Java多线程(3)：wait()/notify()实例

下面是代码实例

public class WaitDemo implements Runnable {

    private Object lock;

    public WaitDemo(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        synchronized (lock) {
            System.out.println("Begin wait() ThreadName=" + Thread.currentThread().getName());
            try {
                // [STEP-1] WaitDemo线程立刻释放获得的对象锁lock，并放弃CPU，进入等待队列。
                lock.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // [STEP-4] WaitDemo有机会获得另一个线程释放的锁，并从等待的地方起开始执行。
            System.out.println("End wait() ThreadName=" + Thread.currentThread().getName());
        }
    }
}

public class NotifyDemo implements Runnable {

    private Object lock;

    public NotifyDemo(Object lock) {
        this.lock = lock;
    }

    @Override
    public void run() {
        synchronized (lock) {
            System.out.println("Begin notify() ThreadName=" + Thread.currentThread().getName());
            // [STEP-2] NotifyDemo线程唤醒其他挂起的线程(WaitDemo线程)。但是此时它并不立即释放锁。
            lock.notify();
            try {
                System.out.println("Sleep 3s...");
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("End notify() ThreadName=" + Thread.currentThread().getName());
            // [STEP-3] 释放锁。
        }
    }
}

// 该实例无法保证假设在线程WaitDemo和NotifyDemo的先后执行顺序。如果线程B先执行了notify()然后结束了，线程A才去执行wait()，那此时，线程A将无法被正常唤醒了
public class DemoTest {

    public static void main(String[] args) throws InterruptedException {
        Object o = new Object();
        // WaitDemo
        Thread a = new Thread(new WaitDemo(o));
        a.setName("WaitDemo");
        a.start();
        Thread.sleep(2000);
        // NotifyDemo
        Thread b = new Thread(new NotifyDemo(o));
        b.setName("NotifyDemo");
        b.start();
    }
}