併發-新類庫中的構件 CountDownLatch

CountDownLatch

• 用來同步一個或多個任務，強制它們等待由其他任務執行的一組操作完成。
• 可以向CountDownLatch對象設置一個初始計數值，任何在這個對象上調用wait（）的方法都將阻塞，直至這個計數值到達0。其他任務在結束其工作時，可以在該對象上調用countDown（）來減小這個計數值。CountDownLathc被設計爲只觸發一次，計數值不能被重置。如果需要能夠重置計數值的版本，可以使用CyclicBarrier。
• 調用countDown（）的任務在產生這個調用時並沒有被阻塞，只有對await的調用會被阻塞，直至計數值達到0。
• CountDownLatch的典型用法是：將一個程序分爲n個互相獨立的可解決任務，並創建值爲0的CountDownLatch。當每個任務完成時，都會在這個鎖存器上調用countDown()。等待問題別解決的任務在這個鎖存器上調用await（），將它們自己攔住，直至鎖存器技術結束。

package com21concurrent;

import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

/**
 * Created by Panda on 2018/6/1.
 */
class TaskPortion implements Runnable{
    private static int counter=0;
    private final int id=counter++;
    private static Random random = new Random(47);
    private final CountDownLatch countDownLatch;
    public TaskPortion(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
    @Override
    public void run() {
        try{
            doWork();
            countDownLatch.countDown();
            System.out.println(countDownLatch.getCount());
        }catch (InterruptedException e){

        }
    }

    public void doWork() throws InterruptedException{
        TimeUnit.MILLISECONDS.sleep(random.nextInt(2000));
        System.out.println(this+"completed");
    }

    public String toString(){
        return String.format("%1$-3d ",id);
    }
}

class WaitingTask implements Runnable{
    private static int counter=0;
    private final int id=counter++;
    private final CountDownLatch countDownLatch;
    public WaitingTask(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
    @Override
    public void run() {
     try{
         //任何在這個對象上調用wait()的方法都將阻塞，直至這個計數值到達0
         countDownLatch.await();
         System.out.println(Thread.currentThread().getName());
         System.out.println("Latch barrier passed for "+this);
     }catch (InterruptedException e){
         System.out.println(this+"interrupted");
     }
    }
    public String toString(){
        return String.format("WaitingTask %1$-3d",id);
    }
}
public class CountDownLatchDemo {
    static final int SIZE=100;
    public static void main(String[] args) throws Exception{
        ExecutorService executorService= Executors.newCachedThreadPool();
        CountDownLatch countDownLatch = new CountDownLatch(SIZE);
        for (int i = 0; i < 10; i++) {
            executorService.execute(new WaitingTask(countDownLatch));
        }
        for (int i = 0; i <SIZE ; i++) {
            executorService.execute(new TaskPortion(countDownLatch));
        }
        System.out.println("Launched all tasks");
        executorService.shutdown();
    }
}