举例讲解线程的wait()和notify()

通常，多线程之间需要协调工作，例如两个人公用一个卫生间（每次只能一个人用），一个人必须等待另一个人用完，得知没有人使用的时候才能使用卫生间。
以上逻辑简单说就是：如果条件不满足，则等待。当条件满足时，等待该条件的线程将被唤醒。在java中，这个机制的实现依赖于wait() 和notify()。等待机制与锁机制密切关联的。例如

synchronized(obj){
while(!condition){
obj.wait();
}
}

当线程A获得了obj锁后，发现条件condition不满足，就无法下一处理，于是线程A就等待（调用wait()方法）。
在另外一线程B中，如果B更改了某些条件，使得线程A的condition条件满足了，就可以唤醒线程A：

synchronized(obj){
condition = true;
obj.notify();
}

如果以使用卫生间作为示例，假设他们都要刷牙和洗脸。他们是这样约定的：轮流用，第一个人现刷牙，然后第二人刷牙；第一个人洗脸，然后第二个人洗脸。程序代码如下所示：

package tarena;

public class Syn {

	public static void main(String[] args) {
		TwoPeople.ONE.start();
		TwoPeople.TWO.start();
	}
}

class TwoPeople extends Thread {

	private int i = 0;
	static Thread ONE = new TwoPeople(1);
	static Thread TWO = new TwoPeople(2);
	static Object lock = new Object();

	private TwoPeople(int i) {
		this.i = i;
	}

	public void run() {
		synchronized (lock) {
			try {
				if (i == 1) {
					this.brush();//1  people one brush
					lock.wait();//2  释放当前对象，进入等待状态
					this.release();//6 线程1 获取当前锁对象，并开始 people one release
					lock.notify();//7 注意：对象锁只能单一操作不能将该对象锁锁住，
																//所以这里只需要唤醒 线程2，进入就绪状态

				} else {
					this.brush();//3 线程2 people two brush
					lock.notify();//4  唤醒 线程1 并进入就绪状态
					lock.wait();//5  释放当前锁对象，让出CPU资源，使线程1 获取锁对象
					this.release();//8 获取对象锁，进入执行状态，最后run()结束
				}
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	private void brush() {
		System.out.println("People  " + i + "  is brushing!");
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("People  " + i + "  is brushed!");
	}

	private void release() {
		System.out.println("People  " + i + "  is releasing!");
		try {
			Thread.sleep(2000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("People  " + i + "  is released!");
	}
}

运行该示例，结果如下
[quote]
People 1 is brushing!
People 1 is brushed!
People 2 is brushing!
People 2 is brushed!
People 1 is releasing!
People 1 is released!
People 2 is releasing!
People 2 is released!
[/quote]