java.util.concurrent
Class CountDownLatch

java.lang.Object sample code for java.lang.Object definition code for java.lang.Object 
  extended by java.util.concurrent.CountDownLatch

public class CountDownLatch
extends Object sample code for java.lang.Object definition code for java.lang.Object

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

A CountDownLatch is initialized with a given count. The await sample code for java.util.concurrent.CountDownLatch.await() definition code for java.util.concurrent.CountDownLatch.await() methods block until the current count sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() reaches zero due to invocations of the countDown() sample code for java.util.concurrent.CountDownLatch.countDown() definition code for java.util.concurrent.CountDownLatch.countDown() method, after which all waiting threads are released and any subsequent invocations of await sample code for java.util.concurrent.CountDownLatch.await() definition code for java.util.concurrent.CountDownLatch.await() 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 sample code for java.util.concurrent.CyclicBarrier definition code for java.util.concurrent.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 await sample code for java.util.concurrent.CountDownLatch.await() definition code for java.util.concurrent.CountDownLatch.await() wait at the gate until it is opened by a thread invoking countDown() sample code for java.util.concurrent.CountDownLatch.countDown() definition code for java.util.concurrent.CountDownLatch.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 await sample code for java.util.concurrent.CountDownLatch.await() definition code for java.util.concurrent.CountDownLatch.await() until all threads could pass.

Sample usage: Here is a pair of classes in which a group of worker threads use two countdown latches:

 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() { ... }
 }

 

Another typical usage would be to divide a problem into N parts, describe each part with a Runnable that executes that portion and counts down on the latch, and queue all the Runnables to an Executor. When all sub-parts are complete, the coordinating thread will be able to pass through await. (When threads must repeatedly count down in this way, instead use a CyclicBarrier sample code for java.util.concurrent.CyclicBarrier definition code for java.util.concurrent.CyclicBarrier .)

 class Driver2 { // ...
   void main() throws InterruptedException {
     CountDownLatch doneSignal = new CountDownLatch(N);
     Executor e = ...

     for (int i = 0; i < N; ++i) // create and start threads
       e.execute(new WorkerRunnable(doneSignal, i));

     doneSignal.await();           // wait for all to finish
   }
 }

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

   void doWork() { ... }
 }

 

Since:
1.5

Constructor Summary
CountDownLatch sample code for java.util.concurrent.CountDownLatch.CountDownLatch(int) definition code for java.util.concurrent.CountDownLatch.CountDownLatch(int) (int count)
          Constructs a CountDownLatch initialized with the given count.
 
Method Summary
 void await sample code for java.util.concurrent.CountDownLatch.await() definition code for java.util.concurrent.CountDownLatch.await() ()
          Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted sample code for java.lang.Thread.interrupt() definition code for java.lang.Thread.interrupt() .
 boolean await sample code for java.util.concurrent.CountDownLatch.await(long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.CountDownLatch.await(long, java.util.concurrent.TimeUnit) (long timeout, TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.TimeUnit  unit)
          Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted sample code for java.lang.Thread.interrupt() definition code for java.lang.Thread.interrupt() , or the specified waiting time elapses.
 void countDown sample code for java.util.concurrent.CountDownLatch.countDown() definition code for java.util.concurrent.CountDownLatch.countDown() ()
          Decrements the count of the latch, releasing all waiting threads if the count reaches zero.
 long getCount sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() ()
          Returns the current count.
 String sample code for java.lang.String definition code for java.lang.String toString sample code for java.util.concurrent.CountDownLatch.toString() definition code for java.util.concurrent.CountDownLatch.toString() ()
          Returns a string identifying this latch, as well as its state.
 
