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(){}
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class A{}
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public static void main(String[] args) {
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A object = new A();
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System.out.println("Main Thread Id = " + Thread.currentThread().getId());
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synchronized (object) {
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try {
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System.out.println("This Thread Id = " + Thread.currentThread().getId());
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object.wait();
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} catch (InterruptedException e) {
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</span>e.printStackTrace();
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}
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}
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}
输出
（2）不加synchronized(){}
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class A{}
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public static void main(String[] args) {
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A object = new A();
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System.out.println("Main Thread Id = " + Thread.currentThread().getId());
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//synchronized (object) {
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try {
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</span>System.out.println("This Thread Id = " + Thread.currentThread().getId());
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</span>object.wait();
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} catch (InterruptedException e) {
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</span>e.printStackTrace();
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}
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//}
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}
2、同步方法测试
（1）synchronized方法前缀
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class A
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{
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public synchronized void test() throws InterruptedException {
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//synchronized (this) {
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System.out.println("Object Thread Id = " + Thread.currentThread().getId());
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wait();
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//}
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};
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}
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public static void main(String[] args) {
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A object = new A();
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System.out.println("Main Thread Id = " + Thread.currentThread().getId());
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//synchronized (object) {
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try {
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System.out.println("This Thread Id = " + Thread.currentThread().getId());
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//object.wait();
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object.test();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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//}
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}
输出
（2）不synchronized方法前缀
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class A
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{
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public void test() throws InterruptedException {
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//synchronized (this) {
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System.out.println("Object Thread Id = " + Thread.currentThread().getId());
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wait();
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//}
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};
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}
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public static void main(String[] args) {
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A object = new A();
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System.out.println("Main Thread Id = " + Thread.currentThread().getId());
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//synchronized (object) {
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try {
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System.out.println("This Thread Id = " + Thread.currentThread().getId());
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//object.wait();
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object.test();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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//}
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}
输出
3、使用其它锁对象测试
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class A{}
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public static void main(String[] args) {
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A object = new A();
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Object object2 = new Object();
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System.out.println("Main Thread Id = " + Thread.currentThread().getId());
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synchronized (object) {
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//synchronized (object2) {
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try {
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System.out.println("This Thread Id = " + Thread.currentThread().getId());
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object.wait();
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//object.test();
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}
输出：
4、wait、notify进行线程同步
（1）一（wait）对一（notify）模式
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class A{}
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public static void main(String[] args) {
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final A object = new A();
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System.out.println("main thread id = " + Thread.currentThread().getId());
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new Thread(new Runnable() {
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@Override
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public void run() {
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synchronized (object) {
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try {
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System.out.println("before wait thread id = " + Thread.currentThread().getId());
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object.wait();
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System.out.println("after wait thread id = " + Thread.currentThread().getId());
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}
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}).start();
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new Thread(new Runnable() {
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@Override
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public void run() {
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try {
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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synchronized (object) {
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System.out.println("before notify thread id = " + Thread.currentThread().getId());
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object.notify();
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System.out.println("after notify thread id = " + Thread.currentThread().getId());
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}
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}
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}).start();
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}
输出
（2）多（wait）对一（notify）模式
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class A{}
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public static void main(String[] args) {
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final A object = new A();
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System.out.println("main thread id = " + Thread.currentThread().getId());
-
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new Thread(new Runnable() {
-
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@Override
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public void run() {
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synchronized (object) {
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try {
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System.out.println("before wait1 thread id = " + Thread.currentThread().getId());
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object.wait();
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System.out.println("after wait1 thread id = " + Thread.currentThread().getId());
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}
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}).start();
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new Thread(new Runnable() {
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@Override
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public void run() {
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synchronized (object) {
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try {
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System.out.println("before wait2 thread id = " + Thread.currentThread().getId());
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object.wait();
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System.out.println("after wait2 thread id = " + Thread.currentThread().getId());
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}
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}).start();
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new Thread(new Runnable() {
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@Override
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public void run() {
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try {
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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synchronized (object) {
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System.out.println("before notify thread id = " + Thread.currentThread().getId());
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object.notify();
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System.out.println("after notify thread id = " + Thread.currentThread().getId());
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}
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}
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}).start();
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}
5、notifyAll
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class A{}
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public static void main(String[] args) {
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final A object = new A();
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System.out.println("main thread id = " + Thread.currentThread().getId());
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new Thread(new Runnable() {
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@Override
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public void run() {
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synchronized (object) {
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try {
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System.out.println("before wait1 thread id = " + Thread.currentThread().getId());
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object.wait();
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System.out.println("after wait1 thread id = " + Thread.currentThread().getId());
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}
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}).start();
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new Thread(new Runnable() {
-
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@Override
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public void run() {
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synchronized (object) {
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try {
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System.out.println("before wait2 thread id = " + Thread.currentThread().getId());
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object.wait();
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System.out.println("after wait2 thread id = " + Thread.currentThread().getId());
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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}
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}
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}).start();
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new Thread(new Runnable() {
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@Override
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public void run() {
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try {
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Thread.sleep(1000);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}
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synchronized (object) {
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System.out.println("before notify thread id = " + Thread.currentThread().getId());
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object.notifyAll();
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System.out.println("after notify thread id = " + Thread.currentThread().getId());
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}
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}
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}).start();
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}
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。
文章来源: blog.csdn.net，作者：福州-司马懿，版权归原作者所有，如需转载，请联系作者。
原文链接：blog.csdn.net/chy555chy/article/details/51864773
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