CountDownLatch類是一個同步計數器,構造時傳入int參數,該參數就是計數器的初始值，每調用一次countDown()方法，計數器減1,計數器大于0 時，await()方法會阻塞程序繼續執行
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。
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import java.util.concurrent.CountDownLatch;import java.util.concurrent.Executor;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CountDownLatchDemo { private static final int PLAYER_AMOUNT = 5; public CountDownLatchDemo() { // TODO Auto-generated constructor stub } /** * @param args */ public static void main(String[] args) { // TODO Auto-generated method stub //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 System.out.println("Race begins!"); begin.countDown(); 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(); }} |
接下來是Player類
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import java.util.concurrent.CountDownLatch; public 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; } @Override 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 } } } |
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