A CountDownLatch is initialized with a given count. The wait methods block until the current count reaches zero due to invocations of the countDown method, after which all waiting threads are released and any subsequent invocations of wait return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.

A CountDownLatch is a versatile synchronization tool and can be used for a number of purposes. A CountDownLatch initialized with a count of one serves as a simple on/off latch, or gate: all threads invoking wait wait at the gate until it is opened by a thread invoking countDown. A CountDownLatch initialized to N can be used to make one thread wait until N threads have completed some action, or some action has been completed N times.

A useful property of a CountDownLatch is that it doesn't require that threads calling countDown wait for the count to reach zero before proceeding, it simply prevents any thread from proceeding past an wait until all threads could pass.

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.

 class Driver {
   void main() {
     CountDownLatch startSignal;
     CountDownLatch doneSignal;

     this() {
       startSignal = new CountDownLatch(1);
       doneSignal = new CountDownLatch(N);
     }
     for (int i = 0; i < N; ++i) { // create and start threads
       Worker work = new Worker(startSignal, doneSignal);
       new Thread(&work.run).start();

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

 class Worker {
   private CountDownLatch startSignal;
   private CountDownLatch doneSignal;
   this(CountDownLatch startSignal, CountDownLatch doneSignal) {
      this.startSignal = startSignal;
      this.doneSignal = doneSignal;
   }
   int run() {
      startSignal.wait();
      doWork();
      doneSignal.countDown();
      return 0;
   }

   void doWork() { ... }
 }