浅析Java中CountDownLatch用法

CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值，每调用一次countDown()方法，计数器减1,计数器大于0 时，await()方法会阻塞程序继续执行。

CountDownLatch如其所写，是一个倒计数的锁存器，当计数减至0时触发特定的事件。利用这种特性，可以让主线程等待子线程的结束。下面以一个模拟运动员比赛的例子加以说明。

package pool;

import java.util.concurrent.CountDownLatch;

public class Player implements Runnable
{

// 运动员编号号
private int id;
private final CountDownLatch begin;
private final CountDownLatch end;

public Player(int id, CountDownLatch begin, CountDownLatch end)
{
this.id = id;
this.begin = begin;
this.end = end;
}

@Override
public void run()
{
try
{
// 等待begin的状态为0
begin.await();

final Long time = (long) (Math.random() * 100);
//// 随机分配时间，即运动员完成时间
Thread.sleep(time);

System.out.println("Play" + id + "use " + time + " arrived.");
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
// 使end状态减1，最终减至0
end.countDown();
}

}

}

package pool;

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()
{

}

public static void main(String[] args)
{
// 对于每位运动员，CountDownLatch减1后开始比赛
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)
{
e.printStackTrace();
}
finally
{
System.out.println("Race ends!");
}
exe.shutdown();
}
}