java CountDownLatch類的使用

CountDownLatch 是一個同步輔助類，猶如倒計時計數器，創建對象時通過構造方法設置初始值，調用CountDownLatch對象的await()方法則處于等待狀態，調用countDown()方法就將計數器減1，當計數到達0時，則所有等待者或單個等待者開始執行。

主要方法

 public CountDownLatch(int count);

 public void countDown();

 public void await() throws InterruptedException

以下舉例子說明用法：

1. public class CountDownLatchDemo {
final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    public static void main(String[] args) throws InterruptedException {
    CountDownLatch latch=new CountDownLatch(2);//兩個工人的協作
    Worker worker1=new Worker("zhang san", 5000, latch);
    Worker worker2=new Worker("li si", 8000, latch);
    worker1.start();//
    worker2.start();//
    latch.await();//等待所有工人完成工作
        System.out.println("all work done at "+sdf.format(new Date()));
}


    static class Worker extends Thread{
    String workerName; 
    int workTime;
    CountDownLatch latch;
    public Worker(String workerName ,int workTime ,CountDownLatch latch){
    this.workerName=workerName;
    this.workTime=workTime;
    this.latch=latch;
    }
    public void run(){
    System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));
    doWork();//工作了
    System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));
    latch.countDown();//工人完成工作，計數器減一


    }

    private void doWork(){
    try {
Thread.sleep(workTime);
} catch (InterruptedException e) {
e.printStackTrace();
}
    }
    }
}

輸出：

Worker zhang san do work begin at 2011-04-14 11:05:11
Worker li si do work begin at 2011-04-14 11:05:11
Worker zhang san do work complete at 2011-04-14 11:05:16
Worker li si do work complete at 2011-04-14 11:05:19
all work done at 2011-04-14 11:05:19

2. 

package com.thread;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
 * 
 * @author Administrator
 *該程序用來模擬發送命令與執行命令，主線程代表指揮官，新建3個線程代表戰士，戰士一直等待著指揮官下達命令，
 *若指揮官沒有下達命令，則戰士們都必須等待。一旦命令下達，戰士們都去執行自己的任務，指揮官處于等待狀態，戰士們任務執行完畢則報告給
 *指揮官，指揮官則結束等待。
 */
public class CountdownLatchTest {


    public static void main(String[] args) {
        ExecutorService service = Executors.newCachedThreadPool(); //創建一個線程池
        final CountDownLatch cdOrder = new CountDownLatch(1);//指揮官的命令，設置為1，指揮官一下達命令，則cutDown,變為0，戰士們執行任務
        final CountDownLatch cdAnswer = new CountDownLatch(3);//因為有三個戰士，所以初始值為3，每一個戰士執行任務完畢則cutDown一次，當三個都執行完畢，變為0，則 指揮官停止等待。        
        for(int i=0;i<3;i++){
            Runnable runnable = new Runnable(){
                    public void run(){
                    try {
                        System.out.println("線程" + Thread.currentThread().getName() + 
                                "正準備接受命令");                        
                        cdOrder.await(); //戰士們都處于等待命令狀態
                        System.out.println("線程" + Thread.currentThread().getName() + 
                        "已接受命令");                                
                        Thread.sleep((long)(Math.random()*10000));    
                        System.out.println("線程" + Thread.currentThread().getName() + 
                                "回應命令處理結果");                        
                        cdAnswer.countDown(); //任務執行完畢，返回給指揮官，cdAnswer減1。                    
                    } catch (Exception e) {
                        e.printStackTrace();
                    }                
                }
            };
            service.execute(runnable);//為線程池添加任務
        }        
        try {
            Thread.sleep((long)(Math.random()*10000));

            System.out.println("線程" + Thread.currentThread().getName() + 
                    "即將發布命令");                        
            cdOrder.countDown(); //發送命令，cdOrder減1，處于等待的戰士們停止等待轉去執行任務。
            System.out.println("線程" + Thread.currentThread().getName() + 
            "已發送命令，正在等待結果");    
            cdAnswer.await(); //命令發送后指揮官處于等待狀態，一旦cdAnswer為0時停止等待繼續往下執行
            System.out.println("線程" + Thread.currentThread().getName() + 
            "已收到所有響應結果");    
        } catch (Exception e) {
            e.printStackTrace();
        }                
        service.shutdown(); //任務結束，停止線程池的所有線程
    }
}