CountDownLatch介绍
CountDownLatch是java.util.concurrent并发包中提供的一个可用于控制多线程同时开始某动作的类。其采用的方式为减计数的方式。当计数减至零时，位于await后的代码才会执行。
CountDownLatch(int)
??? 创建内部类Sync的对象实例，并将state设置为传入的参数。
? await()
???? 调用Sync继承的AbstractQueuedSynchronizer 的acquireShareInterruptibly完成。
acquireShareInterruptibly首先调用Sync的tryAcquireShared方法，该方法判断当前的state是否为零。如果为零，则返回1，否则返回-1。如果返回1，则await直接返回，如果返回-1，则将此线程放入队列进行等待，知道tryAcquireSharedfan返回-1或线程被interrupt
countdown()
调用Sync的tryReleaseShared方法，如果state不为零，基于CAS将state的值设置为减1后的值；如果减1后的值为零，则返回true，否则返回false
如果为true，则通知所有在队列等待的线程。
下面是一个例子
package DownTest;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class CountDownLatchDemo {
private static final int PLAYER_AMOUNT = 5;
public CountDownLatchDemo() {
// TODO Auto-generated constructor stub
}
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
// 对于每位运动员，CountDownLatch减1后即结束比赛
CountDownLatch begin = new CountDownLatch(1);
// 对于整个比赛，所有运动员结束后才算结束
CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
Player[] plays = new Player[PLAYER_AMOUNT];
for (int i = 0; i < PLAYER_AMOUNT; i++)
plays[i] = new Player(i + 1, begin, end);
// 设置特定的线程池，大小为5
ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
for (Player p : plays)
exe.execute(p); // 分配线程
System.out.println("Race begins!");
begin.countDown();
try {
end.await();
// 等待end状态变为0，即为比赛结束
} catch (InterruptedException e) {
// TODO: handle exception
e.printStackTrace();
} finally {
System.out.println("Race ends!");
}
exe.shutdown();
}
}
对于Player类如下
package DownTest;
import java.util.concurrent.CountDownLatch;
public class Player implements Runnable {
???? private int id;
???? private CountDownLatch begin;
???? private CountDownLatch end;
???? public Player(int i, CountDownLatch begin, CountDownLatch end) {
???????? // TODO Auto-generated constructor stub
???????? super();
???????? this.id = i;
???????? this.begin = begin;
???????? this.end = end;
???? }
???? @Override
???? public void run() {
???????? // TODO Auto-generated method stub
???????? try{
???????????? begin.await();??????? //等待begin的状态为0
???????????? Thread.sleep((long)(Math.random()*100));??? //随机分配时间，即运动员完成时间
???????????? System.out.println("Play"+id+" arrived.");
???????? }catch (InterruptedException e) {
???????????? // TODO: handle exception
???????????? e.printStackTrace();
???????? }finally{
???????????? end.countDown();??? //使end状态减1，最终减至0
???????? }
???? }
}
?执行结果：
Race begins!
Play2 arrived.
Play3 arrived.
Play5 arrived.
Play4 arrived.
Play1 arrived.
Race ends!
