如果你在调用latch.countDown()后添加一些调试，这可能会帮助你更好地理解它的行为。

latch.countDown();
System.out.println("DONE "+this.latch); // Add this debug

输出将显示Count正在递减。这个“计数”实际上是你启动的可运行任务(处理器对象)的数量，其中countDown()没有被调用，因此在主线程调用latch.await()时被阻塞。

DONE java.util.concurrent.CountDownLatch@70e69696[Count = 2]
DONE java.util.concurrent.CountDownLatch@70e69696[Count = 1]
DONE java.util.concurrent.CountDownLatch@70e69696[Count = 0]

CountDownLatch在Java中是一种同步器，它允许一个线程在开始处理之前等待一个或多个线程。

CountDownLatch工作在闩锁原理，线程将等待直到门打开。在创建CountDownLatch时，一个线程等待指定的n个线程。

例如:final CountDownLatch = new CountDownLatch(3);

这里我们将计数器设置为3。

任何调用CountDownLatch.await()的线程(通常是应用程序的主线程)都将等待，直到count为零或被另一个线程中断。所有其他线程都需要在完成或准备工作后调用CountDownLatch.countDown()来进行倒计时。一旦count达到0，等待的线程就开始运行。

这里的计数是通过CountDownLatch.countDown()方法递减的。

调用await()方法的线程将一直等待，直到初始计数为零。

为了使计数为零，其他线程需要调用countDown()方法。 一旦计数为零，调用await()方法的线程将恢复(开始执行)。

CountDownLatch的缺点是它不可重用:一旦计数为零，它就不再可用。

package practice;

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample {

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch c= new CountDownLatch(3);  // need to decrements the count (3) to zero by calling countDown() method so that main thread will wake up after calling await() method 
        Task t = new Task(c);
        Task t1 = new Task(c);
        Task t2 = new Task(c);
        t.start();
        t1.start();
        t2.start();
        c.await(); // when count becomes zero main thread will wake up 
        System.out.println("This will print after count down latch count become zero");
    }
}

class Task extends Thread{
    CountDownLatch c;

    public Task(CountDownLatch c) {
        this.c = c;
    }

    @Override
    public void run() {
        try {
            System.out.println(Thread.currentThread().getName());
            Thread.sleep(1000);
            c.countDown();   // each thread decrement the count by one 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

来自oracle关于CountDownLatch的文档:

一种同步辅助工具，允许一个或多个线程等待在其他线程中执行的一组操作完成。

CountDownLatch使用给定的计数进行初始化。由于调用了countDown()方法，await方法阻塞直到当前计数为零，在此之后所有等待的线程都被释放，并且await的任何后续调用立即返回。这是一个一次性现象——计数不能重置。

CountDownLatch是一种通用的同步工具，可用于许多目的。

初始化为1的CountDownLatch用作简单的开/关锁存器或门:所有调用await的线程都在门处等待，直到由调用countDown()的线程打开。

初始化为N的CountDownLatch可用于使一个线程等待到N个线程完成某个操作，或者某个操作已完成N次。

public void await()
           throws InterruptedException

导致当前线程等待，直到锁存器计数到零，除非线程被中断。

如果当前计数为零，则此方法立即返回。

public void countDown()

减少锁存器的计数，如果计数为零，则释放所有等待线程。

如果当前计数大于零，则递减。如果新的计数为零，则所有等待的线程都将重新启用以进行线程调度。

解释你的例子。

You have set count as 3 for latch variable CountDownLatch latch = new CountDownLatch(3); You have passed this shared latch to Worker thread : Processor Three Runnable instances of Processor have been submitted to ExecutorService executor Main thread ( App ) is waiting for count to become zero with below statement latch.await(); Processor thread sleeps for 3 seconds and then it decrements count value with latch.countDown() First Process instance will change latch count as 2 after it's completion due to latch.countDown(). Second Process instance will change latch count as 1 after it's completion due to latch.countDown(). Third Process instance will change latch count as 0 after it's completion due to latch.countDown(). Zero count on latch causes main thread App to come out from await App program prints this output now : Completed

As mentioned in JavaDoc (https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CountDownLatch.html), CountDownLatch is a synchronization aid, introduced in Java 5. Here the synchronization does not mean restricting access to a critical section. But rather sequencing actions of different threads. The type of synchronization achieved through CountDownLatch is similar to that of Join. Assume that there is a thread "M" which needs to wait for other worker threads "T1", "T2", "T3" to complete its tasks Prior to Java 1.5, the way this can be done is, M running the following code

    T1.join();
    T2.join();
    T3.join();

上面的代码确保线程M在T1、T2、T3完成它的工作之后恢复它的工作。T1, T2, T3可以按任意顺序完成它们的工作。 同样可以通过CountDownLatch实现，其中T1、T2、T3和线程M共享相同的CountDownLatch对象。 "M"请求:countDownLatch.await(); 其中“T1”，“T2”，“T3”是countDownLatch.countdown();

连接方法的一个缺点是M必须知道T1、T2、T3。如果后来添加了一个新的工作线程T4，那么M也必须知道它。使用CountDownLatch可以避免这种情况。 实现后的动作顺序为[T1,T2,T3](T1,T2,T3的顺序可以任意)-> [M]

最好的选择是CyclicBarrier，如https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CountDownLatch.html 看到的:

CountDownLatch使用给定的计数进行初始化。由于调用了countDown()方法，await方法阻塞直到当前计数为零，在此之后所有等待的线程都被释放，并且await的任何后续调用立即返回。这是一个一次性现象——计数不能重置。如果您需要一个重置计数的版本，请考虑使用CyclicBarrier。