Java並發之CountDownLatch

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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。

本質上是一個訊號量，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，工人們才可以幹活。所以工人幹活前調用startSignal.await()， 直到監工調用startSignal.countDown() 後Worker後續doWork才開始執行。 
• 工人們幹完活後通知監工，監工才可以接著發號。所以監工調用doneSignal.await()在哪兒等著，直到工人們都幹完活（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

Java並發之CountDownLatch