Java线程中的wait, notify and notifyAll

Java线程中的wait, notify and notifyAll

Object对象中有wait, notify and notifyAll三个方法，其主要用来进行线程间的通讯（communicate about their locking status）。需要注意的是，这这些方法需要在同步块中调用。

wait

wait有三个重载方法，一个是一直等待，在调用wait方法后，其释放该对象上的锁，直到另一线程调用该对象上的notify() 或 notifyAll()方法来唤醒该线程，另外两个是让调用wait()的线程等待指定的时间后自动唤醒或者在此之前被另一线程的notify() 或 notifyAll()唤醒。调用wait()方法后，该线程释放了该对象上的monitor。

notify

notify()只能唤醒等待在该对象上的一个线程，唤醒后，该线程开始执行，如果有多个线程等待在该对象上，那么notify()只能唤醒其中一个。具体唤醒哪一个，依赖于OS对线程管理的实现。

notifyAll

唤醒所有等待在该对象上的线程，具体哪一个线程先进入Running状态，依赖于OS对线程管理的实现。

package Test;

/**
 * Created by siege on 2015-09-14.
 */
public class Message {
    private String msg;

    public Message(String str){
        this.msg=str;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String str) {
        this.msg=str;
    }

}

---

package Test;

/**
 * Created by siege on 2015-09-14.
 */
public class Waiter implements Runnable {
    private Message msg;

    public Waiter(Message msg) {
        this.msg = msg;
    }

    @Override
    public void run() {
        String name=Thread.currentThread().getName();
        synchronized (msg){
            try {
                System.out.println(name+" waiting to get notified at time:"+System.currentTimeMillis());
                msg.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(name+" waiter thread got notified at time:"+System.currentTimeMillis());
            System.out.println(name+" processed: "+msg.getMsg());
        }

    }
}

---

package Test;

/**
 * Created by siege on 2015-09-14.
 */
public class Notifier implements Runnable {

    private Message msg;

    public Notifier(Message msg) {
        this.msg = msg;
    }

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        System.out.println(name+" started");
        try {
            Thread.sleep(1000);
            synchronized (msg){
                msg.setMsg(name+" Notifier work done");
                msg.notify();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

---

package Test;

/**
 * Created by siege on 2015-09-14.
 */
public class WaitNotifyTest {
    public static void main(String[] args) {
        Message msg=new Message("process it");
        Waiter waiter=new Waiter(msg);
        new Thread(waiter,"waiter").start();

        Waiter waiter1=new Waiter(msg);
        new Thread(waiter1,"waiter1").start();

        Notifier notifier=new Notifier(msg);
        new Thread(notifier,"notifier").start();
        System.out.println("All the threads are started");
    }

}

输出结果可能如下所示：

All the threads are started
waiter waiting to get notified at time:1442232184486
waiter1 waiting to get notified at time:1442232184486
notifier started
waiter waiter thread got notified at time:1442232185495
waiter processed: notifier Notifier work done