public class CountDownLatch extends Object
A CountDownLatch is initialized with a given count.
The await methods block until the current 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: Until the count reaches
zero, actions in a thread prior to calling
countDown()
happen-before
actions following a successful return from a corresponding
await() in another thread.
| Constructor and Description |
|---|
CountDownLatch(int count)
Constructs a
CountDownLatch initialized with the given count. |
| Modifier and Type | Method and Description |
|---|---|
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.
|
public CountDownLatch(int count)
CountDownLatch initialized with the given count.count - the number of times countDown() must be invoked
before threads can pass through await()IllegalArgumentException - if count is negativepublic void await()
throws InterruptedException
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; or
If 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)
throws InterruptedException
If the current count is zero then this method returns immediately
with the value 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:
countDown() method; or
If the count reaches zero then the method returns with the
value true.
If the current thread:
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.
timeout - the maximum time to waitunit - the time unit of the timeout argumenttrue if the count reached zero and false
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.