public classCountDownLatchextendsObject
ACountDownLatch is initialized with a givencount. Theawait methods block until the current count reaches zero due to invocations of thecountDown() method, after which all waiting threads are released and any subsequent invocations ofawait return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using aCyclicBarrier.
ACountDownLatch is a versatile synchronization tool and can be used for a number of purposes. ACountDownLatch initialized with a count of one serves as a simple on/off latch, or gate: all threads invokingawait wait at the gate until it is opened by a thread invokingcountDown(). ACountDownLatch initialized toN can be used to make one thread wait untilN threads have completed some action, or some action has been completed N times.
A useful property of aCountDownLatch is that it doesn't require that threads callingcountDown wait for the count to reach zero before proceeding, it simply prevents any thread from proceeding past anawait 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 aCyclicBarrier.)
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: Until the count reaches zero, actions in a thread prior to callingcountDown()happen-before actions following a successful return from a correspondingawait() in another thread.
| Constructor | Description |
|---|---|
CountDownLatch(int count) | Constructs a CountDownLatch initialized with the given count. |
| Modifier and Type | Method | Description |
|---|---|---|
void | await() | Causes the current thread to wait until the latch has counted down to zero, unless the thread isinterrupted. |
boolean | await(long timeout,TimeUnit unit) | Causes the current thread to wait until the latch has counted down to zero, unless the thread isinterrupted, 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. |
public CountDownLatch(int count)
CountDownLatch initialized with the given count.count - the number of timescountDown() must be invoked before threads can pass throughawait()IllegalArgumentException - ifcount is negativepublic void await() throwsInterruptedException
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:
countDown() method; orIf the current thread:
InterruptedException is thrown and the current thread's interrupted status is cleared.InterruptedException - if the current thread is interrupted while waitingpublic boolean await(long timeout,TimeUnit unit) throwsInterruptedException
If the current count is zero then this method returns immediately with the valuetrue.
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:
countDown() method; orIf the count reaches zero then the method returns with the valuetrue.
If the current thread:
InterruptedException is thrown and the current thread's interrupted status is cleared.If the specified waiting time elapses then the valuefalse is returned. If the time is less than or equal to zero, the method will not wait at all.
timeout - the maximum time to waitunit - the time unit of thetimeout argumenttrue if the count reached zero andfalse if the waiting time elapsed before the count reached zeroInterruptedException - if the current thread is interrupted while waitingpublic void countDown()
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.
public long getCount()
This method is typically used for debugging and testing purposes.