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edu.emory.mathcs.util.concurrent
Class CountDownLatch  

java.lang.Object
  edu.emory.mathcs.util.concurrent.CountDownLatch

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public class CountDownLatch

extends 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 55 methods block until the current count 55 reaches zero due to invocations of the countDown() 55 method, after which all waiting threads are released and any subsequent invocations of await 55 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 await 55 wait at the gate until it is opened by a thread invoking countDown() 55 . 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 55 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() 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.)

 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

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Field Summary
protected  int	count
protected  int	initialCount

 

Constructor Summary

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

 

Method Summary
void	await()           Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted 55 .
boolean	await(long timeout, TimeUnit unit)           Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted 55 , or the specified waiting time elapses.
void	countDown()           Decrements the count of the latch, releasing all waiting threads if the count reaches zero.
int	getCount()           Returns the current count.
java.lang.String	toString()           Returns a string identifying this latch, as well as its state.

 

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

 

Field Detail

initialCount

protected final int initialCount

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count

protected int count

Constructor Detail

CountDownLatch

public CountDownLatch(int count)

Constructs a CountDownLatch initialized with the given count.

Method Detail

await

public void await()
           throws java.lang.InterruptedException

Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted 55 .

If the current count 55 is zero then this method returns immediately.

If the current count 55 is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happen:

• The count reaches zero due to invocations of the countDown() 55 method; or
• Some other thread interrupts 55 the current thread.

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted 55 while waiting,

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

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await

public boolean await(long timeout,
                     TimeUnit unit)
              throws java.lang.InterruptedException

Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted 55 , or the specified waiting time elapses.

If the current count 55 is zero then this method returns immediately with the value true.

If the current count 55 is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happen:

• The count reaches zero due to invocations of the countDown() 55 method; or
• Some other thread interrupts 55 the current thread; or
• The specified waiting time elapses.

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

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted 55 while waiting,

then 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.

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countDown

public void countDown()

Decrements the count of the latch, releasing all waiting threads if the count reaches zero.

If the current count 55 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 55 equals zero then nothing happens.

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getCount

public int getCount()

Returns the current count.

This method is typically used for debugging and testing purposes.

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toString

public 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.

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