java.lang.Object java.util.concurrent.CountDownLatch
public class CountDownLatch
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
methods block until the current
count reaches zero due to invocations of the
countDown()
count
reaches zero due to invocations of the
countDown()
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 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 wait at the gate until it is opened by a thread invoking 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 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 countDown() happen-before actions following a successful return from a corresponding await() in another thread.
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 . |
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 , 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. |
Methods inherited from class java.lang. Object |
---|
clone , equals , finalize , getClass , hashCode , notify , notifyAll , wait , wait , wait |
Constructor Detail |
---|
public CountDownLatch(int count)
Method Detail |
---|
public void await() throws InterruptedException
If the current
count
count
is zero then this method returns immediately.
If the current
count
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:
public boolean await(long timeout, TimeUnit unit) throws InterruptedException
If the current
count
count
is zero then this method returns immediately with the value
true.
true
.
If the current
count
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
true.
true
.
If the current thread:
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.
public void countDown()
If the current
count
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
count
equals zero then nothing happens.
public long getCount()
This method is typically used for debugging and testing purposes.
public String toString()