Java 线程同步（wait、notify、notifyAll）

一、方法介绍

1、void wait()

使得线程进入等待状态，直到它被其他线程通过notify()或者notifyAll唤醒。该方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用。如果当前线程不是锁的持有者，该方法会抛出一个IllegalMonitorStateException异常。

即如果锁住的是object，那么你只能调用object的wait()方法。

2、void notify()

随机选择一个在该对象上调用wait方法的线程，解除其阻塞状态。该方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用。如果当前线程不是锁的持有者，该方法会抛出一个IllegalMonitorStateException异常。

3、void notifyAll()

解除所有那些在该对象上调用wait方法的线程的阻塞状态。该方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用。如果当前线程不是锁的持有者，该方法会抛出一个IllegalMonitorStateException异常。

二、范例讲解（ 注意测试类A只能声明为外部类，下面代码在 class A 和 main方法 之间 省略了MainClass { ... } ）

1、“同步块”测试

（1）加synchronized(){}

class A{}public static void main(String[] args) {A object = new A();System.out.println("Main Thread Id = " + Thread.currentThread().getId());synchronized (object) {try {System.out.println("This Thread Id = " + Thread.currentThread().getId());object.wait();} catch (InterruptedException e) {</span>e.printStackTrace();}}}

输出

（2）不加synchronized(){}

class A{}public static void main(String[] args) {A object = new A();System.out.println("Main Thread Id = " + Thread.currentThread().getId());//synchronized (object) {try {</span>System.out.println("This Thread Id = " + Thread.currentThread().getId());</span>object.wait();} catch (InterruptedException e) {</span>e.printStackTrace();}//}}

输出如下，L24指的是object.wait()的那行

2、同步方法测试

（1）synchronized方法前缀

class A{public synchronized void test() throws InterruptedException {//synchronized (this) {System.out.println("Object Thread Id = " + Thread.currentThread().getId());wait();//}};}public static void main(String[] args) {A object = new A();System.out.println("Main Thread Id = " + Thread.currentThread().getId());//synchronized (object) {try {System.out.println("This Thread Id = " + Thread.currentThread().getId());//object.wait();object.test();} catch (InterruptedException e) {e.printStackTrace();}//}}

输出

（2）不synchronized方法前缀

class A{public void test() throws InterruptedException {//synchronized (this) {System.out.println("Object Thread Id = " + Thread.currentThread().getId());wait();//}};}public static void main(String[] args) {A object = new A();System.out.println("Main Thread Id = " + Thread.currentThread().getId());//synchronized (object) {try {System.out.println("This Thread Id = " + Thread.currentThread().getId());//object.wait();object.test();} catch (InterruptedException e) {e.printStackTrace();}//}}

输出

3、使用其它锁对象测试

class A{}public static void main(String[] args) {A object = new A();Object object2 = new Object();System.out.println("Main Thread Id = " + Thread.currentThread().getId());synchronized (object) {//synchronized (object2) {try {System.out.println("This Thread Id = " + Thread.currentThread().getId());object.wait();//object.test();} catch (InterruptedException e) {e.printStackTrace();}}}

输出：

4、wait、notify进行线程同步

（1）一（wait）对一（notify）模式

class A{}public static void main(String[] args) {final A object = new A();System.out.println("main thread id = " + Thread.currentThread().getId());new Thread(new Runnable() {@Overridepublic void run() {synchronized (object) {try {System.out.println("before wait thread id = " + Thread.currentThread().getId());object.wait();System.out.println("after wait thread id = " + Thread.currentThread().getId());} catch (InterruptedException e) {e.printStackTrace();}}}}).start();new Thread(new Runnable() {@Overridepublic void run() {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (object) {System.out.println("before notify thread id = " + Thread.currentThread().getId());object.notify();System.out.println("after notify thread id = " + Thread.currentThread().getId());}}}).start();}

输出

（2）多（wait）对一（notify）模式

class A{}public static void main(String[] args) {final A object = new A();System.out.println("main thread id = " + Thread.currentThread().getId());new Thread(new Runnable() {@Overridepublic void run() {synchronized (object) {try {System.out.println("before wait1 thread id = " + Thread.currentThread().getId());object.wait();System.out.println("after wait1 thread id = " + Thread.currentThread().getId());} catch (InterruptedException e) {e.printStackTrace();}}}}).start();new Thread(new Runnable() {@Overridepublic void run() {synchronized (object) {try {System.out.println("before wait2 thread id = " + Thread.currentThread().getId());object.wait();System.out.println("after wait2 thread id = " + Thread.currentThread().getId());} catch (InterruptedException e) {e.printStackTrace();}}}}).start();new Thread(new Runnable() {@Overridepublic void run() {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (object) {System.out.println("before notify thread id = " + Thread.currentThread().getId());object.notify();System.out.println("after notify thread id = " + Thread.currentThread().getId());}}}).start();}

输出

5、notifyAll

class A{}public static void main(String[] args) {final A object = new A();System.out.println("main thread id = " + Thread.currentThread().getId());new Thread(new Runnable() {@Overridepublic void run() {synchronized (object) {try {System.out.println("before wait1 thread id = " + Thread.currentThread().getId());object.wait();System.out.println("after wait1 thread id = " + Thread.currentThread().getId());} catch (InterruptedException e) {e.printStackTrace();}}}}).start();new Thread(new Runnable() {@Overridepublic void run() {synchronized (object) {try {System.out.println("before wait2 thread id = " + Thread.currentThread().getId());object.wait();System.out.println("after wait2 thread id = " + Thread.currentThread().getId());} catch (InterruptedException e) {e.printStackTrace();}}}}).start();new Thread(new Runnable() {@Overridepublic void run() {try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (object) {System.out.println("before notify thread id = " + Thread.currentThread().getId());object.notifyAll();System.out.println("after notify thread id = " + Thread.currentThread().getId());}}}).start();}

输出

三、总结：

（1）由1和2可以看出：object.wait()方法只能在同步方法（void synchronized methodName( args... ){ ... }）或者 “同步块内部“ （synchronized(object){ ... }）被调用，否则会抛出一个IllegalMonitorStateException异常。

（2）由3可以看出：object.wait()方法只能在锁住自己的情况下被调用，否则会抛出一个IllegalMonitorStateException异常。

（3）由4可得：一个notify只能唤醒一个wait的线程

（4）由5可得：一个notifyAll可以唤醒多个wait的线程

四、注意点：（网上很多博客都讲的不清不楚的，龙神我估计他们就没有花时间真正的去测试过）

wait、notify和notifyAll方法是Object类的final native方法。所以这些方法不能被子类重写，Object类是所有类的超类，因此在程序中有以下三种形式调用wait等方法：

1、wait()

2、this.wait()

3、super.wait()

这三个方法是等价的。