Java:使用wait()与notify()实现线程间协作

Java:使用wait()与notify()实现线程间协作

原创作品，允许转载，转载时请务必以超链接形式标明文章 原始出处 、作者信息和本声明。否则将追究法律责任。http://zhangjunhd.blog.51cto.com/113473/71387

使用wait()与notify()/notifyAll()可以使得多个任务之间彼此协作。

1. wait()与notify()/notifyAll()

调用sleep()和yield()的时候锁并没有被释放，而调用wait()将释放锁。这样另一个任务（线程）可以获得当前对象的锁，从而进入它的synchronized方法中。可以通过notify()/notifyAll()，或者时间到期，从wait()中恢复执行。

只能在同步控制方法或同步块中调用wait()、notify()和notifyAll()。如果在非同步的方法里调用这些方法，在运行时会抛出IllegalMonitorStateException异常。

2.模拟单个线程对多个线程的唤醒

模拟线程之间的协作。Game类有2个同步方法prepare()和go()。标志位start用于判断当前线程是否需要wait()。Game类的实例首先启动所有的Athele类实例，使其进入wait()状态，在一段时间后，改变标志位并notifyAll()所有处于wait状态的Athele线程。

Game.java

package concurrency;

import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

class Athlete implements Runnable {
    private final int id;
    private Game game;

    public Athlete(int id, Game game) {
      this.id = id;
      this.game = game;
    }

    public boolean equals(Object o) {
      if (!(o instanceof Athlete))
        return false;
      Athlete athlete = (Athlete) o;
      return id == athlete.id;
    }

    public String toString() {
      return "Athlete<" + id + ">";
    }

    public int hashCode() {
      return new Integer(id).hashCode();
    }

    public void run() {
      try {
        game.prepare(this);
      } catch (InterruptedException e) {
        System.out.println(this + " quit the game");
      }
    }
  }

public class Game implements Runnable {
    private Set<Athlete> players = new HashSet<Athlete>();
    private boolean start = false;

    public void addPlayer(Athlete one) {
      players.add(one);
    }

    public void removePlayer(Athlete one) {
      players.remove(one);
    }

    public Collection<Athlete> getPlayers() {
      return Collections.unmodifiableSet(players);
    }

    public void prepare(Athlete athlete) throws InterruptedException {
      System.out.println(athlete + " ready!");
      synchronized (this) {
        while (!start)
        wait();
        if (start)
          System.out.println(athlete + " go!");
      }
    }

    public synchronized void go() {
      notifyAll();
    }
    
    public void ready() {
      Iterator<Athlete> iter = getPlayers().iterator();
      while (iter.hasNext())
        new Thread(iter.next()).start();
    }

    public void run() {
      start = false;
      System.out.println("Ready......");
      System.out.println("Ready......");
      System.out.println("Ready......");
      ready();
      start = true;
      System.out.println("Go!");
      go();
    }

    public static void main(String[] args) {
      Game game = new Game();
      for (int i = 0; i < 10; i++)
        game.addPlayer(new Athlete(i, game));
      new Thread(game).start();
    }
}

结果：
Ready......
Ready......
Ready......
Athlete<0> ready!
Athlete<1> ready!
Athlete<2> ready!
Athlete<3> ready!
Athlete<4> ready!
Athlete<5> ready!
Athlete<6> ready!
Athlete<7> ready!
Athlete<8> ready!
Athlete<9> ready!
Go!
Athlete<9> go!
Athlete<8> go!
Athlete<7> go!
Athlete<6> go!
Athlete<5> go!
Athlete<4> go!
Athlete<3> go!
Athlete<2> go!
Athlete<1> go!
Athlete<0> go!

3.模拟忙等待过程

MyObject类的实例是被观察者，当观察事件发生时，它会通知一个Monitor类的实例（通知的方式是改变一个标志位）。而此Monitor类的实例是通过忙等待来不断的检查标志位是否变化。

BusyWaiting.java

import java.util.concurrent.TimeUnit;

class MyObject implements Runnable {
    private Monitor monitor;

    public MyObject(Monitor monitor) {
      this.monitor = monitor;
    }

    public void run() {
      try {
        TimeUnit.SECONDS.sleep(3);
        System.out.println("i'm going.");
        monitor.gotMessage();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
}

class Monitor implements Runnable {
    private volatile boolean go = false;

    public void gotMessage() throws InterruptedException {
      go = true;
    }

    public void watching() {
      while (go == false)
        ;
      System.out.println("He has gone.");
    }

    public void run() {
      watching();
    }
}

public class BusyWaiting {
    public static void main(String[] args) {
      Monitor monitor = new Monitor();
      MyObject o = new MyObject(monitor);
      new Thread(o).start();
      new Thread(monitor).start();
    }
}

结果：

i'm going.

He has gone.

4.使用wait()与notify()改写上面的例子

下面的例子通过wait()来取代忙等待机制，当收到通知消息时，notify当前Monitor类线程。

Wait.java

package concurrency.wait;

import java.util.concurrent.TimeUnit;

class MyObject implements Runnable {
    private Monitor monitor;

    public MyObject(Monitor monitor) {
      this.monitor = monitor;
    }

    public void run() {
      try {
        TimeUnit.SECONDS.sleep(3);
        System.out.println("i'm going.");
        monitor.gotMessage();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
}

class Monitor implements Runnable {
    private volatile boolean go = false;

    public synchronized void gotMessage() throws InterruptedException {
      go = true;
      notify();
    }

    public synchronized void watching() throws InterruptedException {
      while (go == false)
        wait();
      System.out.println("He has gone.");
    }

    public void run() {
      try {
        watching();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
}

public class Wait {
    public static void main(String[] args) {
      Monitor monitor = new Monitor();
      MyObject o = new MyObject(monitor);
      new Thread(o).start();
      new Thread(monitor).start();
    }
}