/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
/**
* A synchronization aid that allows one or more threads to wait until
* a set of operations being performed in other threads completes.
*
* <p>A <tt>CountDownLatch</tt> is initialized with a given
* <em>count</em>. The {@link #await await} methods block until the current
* {@link #getCount count} reaches zero due to invocations of the
* {@link #countDown} method, after which all waiting threads are
* released and any subsequent invocations of {@link #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 {@link CyclicBarrier}.
*
* <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool
* and can be used for a number of purposes. A
* <tt>CountDownLatch</tt> initialized with a count of one serves as a
* simple on/off latch, or gate: all threads invoking {@link #await await}
* wait at the gate until it is opened by a thread invoking {@link
* #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em>
* can be used to make one thread wait until <em>N</em> threads have
* completed some action, or some action has been completed N times.
* <p>A useful property of a <tt>CountDownLatch</tt> is that it
* doesn't require that threads calling <tt>countDown</tt> wait for
* the count to reach zero before proceeding, it simply prevents any
* thread from proceeding past an {@link #await await} until all
* threads could pass.
*
* <p><b>Sample usage:</b> Here is a pair of classes in which a group
* of worker threads use two countdown latches:
* <ul>
* <li>The first is a start signal that prevents any worker from proceeding
* until the driver is ready for them to proceed;
* <li>The second is a completion signal that allows the driver to wait
* until all workers have completed.
* </ul>
*
* <pre>
* 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() { ... }
* }
*
* </pre>
*
* <p>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 {@link CyclicBarrier}.)
*
* <pre>
* 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() { ... }
* }
*
* </pre>
*
* @since 1.5
* @author Doug Lea
*/
public class CountDownLatch {
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
public int tryAcquireShared(int acquires) {
return getState() == 0? 1 : -1;
}
public boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c-1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
private final Sync sync;
/**
* Constructs a <tt>CountDownLatch</tt> initialized with the given
* count.
*
* @param count the number of times {@link #countDown} must be invoked
* before threads can pass through {@link #await}.
*
* @throws IllegalArgumentException if <tt>count</tt> is less than zero.
*/
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@link Thread#interrupt interrupted}.
*
* <p>If the current {@link #getCount count} is zero then this method
* returns immediately.
* <p>If the current {@link #getCount count} is greater than zero then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of two things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@link Thread#interrupt interrupts} the current
* thread.
* </ul>
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@link Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting.
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@link Thread#interrupt interrupted},
* or the specified waiting time elapses.
*
* <p>If the current {@link #getCount count} is zero then this method
* returns immediately with the value <tt>true</tt>.
*
* <p>If the current {@link #getCount count} is greater than zero then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@link Thread#interrupt interrupts} the current
* thread; or
* <li>The specified waiting time elapses.
* </ul>
* <p>If the count reaches zero then the method returns with the
* value <tt>true</tt>.
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@link Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value <tt>false</tt>
* is returned.
* If the time is
* less than or equal to zero, the method will not wait at all.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the <tt>timeout</tt> argument.
* @return <tt>true</tt> if the count reached zero and <tt>false</tt>
* if the waiting time elapsed before the count reached zero.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting.
*/
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
* <p>If the current {@link #getCount 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.
* <p>If the current {@link #getCount count} equals zero then nothing
* happens.
*/
public void countDown() {
sync.releaseShared(1);
}
/**
* Returns the current count.
* <p>This method is typically used for debugging and testing purposes.
* @return the current count.
*/
public long getCount() {
return sync.getCount();
}
/**
* Returns a string identifying this latch, as well as its state.
* The state, in brackets, includes the String
* "Count =" followed by the current count.
* @return a string identifying this latch, as well as its
* state
*/
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}
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