Java並發之CountDownLatch，javacountdownlatch

Java並發之CountDownLatch，javacountdownlatch

CountDownLatch是Java concurrent包下的一個同步工具。它可以讓一個（或多個）線程等待，直到其他線程中的某些操作完成。

本質上是一個信號量，我們把它比作一個有N個插銷的大門，它把等待（調用await）的線程擋住了, 直到其他線程把插銷去完了（調用countDown減到0），這批線程才能執行。

下面是根據oracle官方文檔改的一個例子：

/**
 * 
 * Sample usage: Here is a pair of classes in which a group of worker threads use two countdown latches:

 The first is a start signal that prevents any worker from proceeding until the driver is ready for them to proceed;
 The second is a completion signal that allows the driver to wait until all workers have completed.
 */
public class CountDownLatchDemo {

    public static void main(String[] args) throws InterruptedException {
        new Driver().run();
    }
}

class Driver {
    static final int N = 5;
    public void run() throws InterruptedException {
        CountDownLatch startSignal = new CountDownLatch(1);

        CountDownLatch doneSignal = new CountDownLatch(N);
        for (int i=0; i<N; i++) {
            new Thread(new Worker(startSignal, doneSignal))
                    .start();
        }
        doSomething(); // don't let run yet
        startSignal.countDown(); // let all threads proceed
        doneSignal.await(); // wait for all to finish
        doSomething();
    }

    private void doSomething() {
        System.out.println("doSomething");
    }
}



class Worker implements Runnable {
    private CountDownLatch startSignal;
    private CountDownLatch doneSignal;

    public Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
        this.startSignal = startSignal;
        this.doneSignal = doneSignal;
    }

    public void run() {
        try {
            startSignal.await();
            doWork();
            doneSignal.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void doWork() {
        System.out.println("worker work.");
    }
}

把Driver看作是一個監工，Worker看作是工人，有5個。

• 監工發號施令後，工人們才可以干活。所以工人干活前調用startSignal.await()， 直到監工調用startSignal.countDown() 後Worker後續doWork才開始執行。 
• 工人們干完活後通知監工，監工才可以接著發號。所以監工調用doneSignal.await()在哪兒等著，直到工人們都干完活（調用5次doneSignal.countDown()）後，才能接著執行。

看下執行結果，有助於理解整個工作過程和latch機制：

doSomething
worker work.
worker work.
worker work.
worker work.
worker work.
doSomething