使用wait()與notify()/notifyAll()可以使得多個任務之間彼 此協作。
1. wait()與notify()/notifyAll()
調用sleep()和yield()的時候鎖並沒有被釋放，而調用wait() 將釋放鎖。這樣另一個任務（線程）可以獲得當前對象的鎖，從而 進入它的synchronized方法中。可以通過notify()/notifyAll()， 或者時間到期，從wait()中恢復執行。
只能在同步控制方法或同步塊中調用wait()、notify()和 notifyAll()。如果在非同步的方法裡調用這些方法，在運行時會 拋出IllegalMonitorStateException異常。
2.模擬單個線程對多個線程的喚醒
模擬線程之間的協作。Game類有2個同步方法prepare()和go() 。標志位start用於判斷當前線程是否需要wait()。Game類的實例 首先啟動所有的Athele類實例，使其進入wait()狀態，在一段時間 後，改變標志位並notifyAll()所有處於wait狀態的Athele線程。
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();
}
}
結果：
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.模擬忙等待過程
MyObject類的實例是被觀察者，當觀察事件發生時，它會通知 一個Monitor類的實例（通知的方式是改變一個標志位）。而此 Monitor類的實例是通過忙等待來不斷的檢查標志位是否變化。
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();
}
}
結果：
i'm going.
He has gone.
4.使用wait()與notify()改寫上面的例子
下面的例子通過wait()來取代忙等待機制，當收到通知消息時 ，notify當前Monitor類線程。
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();
}
}
結果：
i'm going.
He has gone.
本文出自 “子 孑” 博客，請務必保留此出處 http://zhangjunhd.blog.51cto.com/113473/71387
