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() {

@Override
public 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() {

@Override
public 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() {

@Override
public 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() {

@Override
public 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() {

@Override
public 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() {

@Override
public 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() {

@Override
public 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() {

@Override
public 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。