jaroslav@1890: /*
jaroslav@1890:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@1890:  *
jaroslav@1890:  * This code is free software; you can redistribute it and/or modify it
jaroslav@1890:  * under the terms of the GNU General Public License version 2 only, as
jaroslav@1890:  * published by the Free Software Foundation.  Oracle designates this
jaroslav@1890:  * particular file as subject to the "Classpath" exception as provided
jaroslav@1890:  * by Oracle in the LICENSE file that accompanied this code.
jaroslav@1890:  *
jaroslav@1890:  * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@1890:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@1890:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
jaroslav@1890:  * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@1890:  * accompanied this code).
jaroslav@1890:  *
jaroslav@1890:  * You should have received a copy of the GNU General Public License version
jaroslav@1890:  * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@1890:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@1890:  *
jaroslav@1890:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@1890:  * or visit www.oracle.com if you need additional information or have any
jaroslav@1890:  * questions.
jaroslav@1890:  */
jaroslav@1890: 
jaroslav@1890: /*
jaroslav@1890:  * This file is available under and governed by the GNU General Public
jaroslav@1890:  * License version 2 only, as published by the Free Software Foundation.
jaroslav@1890:  * However, the following notice accompanied the original version of this
jaroslav@1890:  * file:
jaroslav@1890:  *
jaroslav@1890:  * Written by Doug Lea with assistance from members of JCP JSR-166
jaroslav@1890:  * Expert Group and released to the public domain, as explained at
jaroslav@1890:  * http://creativecommons.org/publicdomain/zero/1.0/
jaroslav@1890:  */
jaroslav@1890: 
jaroslav@1890: package java.util.concurrent;
jaroslav@1890: import java.util.concurrent.locks.*;
jaroslav@1890: import java.util.concurrent.atomic.*;
jaroslav@1890: 
jaroslav@1890: /**
jaroslav@1890:  * A synchronization aid that allows one or more threads to wait until
jaroslav@1890:  * a set of operations being performed in other threads completes.
jaroslav@1890:  *
jaroslav@1890:  * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
jaroslav@1890:  * The {@link #await await} methods block until the current count reaches
jaroslav@1890:  * zero due to invocations of the {@link #countDown} method, after which
jaroslav@1890:  * all waiting threads are released and any subsequent invocations of
jaroslav@1890:  * {@link #await await} return immediately.  This is a one-shot phenomenon
jaroslav@1890:  * -- the count cannot be reset.  If you need a version that resets the
jaroslav@1890:  * count, consider using a {@link CyclicBarrier}.
jaroslav@1890:  *
jaroslav@1890:  * <p>A {@code CountDownLatch} is a versatile synchronization tool
jaroslav@1890:  * and can be used for a number of purposes.  A
jaroslav@1890:  * {@code CountDownLatch} initialized with a count of one serves as a
jaroslav@1890:  * simple on/off latch, or gate: all threads invoking {@link #await await}
jaroslav@1890:  * wait at the gate until it is opened by a thread invoking {@link
jaroslav@1890:  * #countDown}.  A {@code CountDownLatch} initialized to <em>N</em>
jaroslav@1890:  * can be used to make one thread wait until <em>N</em> threads have
jaroslav@1890:  * completed some action, or some action has been completed N times.
jaroslav@1890:  *
jaroslav@1890:  * <p>A useful property of a {@code CountDownLatch} is that it
jaroslav@1890:  * doesn't require that threads calling {@code countDown} wait for
jaroslav@1890:  * the count to reach zero before proceeding, it simply prevents any
jaroslav@1890:  * thread from proceeding past an {@link #await await} until all
jaroslav@1890:  * threads could pass.
jaroslav@1890:  *
jaroslav@1890:  * <p><b>Sample usage:</b> Here is a pair of classes in which a group
jaroslav@1890:  * of worker threads use two countdown latches:
jaroslav@1890:  * <ul>
jaroslav@1890:  * <li>The first is a start signal that prevents any worker from proceeding
jaroslav@1890:  * until the driver is ready for them to proceed;
jaroslav@1890:  * <li>The second is a completion signal that allows the driver to wait
jaroslav@1890:  * until all workers have completed.
jaroslav@1890:  * </ul>
jaroslav@1890:  *
jaroslav@1890:  * <pre>
jaroslav@1890:  * class Driver { // ...
