Java线程的等待/通知机制

Java线程的等待通知是由wait()和notify()实现的。

关键点：

1.只能在同步方法块中调用wait()方法。

2.只有等通知的线程运行结束，被通知的线程才开始运行。

3.只能唤醒同一共享资源的线程，即只能notify相同对象的wait()方法。

下面是Demo：

SynchronizedDemo启动了三个线程，ThreadA和ThreadB两个是等待线程，ThreadC是通知线程。

ThreadC通知ThreadA和ThreadB停止等待继续执行。

package com.example.demo.synchronize;

import lombok.extern.slf4j.Slf4j;

@Slf4j
public class SynchronizedDemo {

    public static void main(String[] args) {
        Object lock = new Object();
        Thread threadA = new Thread(new ThreadA(lock));
        Thread threadB = new Thread(new ThreadB(lock));
        Thread threadC = new Thread(new ThreadC(lock));

        threadA.start();
        threadB.start();
        threadC.start();
    }

}

等待线程ThreadA： 

package com.example.demo.synchronize;

import lombok.extern.slf4j.Slf4j;

import java.util.Random;
import java.util.concurrent.TimeUnit;

@Slf4j
public class ThreadA implements Runnable {
    private Object lock;
    public ThreadA(Object obj) {
        lock = obj;
    }

    @Override
    public void run() {
        log.info("begin---------");
        try {
            synchronized (lock) {
                log.info("wait------");
                lock.wait();
                //通知线程结束后才开始唤起其它线程。
                log.info("being notified------");
                TimeUnit.SECONDS.sleep(new Random().nextInt(3));
            }
        }catch (Exception e){
            e.printStackTrace();
        }
        log.info("end---------");
    }
}

等待线程ThreadB：

package com.example.demo.synchronize;

import lombok.extern.slf4j.Slf4j;

import java.util.Random;
import java.util.concurrent.TimeUnit;

@Slf4j
public class ThreadB implements Runnable {
    private Object lock;
    public ThreadB(Object obj) {
        lock = obj;
    }
    @Override
    public void run() {
        log.info("begin---------");
        try {
            synchronized (lock) {
                log.info("wait------");
                lock.wait();
                //通知线程结束后才开始唤起其它线程。
                log.info("being notified------");
                TimeUnit.SECONDS.sleep(new Random().nextInt(3));
            }
        }catch (Exception e){
            e.printStackTrace();
        }
        log.info("end---------");
    }
}

 通知线程ThreadC：

package com.example.demo.synchronize;

import lombok.extern.slf4j.Slf4j;

import java.util.Random;
import java.util.concurrent.TimeUnit;

@Slf4j
public class ThreadC implements Runnable {
    private Object lock;
    public ThreadC(Object obj) {
        lock = obj;
    }

    @Override
    public void run() {
        log.info("通知线程begin---------");
        try {
            synchronized (lock) {
                TimeUnit.SECONDS.sleep(new Random().nextInt(5)+5);
                log.info("通知线程notify other------");
                lock.notifyAll();
            }
        }catch (Exception e){
            e.printStackTrace();
        }
        log.info("通知线程end---------");
    }
}

 执行结果是：ThreadC结束后，ThreadA和ThreadB才会继续执行

00:58:58.876 [Thread-0] INFO com.example.demo.synchronize.ThreadA - begin---------
00:58:58.876 [Thread-1] INFO com.example.demo.synchronize.ThreadB - begin---------
00:58:58.876 [Thread-2] INFO com.example.demo.synchronize.ThreadC - 通知线程begin---------
00:58:58.876 [Thread-0] INFO com.example.demo.synchronize.ThreadA - wait------
00:58:58.876 [Thread-1] INFO com.example.demo.synchronize.ThreadB - wait------
00:59:06.878 [Thread-2] INFO com.example.demo.synchronize.ThreadC - 通知线程notify other------
00:59:06.878 [Thread-2] INFO com.example.demo.synchronize.ThreadC - 通知线程end---------
00:59:06.878 [Thread-1] INFO com.example.demo.synchronize.ThreadB - being notified------
00:59:08.879 [Thread-0] INFO com.example.demo.synchronize.ThreadA - being notified------
00:59:08.879 [Thread-1] INFO com.example.demo.synchronize.ThreadB - end---------
00:59:09.880 [Thread-0] INFO com.example.demo.synchronize.ThreadA - end---------