Java:使用wait()与notify()实现线程间协作

使用wait()与notify()/notifyAll()可以使得多个任务之间彼此协作。
1. wait()[/b]与[/b]notify()/notifyAll()[/b]
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.[/b]模拟单个线程对多个线程的唤醒[/b][/b]
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.javapackage 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!
[align=left][/align]
3.[/b]模拟忙等待过程[/b][/b]
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.javaimport 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：
[align=left]i'm going.[/align]
He has gone.
4.[/b]使用[/b]wait()[/b]与[/b]notify()[/b]改写上面的例子[/b][/b]
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.javapackage 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();
}
}结果：
[align=left]i'm going.[/align]
He has gone.