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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()<o:p></o:p>
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 <o:p></o:p>
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。
<u> Game.java</u> 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!
<o:p></o:p>
3. 模拟忙等待过程 <o:p></o:p>
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。
<u> BusyWaiting.java</u> 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. <o:p></o:p>
He has gone.
4. 使用 wait() 与 notify() 改写上面的例子 <o:p></o:p>
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。
<u> Wait.java</u> 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();
}
}结果：
i'm going. <o:p></o:p>
He has gone.
----------------------------
原文链接:https://blog.51cto.com/zhangjunhd/71387
程序猿的技术大观园：www.javathinker.net
[这个贴子最后由 flybird 在 2020-01-08 15:10:15 重新编辑]
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Java技术网站 → 『 Java开发技术 』 → 『 Java编程技术专题 』 → 使用wait()与notify()实现线程间协作
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