Methods inherited from class java.lang.Object sample code for java.lang.Object definition code for java.lang.Object
clone sample code for java.lang.Object.clone() definition code for java.lang.Object.clone() , equals sample code for java.lang.Object.equals(java.lang.Object) definition code for java.lang.Object.equals(java.lang.Object) , finalize sample code for java.lang.Object.finalize() definition code for java.lang.Object.finalize() , getClass sample code for java.lang.Object.getClass() definition code for java.lang.Object.getClass() , hashCode sample code for java.lang.Object.hashCode() definition code for java.lang.Object.hashCode() , notify sample code for java.lang.Object.notify() definition code for java.lang.Object.notify() , notifyAll sample code for java.lang.Object.notifyAll() definition code for java.lang.Object.notifyAll() , wait sample code for java.lang.Object.wait() definition code for java.lang.Object.wait() , wait sample code for java.lang.Object.wait(long) definition code for java.lang.Object.wait(long) , wait sample code for java.lang.Object.wait(long, int) definition code for java.lang.Object.wait(long, int)
 

Constructor Detail

CountDownLatch sample code for java.util.concurrent.CountDownLatch(int) definition code for java.util.concurrent.CountDownLatch(int)

public CountDownLatch(int count)
Constructs a CountDownLatch initialized with the given count.

Parameters:
count - the number of times countDown() sample code for java.util.concurrent.CountDownLatch.countDown() definition code for java.util.concurrent.CountDownLatch.countDown() must be invoked before threads can pass through await() sample code for java.util.concurrent.CountDownLatch.await() definition code for java.util.concurrent.CountDownLatch.await() .
Throws:
IllegalArgumentException sample code for java.lang.IllegalArgumentException definition code for java.lang.IllegalArgumentException - if count is less than zero.
Method Detail

await sample code for java.util.concurrent.CountDownLatch.await() definition code for java.util.concurrent.CountDownLatch.await()

public void await()
           throws InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException 
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted sample code for java.lang.Thread.interrupt() definition code for java.lang.Thread.interrupt() .

If the current count sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() is zero then this method returns immediately.

If the current count sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happen:

If the current thread:

then InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException is thrown and the current thread's interrupted status is cleared.

Throws:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if the current thread is interrupted while waiting.

await sample code for java.util.concurrent.CountDownLatch.await(long, java.util.concurrent.TimeUnit) definition code for java.util.concurrent.CountDownLatch.await(long, java.util.concurrent.TimeUnit)

public boolean await(long timeout,
                     TimeUnit sample code for java.util.concurrent.TimeUnit definition code for java.util.concurrent.TimeUnit  unit)
              throws InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException 
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted sample code for java.lang.Thread.interrupt() definition code for java.lang.Thread.interrupt() , or the specified waiting time elapses.

If the current count sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() is zero then this method returns immediately with the value true.

If the current count sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happen:

If the count reaches zero then the method returns with the value true.

If the current thread:

then InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all.

Parameters:
timeout - the maximum time to wait
unit - the time unit of the timeout argument.
Returns:
true if the count reached zero and false if the waiting time elapsed before the count reached zero.
Throws:
InterruptedException sample code for java.lang.InterruptedException definition code for java.lang.InterruptedException - if the current thread is interrupted while waiting.

countDown sample code for java.util.concurrent.CountDownLatch.countDown() definition code for java.util.concurrent.CountDownLatch.countDown()

public void countDown()
Decrements the count of the latch, releasing all waiting threads if the count reaches zero.

If the current count sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() is greater than zero then it is decremented. If the new count is zero then all waiting threads are re-enabled for thread scheduling purposes.

If the current count sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount() equals zero then nothing happens.


getCount sample code for java.util.concurrent.CountDownLatch.getCount() definition code for java.util.concurrent.CountDownLatch.getCount()

public long getCount()
Returns the current count.

This method is typically used for debugging and testing purposes.

Returns:
the current count.

toString sample code for java.util.concurrent.CountDownLatch.toString() definition code for java.util.concurrent.CountDownLatch.toString()

public String sample code for java.lang.String definition code for java.lang.String  toString()
Returns a string identifying this latch, as well as its state. The state, in brackets, includes the String "Count =" followed by the current count.

Overrides:
toString sample code for java.lang.Object.toString() definition code for java.lang.Object.toString() in class Object sample code for java.lang.Object definition code for java.lang.Object
Returns:
a string identifying this latch, as well as its state