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。

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]

当我们想要等待多个线程完成其任务时，就会使用它。它类似于线程连接。

我们可以在哪里使用CountDownLatch

考虑这样一个场景:我们有三个线程“a”、“B”和“C”，我们希望只有在“a”和“B”线程完成或部分完成它们的任务时才启动线程“C”。

它可以应用于真实的It场景

考虑这样一个场景，经理将模块划分到开发团队(a和B)之间，他希望仅在两个团队都完成任务时将模块分配给QA团队进行测试。

public class Manager {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        MyDevTeam teamDevA = new MyDevTeam(countDownLatch, "devA");
        MyDevTeam teamDevB = new MyDevTeam(countDownLatch, "devB");
        teamDevA.start();
        teamDevB.start();
        countDownLatch.await();
        MyQATeam qa = new MyQATeam();
        qa.start();
    }   
}

class MyDevTeam extends Thread {   
    CountDownLatch countDownLatch;
    public MyDevTeam (CountDownLatch countDownLatch, String name) {
        super(name);
        this.countDownLatch = countDownLatch;       
    }   
    @Override
    public void run() {
        System.out.println("Task assigned to development team " + Thread.currentThread().getName());
        try {
                Thread.sleep(2000);
        } catch (InterruptedException ex) {
                ex.printStackTrace();
        }
        System.out.println("Task finished by development team " + Thread.currentThread().getName());
        this.countDownLatch.countDown();
    }
}

class MyQATeam extends Thread {   
    @Override
    public void run() {
        System.out.println("Task assigned to QA team");
        try {
            Thread.sleep(2000);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
        System.out.println("Task finished by QA team");
    }
}

以上代码的输出为:

分配给开发团队devB的任务

分配给开发团队devA的任务

任务由开发团队devB完成

由开发团队devA完成的任务

分配给QA团队的任务

QA团队完成的任务

这里await()方法等待countdownlatch标志变为0，而countDown()方法将countdownlatch标志减1。

JOIN的限制: 上面的例子也可以通过JOIN实现，但是JOIN不能在两种情况下使用:

当我们使用ExecutorService而不是Thread类创建线程时。 修改上面的例子，经理希望在开发完成80%的任务后立即将代码移交给QA团队。这意味着CountDownLatch允许我们修改实现，可以用来等待另一个线程的部分执行。

One good example of when to use something like this is with Java Simple Serial Connector, accessing serial ports. Typically you'll write something to the port, and asyncronously, on another thread, the device will respond on a SerialPortEventListener. Typically, you'll want to pause after writing to the port to wait for the response. Handling the thread locks for this scenario manually is extremely tricky, but using Countdownlatch is easy. Before you go thinking you can do it another way, be careful about race conditions you never thought of!!

伪代码:

CountDownLatch latch;
void writeData() { 
   latch = new CountDownLatch(1);
   serialPort.writeBytes(sb.toString().getBytes())
   try {
      latch.await(4, TimeUnit.SECONDS);
    } catch (InterruptedException e) {
   }
}
class SerialPortReader implements SerialPortEventListener {
    public void serialEvent(SerialPortEvent event) {
        if(event.isRXCHAR()){//If data is available
            byte buffer[] = serialPort.readBytes(event.getEventValue());
            latch.countDown();
         }
     }
}

CoundDownLatch使您能够让一个线程等待所有其他线程执行完毕。

伪代码可以是:

// Main thread starts
// Create CountDownLatch for N threads
// Create and start N threads
// Main thread waits on latch
// N threads completes there tasks are returns
// Main thread resume execution

这个来自Java Doc的例子帮助我清楚地理解了这些概念:

class Driver { // ...
  void main() throws InterruptedException {
    CountDownLatch startSignal = new CountDownLatch(1);
    CountDownLatch doneSignal = new CountDownLatch(N);

    for (int i = 0; i < N; ++i) // create and start threads
      new Thread(new Worker(startSignal, doneSignal)).start();

    doSomethingElse();            // don't let run yet
    startSignal.countDown();      // let all threads proceed
    doSomethingElse();
    doneSignal.await();           // wait for all to finish
  }
}

class Worker implements Runnable {
  private final CountDownLatch startSignal;
  private final CountDownLatch doneSignal;
  Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
     this.startSignal = startSignal;
     this.doneSignal = doneSignal;
  }
  public void run() {
     try {
       startSignal.await();
       doWork();
       doneSignal.countDown();
     } catch (InterruptedException ex) {} // return;
  }

  void doWork() { ... }
}

视觉解释:

显然，CountDownLatch允许一个线程(这里是Driver)等待一堆正在运行的线程(这里是Worker)完成它们的执行。