edu.emory.mathcs.backport.java.util.concurrent
public class CountDownLatch extends Object
A {@code CountDownLatch} is initialized with a given count.
The await
methods block until the current count reaches
zero due to invocations of the CountDownLatch method, after which
all waiting threads are released and any subsequent invocations of
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.
A {@code CountDownLatch} is a versatile synchronization tool
and can be used for a number of purposes. A
{@code CountDownLatch} initialized with a count of one serves as a
simple on/off latch, or gate: all threads invoking await
wait at the gate until it is opened by a thread invoking CountDownLatch. A {@code 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 {@code CountDownLatch} is that it
doesn't require that threads calling {@code countDown} wait for
the count to reach zero before proceeding, it simply prevents any
thread from proceeding past an 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.)
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() { ... } }
Memory consistency effects: Actions in a thread prior to calling {@code countDown()} happen-before actions following a successful return from a corresponding {@code await()} in another thread.
Since: 1.5
Constructor Summary | |
---|---|
CountDownLatch(int count)
Constructs a {@code 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 Thread#interrupt interrupted.
|
boolean | await(long timeout, TimeUnit unit)
Causes the current thread to wait until the latch has counted down to
zero, unless the thread is Thread#interrupt interrupted,
or the specified waiting time elapses.
|
void | countDown()
Decrements the count of the latch, releasing all waiting threads if
the count reaches zero.
|
long | getCount()
Returns the current count.
|
String | toString()
Returns a string identifying this latch, as well as its state.
|
Parameters: count the number of times CountDownLatch must be invoked before threads can pass through CountDownLatch
Throws: IllegalArgumentException if {@code count} is negative
If the current count is zero then this method returns immediately.
If the current count 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:
Throws: InterruptedException if the current thread is interrupted while waiting
If the current count is zero then this method returns immediately with the value {@code true}.
If the current count 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 {@code true}.
If the current thread:
If the specified waiting time elapses then the value {@code 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 {@code timeout} argument
Returns: {@code true} if the count reached zero and {@code false} if the waiting time elapsed before the count reached zero
Throws: InterruptedException if the current thread is interrupted while waiting
If the current count 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 equals zero then nothing happens.
This method is typically used for debugging and testing purposes.
Returns: the current count
Returns: a string identifying this latch, as well as its state