來完成，其實wait/notify等方法也依賴于monitor對象，這就是為什么只有在同步的塊或者方法中才能調用wait/notify等方法，否則會拋出java.lang.IllegalMonitorStateException的異常的原因。

我們再來看一下同步方法的反編譯結果：

源代碼：

1 package com.paddx.test.concurrent;

2

3 public class SynchronizedMethod {

4 public synchronized void method() {

5 System.out.println("Hello World!&quot😉;

6 }

7 }

反編譯結果：

從反編譯的結果來看，方法的同步并沒有通過指令monitorenter和monitorexit來完成(理論上其實也可以通過這兩條指令來實現)，不過相對于普通方法，其常量池中多了A

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