閉鎖相當于一扇門，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，這扇門才會打開并容許所有線程通過。它可以使一個或多個線程等待一組事件發生。閉鎖狀態包括一個計數器，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強調的是一個線程（或多個）需要等待另外的n個線程干完某件事情之后才能繼續執行。
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，當最后一個人通過終點的時候，比賽結束。10個運動相當于10個線程，這里關鍵是控制10個線程同時跑起來，還有怎么判斷最后一個線程到達終點?？梢杂?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; 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(); //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
1th people is ready2th people is ready4th people is ready6th people is ready3th people is ready10th people is ready8th people is ready5th people is ready7th people is ready9th people is readyjudge say : run !1th people arrive4th people arrive10th people arrive5th people arrive2th people arrivejudge say : all arrived !9th people arrive7th people arrive8th people arrive3th people arrive6th people arrivespend time: 970933
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