Java多线程之wait和notify

最近在看Java特种兵，看到多线程部分，对wait和notify不是很理解，于是写了代码来帮助理解。特此记录下来，以供回顾。

             wait方法通过参数可以指定等待的时长。如果没有指定参数，默认一直等待直到被通知。notify方法是通知某个正在等待这个对象的控制权的线程可以继续运行。

调用wait方法时候，必须加上synchronized同步块，不然会抛出java.lang.IllegalMonitorStateException异常

public class WaitAndNotify {
private final static Object object = new Object();
	
	static class ThreadA extends Thread{
		public void run(){
			//通过object对象进行加锁
				System.out.println(Thread.currentThread().getName()+":start...");
				try {
					//调用当前对象的wait方法
					Thread.currentThread().wait();
				} catch (InterruptedException e) {
					System.out.println("aaaa");
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName()+":end...");
		}
	}
	public static void main(String[] args) throws InterruptedException {
		ThreadA t1 = new ThreadA();
		t1.setName("t1");
		t1.start();
		Thread.sleep(100);
		System.out.println(t1.getName()+":"+t1.getState());
	}
}

public class WaitAndNotify {
private final static Object object = new Object();
	
	static class ThreadA extends Thread{
		public void run(){
			//通过object对象进行加锁
			synchronized (object) {
				System.out.println(Thread.currentThread().getName()+":start...");
				try {
					//调用当前对象的wait方法
					Thread.currentThread().wait();
				} catch (InterruptedException e) {
					System.out.println("aaaa");
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName()+":end...");
			}
		}
	}
	public static void main(String[] args) throws InterruptedException {
		ThreadA t1 = new ThreadA();
		t1.setName("t1");
		t1.start();
		Thread.sleep(100);
		System.out.println(t1.getName()+":"+t1.getState());
	}
}

 
这段程序是报异常：

t1:start...
Exception in thread "t1" java.lang.IllegalMonitorStateException
	at java.lang.Object.wait(Native Method)
	at java.lang.Object.wait(Unknown Source)
	at Thread.WaitAndNotify$ThreadA.run(WaitAndNotify.java:15)
t1:TERMINATED

 
原因是：锁对象和调用wait方法的对象不是同一个。wait方法是Object的方法，对象和调用wait方法的对象必须是同一个对象。
以下是修改后的正确代码：

public class WaitAndNotify {
private final static Object object = new Object();
	
	static class ThreadA extends Thread{
		public void run(){
			//通过object对象进行加锁
			synchronized (this) {
				System.out.println(Thread.currentThread().getName()+":start...");
				try {
					//调用当前对象的wait方法
					Thread.currentThread().wait();
				} catch (InterruptedException e) {
					System.out.println("aaaa");
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName()+":end...");
			}
		}
	}
	public static void main(String[] args) throws InterruptedException {
		ThreadA t1 = new ThreadA();
		t1.setName("t1");
		t1.start();
		Thread.sleep(100);
		System.out.println(t1.getName()+":"+t1.getState());
	}
}

 或者：

public class WaitAndNotify {
private final static Object object = new Object();
	
	static class ThreadA extends Thread{
		public void run(){
			//通过object对象进行加锁
			synchronized (object) {
				System.out.println(Thread.currentThread().getName()+":start...");
				try {
					//调用object对象的wait方法
					object.wait();
				} catch (InterruptedException e) {
					System.out.println("aaaa");
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName()+":end...");
			}
		}
	}
	public static void main(String[] args) throws InterruptedException {
		ThreadA t1 = new ThreadA();
		t1.setName("t1");
		t1.start();
		Thread.sleep(100);
		System.out.println(t1.getName()+":"+t1.getState());
	}
}

 上面两段代码虽然正确执行了，但是由于线程状态一直是WAITING，会一直等待下去，这个时候，我们使用调用notify方法，让线程继续执行下去

public class WaitAndNotify {
private final static Object object = new Object();
	
	static class ThreadA extends Thread{
		public void run(){
			//通过object对象进行加锁
			synchronized (object) {
				System.out.println(Thread.currentThread().getName()+":start...");
				try {
					//调用object对象的wait方法
					object.wait();
				} catch (InterruptedException e) {
					System.out.println("aaaa");
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName()+":end...");
			}
		}
	}
	public static void main(String[] args) throws InterruptedException {
		ThreadA t1 = new ThreadA();
		t1.setName("t1");
		t1.start();
		Thread.sleep(100);
		System.out.println(t1.getName()+":"+t1.getState());
		//和wait方法一样，需要加锁
		synchronized (object) {
			object.notify();
		}
	}
}