| 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
CountDownLatch.await await methods block until the current count reaches
zero due to invocations of the
CountDownLatch.countDown method, after which
all waiting threads are released and any subsequent invocations of
CountDownLatch.await 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
CountDownLatch.await await wait at the gate until it is opened by a thread invoking
CountDownLatch.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
CountDownLatch.await 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:
- The first is a start signal that prevents any worker from proceeding
until the driver is ready for them to proceed;
- The second is a completion signal that allows the driver to wait
until all workers have completed.
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.
since:
1.5
author:
Doug Lea | 