Java中CountDownLatch用法

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。

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

```java 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
     }
 }

} ```