并发 | 等待/通知机制

等待/通知机制

等待通知机制，是指一个线程A调用了对象O的wait()方法进入等待状态，而另一个线程B调用了对象O的notify()或着notifyAll()方法，线程A收到通知后从对象O的wait()方法返回，进而执行后续操作。
示例代码

package concurrency;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.TimeUnit;

public class WaitNotify {
    static boolean flag = true;
    static Object lock = new Object();
    
    public static void main(String[] args) throws InterruptedException {
        Thread waitThread = new Thread(new Wait(), "WaitThread");
        waitThread.start();
        TimeUnit.SECONDS.sleep(1);
        Thread notifyThread = new Thread(new Notify(), "NotifyThread");
        notifyThread.start();
    }
    
    static class Wait implements Runnable{
        @Override
        public void run(){
            //加锁，用于lock的Monitor
            synchronized(lock){
                //当条件不满足时，继续wait，同时释放了lock的锁
                while(flag){
                    try {
                        System.out.println(Thread.currentThread()+" flag is true. wait@ " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
                        lock.wait();
                    } catch (InterruptedException e) {
                    }
                }
                //条件满足时，完成工作
                System.out.println(Thread.currentThread() + "flag is false. running@ " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
            }
        }
    }
    
    static class Notify implements Runnable{
        @Override
        public void run() {
            synchronized(lock){
                //获取lock的锁，然后进行通知，通知时不会释放lock的锁
                //直到当前线程释放了lock后，WaitThread才能从wait方法中返回
                System.out.println(Thread.currentThread() + " hold lock. notify @ " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
                lock.notifyAll();
                flag = false;
                SleepUtils.second(5);
            }
            
            //再次加锁
            synchronized(lock){
                System.out.println(Thread.currentThread()+ "hold lock again. sleep@ " + new SimpleDateFormat("HH:mm:ss").format(new Date()));
                SleepUtils.second(5);
            }
            
        }
    }
}