閉鎖相當於一扇門，在閉鎖到達結束狀態之前，這扇門一直是關閉著的，沒有任何線程可以通過，當到達結束狀態時，這扇門才會打開並容許所有線程通過。它可以使一個或多個線程等待一組事件發生。閉鎖狀態包含一個計數器，初始化為一個正式，正數表示需要等待的事件數量。 countDown方法遞減計數器，表示一個事件已經發生，而await方法等待計數器到達0，表示等待的事件已經發生。 CountDownLatch強調的是一個執行緒（或多個）需要等待另外的n個執行緒乾完某件事情之後才能繼續執行。
場景應用：
10個運動員準備賽跑，他們等待裁判一聲令下就開始同時跑，當最後一個人通過終點的時候，比賽結束。 10個運動相當於10個線程，這裡關鍵是控制10個線程同時跑起來，還有怎麼判斷最後一個線程到達終點。可以用2個閉鎖，第一個閉鎖用來控制10個線程等待裁判的命令，第二個閉鎖控制比賽結束。
import java.util.concurrent.CountDownLatch; class Aworker implements Runnable { private int num; private CountDownLatch begin; private CountDownLatch end； = begin; this.end = end; } @Override public void run() { // TODO Auto-generated method stub try { System.out.println(num + "th people is ready"); begin.await(); / /準備就緒} catch (InterruptedException e) { e.printStackTrace(); } finally { end.countDown(); //計數器減一，到達終點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(); //裁判一聲令下開始跑long startTime = System.nanoTime(); try { end.await(); //等待結束} 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)); } }}輸出
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