CountDownLatch如其所寫，是一個倒計數的鎖存器，當計數減至0時觸發特定的事件。利用這種特性，可以讓主線程等待子線程的結束。下面以一個模擬運動員比賽的例子加以說明。
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Test {
private static final int PLAYER_AMOUNT = 5;
public static void main(String[] args) {
//對于每位運動員，CountDownLatch減1后即結束比賽
CountDownLatch begin = new CountDownLatch(1);
//對于整個比賽，所有運動員結束后才算結束
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);
//設置特定的線程池，大小為5
ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
for (Player p : plays)
exe.execute(p); //分配線程
begin.countDown();
System.out.println("Race begins!");
try {
end.await(); //等待end狀態變為0，即為比賽結束
} catch (InterruptedException e) {
// TODO: handle exception
e.printStackTrace();
} finally {
System.out.println("Race ends!");
}
exe.shutdown();
}
}
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;
}
public void run() {
// TODO Auto-generated method stub
try {
begin.await(); //等待begin的狀態為0
Thread.sleep((long) (Math.random() * 100)); //隨機分配時間，即運動員完成時間
System.out.println("Play" + id + " arrived.");
} catch (InterruptedException e) {
// TODO: handle exception
e.printStackTrace();
} finally {
end.countDown(); //使end狀態減1，最終減至0
}
}
}