jaroslav@1890:  *   void main() throws InterruptedException {
jaroslav@1890:  *     CountDownLatch startSignal = new CountDownLatch(1);
jaroslav@1890:  *     CountDownLatch doneSignal = new CountDownLatch(N);
jaroslav@1890:  *
jaroslav@1890:  *     for (int i = 0; i < N; ++i) // create and start threads
jaroslav@1890:  *       new Thread(new Worker(startSignal, doneSignal)).start();
jaroslav@1890:  *
jaroslav@1890:  *     doSomethingElse();            // don't let run yet
jaroslav@1890:  *     startSignal.countDown();      // let all threads proceed
jaroslav@1890:  *     doSomethingElse();
jaroslav@1890:  *     doneSignal.await();           // wait for all to finish
jaroslav@1890:  *   }
jaroslav@1890:  * }
jaroslav@1890:  *
jaroslav@1890:  * class Worker implements Runnable {
jaroslav@1890:  *   private final CountDownLatch startSignal;
jaroslav@1890:  *   private final CountDownLatch doneSignal;
jaroslav@1890:  *   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
jaroslav@1890:  *      this.startSignal = startSignal;
jaroslav@1890:  *      this.doneSignal = doneSignal;
jaroslav@1890:  *   }
jaroslav@1890:  *   public void run() {
jaroslav@1890:  *      try {
jaroslav@1890:  *        startSignal.await();
jaroslav@1890:  *        doWork();
jaroslav@1890:  *        doneSignal.countDown();
jaroslav@1890:  *      } catch (InterruptedException ex) {} // return;
jaroslav@1890:  *   }
jaroslav@1890:  *
jaroslav@1890:  *   void doWork() { ... }
jaroslav@1890:  * }
jaroslav@1890:  *
jaroslav@1890:  * </pre>
jaroslav@1890:  *
jaroslav@1890:  * <p>Another typical usage would be to divide a problem into N parts,
jaroslav@1890:  * describe each part with a Runnable that executes that portion and
jaroslav@1890:  * counts down on the latch, and queue all the Runnables to an
jaroslav@1890:  * Executor.  When all sub-parts are complete, the coordinating thread
jaroslav@1890:  * will be able to pass through await. (When threads must repeatedly
jaroslav@1890:  * count down in this way, instead use a {@link CyclicBarrier}.)
jaroslav@1890:  *
jaroslav@1890:  * <pre>
jaroslav@1890:  * class Driver2 { // ...
jaroslav@1890:  *   void main() throws InterruptedException {
jaroslav@1890:  *     CountDownLatch doneSignal = new CountDownLatch(N);
jaroslav@1890:  *     Executor e = ...
jaroslav@1890:  *
jaroslav@1890:  *     for (int i = 0; i < N; ++i) // create and start threads
jaroslav@1890:  *       e.execute(new WorkerRunnable(doneSignal, i));
jaroslav@1890:  *
jaroslav@1890:  *     doneSignal.await();           // wait for all to finish
jaroslav@1890:  *   }
jaroslav@1890:  * }
jaroslav@1890:  *
jaroslav@1890:  * class WorkerRunnable implements Runnable {
jaroslav@1890:  *   private final CountDownLatch doneSignal;
jaroslav@1890:  *   private final int i;
jaroslav@1890:  *   WorkerRunnable(CountDownLatch doneSignal, int i) {
jaroslav@1890:  *      this.doneSignal = doneSignal;
jaroslav@1890:  *      this.i = i;
jaroslav@1890:  *   }
jaroslav@1890:  *   public void run() {
jaroslav@1890:  *      try {
jaroslav@1890:  *        doWork(i);
jaroslav@1890:  *        doneSignal.countDown();
jaroslav@1890:  *      } catch (InterruptedException ex) {} // return;
jaroslav@1890:  *   }
jaroslav@1890:  *
jaroslav@1890:  *   void doWork() { ... }
jaroslav@1890:  * }
jaroslav@1890:  *
jaroslav@1890:  * </pre>
jaroslav@1890:  *
jaroslav@1890:  * <p>Memory consistency effects: Until the count reaches
jaroslav@1890:  * zero, actions in a thread prior to calling
jaroslav@1890:  * {@code countDown()}
jaroslav@1890:  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
jaroslav@1890:  * actions following a successful return from a corresponding
jaroslav@1890:  * {@code await()} in another thread.
jaroslav@1890:  *
jaroslav@1890:  * @since 1.5
jaroslav@1890:  * @author Doug Lea
jaroslav@1890:  */
jaroslav@1890: public class CountDownLatch {
jaroslav@1890:     /**
jaroslav@1890:      * Synchronization control For CountDownLatch.
jaroslav@1890:      * Uses AQS state to represent count.
jaroslav@1890:      */
jaroslav@1890:     private static final class Sync extends AbstractQueuedSynchronizer {
jaroslav@1890:         private static final long serialVersionUID = 4982264981922014374L;
jaroslav@1890: 
jaroslav@1890:         Sync(int count) {
jaroslav@1890:             setState(count);
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         int getCount() {
jaroslav@1890:             return getState();
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         protected int tryAcquireShared(int acquires) {
jaroslav@1890:             return (getState() == 0) ? 1 : -1;
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         protected boolean tryReleaseShared(int releases) {
jaroslav@1890:             // Decrement count; signal when transition to zero
jaroslav@1890:             for (;;) {
jaroslav@1890:                 int c = getState();
jaroslav@1890:                 if (c == 0)
jaroslav@1890:                     return false;
jaroslav@1890:                 int nextc = c-1;
jaroslav@1890:                 if (compareAndSetState(c, nextc))
jaroslav@1890:                     return nextc == 0;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     private final Sync sync;
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Constructs a {@code CountDownLatch} initialized with the given count.
