Java多線程併發經常使用類實例之：Countdownlatch

    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,計數器大於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;

publicclass CountDownLatchDemo {
privatestaticfinalint PLAYER_AMOUNT = 5;
public CountDownLatchDemo() {
// TODO Auto-generated constructor stub    
      }
/**
      * @param args
*/
publicstaticvoid main(String[] args) {
// TODO Auto-generated method stub
//對於每位運動員，CountDownLatch減1後即結束比賽
         CountDownLatch begin = new CountDownLatch(1);
//對於整個比賽，全部運動員結束後纔算結束
         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，大小爲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狀態變爲0，即爲比賽結束
         }catch (InterruptedException e) {
// TODO: handle exception
             e.printStackTrace();
         }finally{
             System.out.println("Race ends!");
         }
         exe.shutdown();
     }
 }
publicclass Player implements Runnable {

privateint 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
publicvoid run() {
// TODO Auto-generated method stub
try{
             begin.await();        //等待begin的狀態爲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
         }
     }
 }