Class CountDown

All Implemented Interfaces:

public class CountDown
extends java.lang.Object
implements Monitor

A CountDown can serve as a simple one-shot barrier. A Countdown is initialized with a given count value. Each release decrements the count. All acquires block until the count reaches zero. Upon reaching zero all current acquires are unblocked and all subsequent acquires pass without blocking. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a Barrier.

Sample usage. Here are a set of classes in which a group of worker threads use a countdown to notify a driver when all threads are complete.

 class Worker implements Runnable {
   private final CountDown done;
   Worker(CountDown d) { done = d; }
   public void run() {

 class Driver { // ...
   void main() {
	 CountDown done = new CountDown(N);
	 for (int i = 0; i < N; ++i)
	   new Thread(new Worker(done)).start();
	 done.acquire(); // wait for all to finish

Field Summary
Fields inherited from interface org.metasyntactic.thread.concurrent.Monitor
Constructor Summary
CountDown(int count)
          Create a new CountDown with given count value
Method Summary
 void acquire()
          This could use double-check, but doesn't out of concern for surprising effects on user programs stemming from lack of memory barriers with lack of synch.
 boolean attempt(long msecs)
          Wait at most msecs to pass; report whether passed.
 int currentCount()
          Return the current count value.
 int initialCount()
          Return the initial count value
 void release()
          Decrement the count.
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail


public CountDown(int count)
Create a new CountDown with given count value

count - The value to start counting down from
Method Detail


public void acquire()
             throws java.lang.InterruptedException
This could use double-check, but doesn't out of concern for surprising effects on user programs stemming from lack of memory barriers with lack of synch.

Specified by:
acquire in interface Monitor


public boolean attempt(long msecs)
                throws java.lang.InterruptedException
Description copied from interface: Monitor
Wait at most msecs to pass; report whether passed.

The method has best-effort semantics: The msecs bound cannot be guaranteed to be a precise upper bound on wait time in Java. Implementations generally can only attempt to return as soon as possible after the specified bound. Also, timers in Java do not stop during garbage collection, so timeouts can occur just because a GC intervened. So, msecs arguments should be used in a coarse-grained manner. Further, implementations cannot always guarantee that this method will return at all without blocking indefinitely when used in unintended ways. For example, deadlocks may be encountered when called in an unintended context.

Specified by:
attempt in interface Monitor
msecs - the number of milleseconds to wait. An argument less than or equal to zero means not to wait at all. However, this may still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention among threads.
true if acquired


public void release()
Decrement the count. After the initialCount'th release, all current and future acquires will pass

Specified by:
release in interface Monitor


public int initialCount()
Return the initial count value

The initial count of this CountDown


public int currentCount()
Return the current count value. This is just a snapshot value, that may change immediately after returning.

THe current count of this CountDown