闭锁相当于一扇门，在闭锁到达结束状态之前，这扇门一直是关闭着的，没有任何线程可以通过，当到达结束状态时，这扇门才会打开并容许所有线程通过。它可以使一个或多个线程等待一组事件发生。闭锁状态包括一个计数器，初始化为一个正式，正数表示需要等待的事件数量。countDown方法递减计数器，表示一个事件已经发生，而await方法等待计数器到达0，表示等待的事件已经发生。CountDownLatch强调的是一个线程（或多个）需要等待另外的n个线程干完某件事情之后才能继续执行。
场景应用：
10个运动员准备赛跑，他们等待裁判一声令下就开始同时跑，当最后一个人通过终点的时候，比赛结束。10个运动相当于10个线程，这里关键是控制10个线程同时跑起来，还有怎么判断最后一个线程到达终点。可以用2个闭锁，第一个闭锁用来控制10个线程等待裁判的命令，第二个闭锁控制比赛结束。
\
import java.util.concurrent.CountDownLatch;
``
class Aworker implements Runnable {
private int num;
private CountDownLatch begin;
private CountDownLatch end;
``
public Aworker(int num, final CountDownLatch begin, final CountDownLatch end) {
this.num = num;
this.begin = begin;
this.end = end;
}
``
@Override
public void run() {
// TODO Auto-generated method stub
try {
System.out.println(num + "th people is ready");
begin.await(); //准备就绪
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
end.countDown(); //计数器减一，到达终点
System.out.println(num + "th people arrive");
}
}
}
``
public class Race {
public static void main(String[] args) {
int num = 10;
CountDownLatch begin = new CountDownLatch(1);
CountDownLatch end = new CountDownLatch(num);
``
for (int i = 1; i <= num; i++) {
new Thread(new Aworker(i, begin, end)).start();
}
``
try {
Thread.sleep((long) (Math.random() * 5000));
} catch (InterruptedException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
System.out.println("judge say : run !");
begin.countDown(); //裁判一声令下开始跑
long startTime = System.nanoTime();
try {
end.await(); //等待结束
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
long endTime = System.nanoTime();
System.out.println("judge say : all arrived !");
System.out.println("spend time: " + (endTime - startTime));
}
}
}
\
输出
1thpeopleisready
2thpeopleisready
4thpeopleisready
6thpeopleisready
3thpeopleisready
10thpeopleisready
8thpeopleisready
5thpeopleisready
7thpeopleisready
9thpeopleisready
judgesay:run!
1thpeoplearrive
4thpeoplearrive
10thpeoplearrive
5thpeoplearrive
2thpeoplearrive
judgesay:allarrived!
9thpeoplearrive
7thpeoplearrive
8thpeoplearrive
3thpeoplearrive
6thpeoplearrive
spendtime:970933