jaroslav@1890:      *
jaroslav@1890:      * @param count the number of times {@link #countDown} must be invoked
jaroslav@1890:      *        before threads can pass through {@link #await}
jaroslav@1890:      * @throws IllegalArgumentException if {@code count} is negative
jaroslav@1890:      */
jaroslav@1890:     public CountDownLatch(int count) {
jaroslav@1890:         if (count < 0) throw new IllegalArgumentException("count < 0");
jaroslav@1890:         this.sync = new Sync(count);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Causes the current thread to wait until the latch has counted down to
jaroslav@1890:      * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current count is zero then this method returns immediately.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current count is greater than zero then the current
jaroslav@1890:      * thread becomes disabled for thread scheduling purposes and lies
jaroslav@1890:      * dormant until one of two things happen:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>The count reaches zero due to invocations of the
jaroslav@1890:      * {@link #countDown} method; or
jaroslav@1890:      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
jaroslav@1890:      * the current thread.
jaroslav@1890:      * </ul>
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>has its interrupted status set on entry to this method; or
jaroslav@1890:      * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
jaroslav@1890:      * </ul>
jaroslav@1890:      * then {@link InterruptedException} is thrown and the current thread's
jaroslav@1890:      * interrupted status is cleared.
jaroslav@1890:      *
jaroslav@1890:      * @throws InterruptedException if the current thread is interrupted
jaroslav@1890:      *         while waiting
jaroslav@1890:      */
jaroslav@1890:     public void await() throws InterruptedException {
jaroslav@1890:         sync.acquireSharedInterruptibly(1);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Causes the current thread to wait until the latch has counted down to
jaroslav@1890:      * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
jaroslav@1890:      * or the specified waiting time elapses.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current count is zero then this method returns immediately
jaroslav@1890:      * with the value {@code true}.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current count is greater than zero then the current
jaroslav@1890:      * thread becomes disabled for thread scheduling purposes and lies
jaroslav@1890:      * dormant until one of three things happen:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>The count reaches zero due to invocations of the
jaroslav@1890:      * {@link #countDown} method; or
jaroslav@1890:      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
jaroslav@1890:      * the current thread; or
jaroslav@1890:      * <li>The specified waiting time elapses.
jaroslav@1890:      * </ul>
jaroslav@1890:      *
jaroslav@1890:      * <p>If the count reaches zero then the method returns with the
jaroslav@1890:      * value {@code true}.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>has its interrupted status set on entry to this method; or
jaroslav@1890:      * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
jaroslav@1890:      * </ul>
jaroslav@1890:      * then {@link InterruptedException} is thrown and the current thread's
jaroslav@1890:      * interrupted status is cleared.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the specified waiting time elapses then the value {@code false}
jaroslav@1890:      * is returned.  If the time is less than or equal to zero, the method
jaroslav@1890:      * will not wait at all.
jaroslav@1890:      *
jaroslav@1890:      * @param timeout the maximum time to wait
jaroslav@1890:      * @param unit the time unit of the {@code timeout} argument
jaroslav@1890:      * @return {@code true} if the count reached zero and {@code false}
jaroslav@1890:      *         if the waiting time elapsed before the count reached zero
jaroslav@1890:      * @throws InterruptedException if the current thread is interrupted
jaroslav@1890:      *         while waiting
jaroslav@1890:      */
jaroslav@1890:     public boolean await(long timeout, TimeUnit unit)
jaroslav@1890:         throws InterruptedException {
jaroslav@1890:         return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Decrements the count of the latch, releasing all waiting threads if
jaroslav@1890:      * the count reaches zero.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current count is greater than zero then it is decremented.
jaroslav@1890:      * If the new count is zero then all waiting threads are re-enabled for
jaroslav@1890:      * thread scheduling purposes.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current count equals zero then nothing happens.
jaroslav@1890:      */
jaroslav@1890:     public void countDown() {
jaroslav@1890:         sync.releaseShared(1);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns the current count.
jaroslav@1890:      *
jaroslav@1890:      * <p>This method is typically used for debugging and testing purposes.
jaroslav@1890:      *
jaroslav@1890:      * @return the current count
jaroslav@1890:      */
jaroslav@1890:     public long getCount() {
jaroslav@1890:         return sync.getCount();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns a string identifying this latch, as well as its state.
jaroslav@1890:      * The state, in brackets, includes the String {@code "Count ="}
jaroslav@1890:      * followed by the current count.
jaroslav@1890:      *
jaroslav@1890:      * @return a string identifying this latch, as well as its state
jaroslav@1890:      */
jaroslav@1890:     public String toString() {
jaroslav@1890:         return super.toString() + "[Count = " + sync.getCount() + "]";
jaroslav@1890:     }
jaroslav@1890: }