Java多线程的通信3

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在使用wait/notify机制实现多线程通信时，需要注意如下方面：

notify只能通知一个线程，如果有多个线程需要被通知的话，需要使用notifyAll。
notify不能通知过早，如果通知过早，会导致wait线程一直等待。

先看等待线程类：

package com.lenovo.plm.dms.p11;public class MyThreadA extends Thread {    private Object lock;        public MyThreadA(Object lock){        this.lock = lock;    }        @Override    public void run() {        // TODO Auto-generated method stub        super.run();        synchronized(lock){                        System.out.println(Thread.currentThread().getName() +" begin to wait..");            try {                lock.wait();            } catch (InterruptedException e) {                // TODO Auto-generated catch block                e.printStackTrace();            }                        System.out.println(Thread.currentThread().getName() +" come to end..");                    }    }    }

再看通知线程类：

package com.lenovo.plm.dms.p11;public class MyThreadB extends Thread {private Object lock;        public MyThreadB(Object lock){        this.lock = lock;    }        @Override    public void run() {        // TODO Auto-generated method stub        super.run();        synchronized(lock){                        System.out.println(Thread.currentThread().getName() +" begin to notify..");            lock.notify();            System.out.println(Thread.currentThread().getName() +" end to notify..");                    }    }    }

运行类如下：

package com.lenovo.plm.dms.p11;public class Main {        public static void main(String[] args) {                Object lock = new Object();        MyThreadA t1 = new MyThreadA(lock);        MyThreadA t2 = new MyThreadA(lock);        t1.start();        t2.start();        try {            Thread.sleep(3000);        } catch (InterruptedException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }        MyThreadB t3 = new MyThreadB(lock);        t3.start();            } }

此处创建了两个等待线程，一个通知线程，当通知线程调用notify后，只有一个等待线程被通知到了。看结果：

Thread-0 begin to wait..
Thread-1 begin to wait..
Thread-2 begin to notify..
Thread-2 end to notify..
Thread-0 come to end..

这里有一个线程Thread-1还在等待中。将通知线程中的notify方法改为notifyAll方法后，运行结果如下：

Thread-0 begin to wait..
Thread-1 begin to wait..
Thread-2 begin to notify..
Thread-2 end to notify..
Thread-1 come to end..
Thread-0 come to end..

还有一种情况是：如果通知线程在其他线程等待之前就通知了，那么等待的线程就一直会等待着，不会停止了。

现在将运行类的main方法改成如下样子：

package com.lenovo.plm.dms.p11;public class Main {        public static void main(String[] args) {                Object lock = new Object();        MyThreadA t1 = new MyThreadA(lock);        MyThreadA t2 = new MyThreadA(lock);        MyThreadB t3 = new MyThreadB(lock);        t1.start();        t2.start();        t3.start();     }}

运行结果如下：

Thread-0 begin to wait..
Thread-2 begin to notify..
Thread-2 end to notify..
Thread-1 begin to wait..
Thread-0 come to end..

这里每次运行结果都会不同，这一次运行，Thread-1还没开始等待，Thread-2就通知了，导致Thread-1一直等待。

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