JDK-CountDownLatch-实例、源码和模拟实现
ConceptionA synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
根据CountDownLatch的document，就是说CountDownLatch作为一个同步的助手，可以阻塞一个线程，等待另一个线程结束了再执行。
所以CountDownLatch的作用在于：通过阻塞来协调线程间的执行顺序。
CountDownLatch最重要的方法为await()和countDown()。首先，初始化指定一个count，每次countDown()都会count--，而await()就是阻塞调用它的方法，直到count==0。
所以可以把CountDownLatch看成同步的计数器，什么时候倒数到0就执行，否则阻塞。

Demo我们首先做一个CountDownLatch的demo，看看它是怎么一个阻塞。
count初始化为2，所以count.await()会阻塞主线程，直到两个processor都countDown()，即count==0的时候才会执行，打印出"Processors has been done."

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public class CountDownLatchSimulator {private AtomicInteger count;public CountDownLatchSimulator(int count) {this.count = new AtomicInteger(count);}public void await() throws InterruptedException {while (this.count.get() != 0) {//Thread.currentThread().sleep(100);}}public void countDown() {this.count.getAndDecrement();}public static void main(String[] args) throws InterruptedException {CountDownLatchSimulator begin = new CountDownLatchSimulator(1);CountDownLatchSimulator end = new CountDownLatchSimulator(3);ExecutorService exc = Executors.newCachedThreadPool();System.out.println("Runners are comming.");exc.submit(new CountProcessor2(begin, end, "Runner_1", 1000));exc.submit(new CountProcessor2(begin, end, "Runner_2", 2000));exc.submit(new CountProcessor2(begin, end, "Runner_3", 3000));System.out.println("Ready.");Thread.currentThread().sleep(2000);System.out.println("Go.");begin.countDown();end.await();System.out.println("All runners Finish the match.");exc.shutdown();}}class CountProcessor2 extends Thread {private CountDownLatchSimulator beginCount;private CountDownLatchSimulator endCount;private String name;private int runningTime;public CountProcessor2(CountDownLatchSimulator beginCount, CountDownLatchSimulator endCount, String name, int runningTime) {this.beginCount = beginCount;this.endCount = endCount;this.name = name;this.runningTime = runningTime;}@Overridepublic void run() {try {this.beginCount.await();System.out.println(this.name + " start.");Thread.currentThread().sleep(this.runningTime);System.out.println(this.name + " breast the tape.");this.endCount.countDown();} catch (InterruptedException e) {e.printStackTrace();}}}