Java中CountDownLatch用法解析

CountDownLatch類是一個同步計數器,結構時傳入int參數,該參數就是計數器的初始值，每調用一次countDown()辦法，計數器減1,計數器大於0 時，await()辦法會阻塞順序持續執行

CountDownLatch如其所寫，是一個倒計數的鎖存器，當計數減至0時觸發特定的事情。應用這種特性，可以讓主線程等候子線程的完畢。上面以一個模仿運發動競賽的例子加以闡明。

 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
     //關於每位運發動，CountDownLatch減1後即完畢競賽
     CountDownLatch begin = new CountDownLatch(1);
     //關於整個競賽，一切運發動完畢後才算完畢
     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();      //等候end形態變為0，即為競賽完畢
     }catch (InterruptedException e) {
       // TODO: handle exception
       e.printStackTrace();
     }finally{
       System.out.println("Race ends!");
     }
     exe.shutdown();
   }
 }

接上去是Player類

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();    //等候begin的形態為0
       Thread.sleep((long)(Math.random()*100));  //隨機分配時間，即運發動完成時間
       System.out.println("Play"+id+" arrived.");
     }catch (InterruptedException e) {
       // TODO: handle exception
       e.printStackTrace();
     }finally{
       end.countDown();  //使end形態減1，最終減至0
     }
   }
 }

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