相关疑难解决方法(0)

有人可以帮我理解Java CountDownLatch是什么以及何时使用它？

我对这个程序的工作原理并不十分清楚.据我所知,所有三个线程立即启动,每个线程将在3000ms后调用CountDownLatch.倒数会逐一减少.在锁存器变为零之后,程序打印"已完成".也许我理解的方式不正确.

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

class Processor implements Runnable {
    private CountDownLatch latch;

    public Processor(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        System.out.println("Started.");

        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        latch.countDown();
    }
}

// ------------------------------------------------ -----

public class App {

    public static void main(String[] args) {

        CountDownLatch latch = new CountDownLatch(3); // coundown from 3 to 0

        ExecutorService executor = Executors.newFixedThreadPool(3); // 3 Threads in pool

        for(int i=0; i …

    ExecutorService pool=Executors.newFixedThreadPool(7);
        List<Future<Hotel>> future=new ArrayList<Future<Hotel>>();
        List<Callable<Hotel>> callList = new ArrayList<Callable<Hotel>>();

        for(int i=0;i<=diff;i++){

            String str="2013-"+(liDates.get(i).get(Calendar.MONTH)+1)+"-"+liDates.get(i).get(Calendar.DATE);

            callList.add(new HotelCheapestFare(str));

        }       
     future=pool.invokeAll(callList);
for(int i=0;i<=future.size();i++){

        System.out.println("name is:"+future.get(i).get().getName());
    }

现在我想invokeAll在进入for循环之前将pool放到所有任务中,但是当我运行这个程序for循环时,在此之前执行invokeAll并抛出此异常:

java.util.concurrent.ExecutionException: java.lang.NullPointerException at 
java.util.concurrent.FutureTask$Sync.innerGet(Unknown Source) at  
java.util.concurrent.FutureTask.get(Unknown Source) at 
com.mmt.freedom.cheapestfare.TestHotel.main(TestHotel.java:6??5)

Caused by: java.lang.NullPointerException at 
com.mmt.freedom.cheapestfare.HotelCheapestFare.getHotelCheap?estFare(HotelCheapes??tFare.java:166) 
at com.mmt.freedom.cheapestfare.HotelCheapestFare.call(HotelChe??apestFare.java:219)
at com.mmt.freedom.cheapestfare.HotelCheapestFare.call(HotelChe??apestFare.java:1) 
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source) at java.util.concurrent.FutureTask.run(Unknown Source) 
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) atjava.util.concurrent.ThreadPoolExecutor$Worker.run(Unknow??n Source)
at java.lang.Thread.run