luni/src/main/java/java/util/concurrent/CountDownLatch.java - platform/libcore - Git at Google

 /*
  * 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/publicdomain/zero/1.0/
  */

 package java.util.concurrent;

 import java.util.concurrent.locks.AbstractQueuedSynchronizer;

 /**
  * A synchronization aid that allows one or more threads to wait until
  * a set of operations being performed in other threads completes.
  *
  * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
  * The {@link #await await} methods block until the current 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 {@code CountDownLatch} is a versatile synchronization tool
  * and can be used for a number of purposes.  A
  * {@code CountDownLatch} 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 {@code CountDownLatch} 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 {@code CountDownLatch} is that it
  * doesn't require that threads calling {@code countDown} 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> {@code
  * 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> {@code
  * 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>
  *
  * <p>Memory consistency effects: Until the count reaches
  * zero, actions in a thread prior to calling
  * {@code countDown()}
  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
  * actions following a successful return from a corresponding
  * {@code await()} in another thread.
  *
  * @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 {
         private static final long serialVersionUID = 4982264981922014374L;

         Sync(int count) {
             setState(count);
         }

         int getCount() {
             return getState();
         }

         protected int tryAcquireShared(int acquires) {
             return (getState() == 0) ? 1 : -1;
         }

         protected 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 {@code CountDownLatch} 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 {@code count} is negative
      */
     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 {@linkplain Thread#interrupt interrupted}.
      *
      * <p>If the current count is zero then this method returns immediately.
      *
      * <p>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:
      * <ul>
      * <li>The count reaches zero due to invocations of the
      * {@link #countDown} method; or
      * <li>Some other thread {@linkplain 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 {@linkplain 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 {@linkplain Thread#interrupt interrupted},
      * or the specified waiting time elapses.
      *
      * <p>If the current count is zero then this method returns immediately
      * with the value {@code true}.
      *
      * <p>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:
      * <ul>
      * <li>The count reaches zero due to invocations of the
      * {@link #countDown} method; or
      * <li>Some other thread {@linkplain 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 {@code true}.
      *
      * <p>If the current thread:
      * <ul>
      * <li>has its interrupted status set on entry to this method; or
      * <li>is {@linkplain 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 {@code false}
      * 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 {@code timeout} argument
      * @return {@code true} if the count reached zero and {@code false}
      *         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 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 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 {@code "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() + "]";
     }
 }
	/*
	* 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/publicdomain/zero/1.0/
	*/

	package java.util.concurrent;

	import java.util.concurrent.locks.AbstractQueuedSynchronizer;

	/**
	* A synchronization aid that allows one or more threads to wait until
	* a set of operations being performed in other threads completes.
	*
	* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
	* The {@link #await await} methods block until the current 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 {@code CountDownLatch} is a versatile synchronization tool
	* and can be used for a number of purposes. A
	* {@code CountDownLatch} 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 {@code CountDownLatch} 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 {@code CountDownLatch} is that it
	* doesn't require that threads calling {@code countDown} 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> {@code
	* 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> {@code
	* 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>
	*
	* <p>Memory consistency effects: Until the count reaches
	* zero, actions in a thread prior to calling
	* {@code countDown()}
	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
	* actions following a successful return from a corresponding
	* {@code await()} in another thread.
	*
	* @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 {
	private static final long serialVersionUID = 4982264981922014374L;

	Sync(int count) {
	setState(count);
	}

	int getCount() {
	return getState();
	}

	protected int tryAcquireShared(int acquires) {
	return (getState() == 0) ? 1 : -1;
	}

	protected 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 {@code CountDownLatch} 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 {@code count} is negative
	*/
	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 {@linkplain Thread#interrupt interrupted}.
	*
	* <p>If the current count is zero then this method returns immediately.
	*
	* <p>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:
	* <ul>
	* <li>The count reaches zero due to invocations of the
	* {@link #countDown} method; or
	* <li>Some other thread {@linkplain 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 {@linkplain 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 {@linkplain Thread#interrupt interrupted},
	* or the specified waiting time elapses.
	*
	* <p>If the current count is zero then this method returns immediately
	* with the value {@code true}.
	*
	* <p>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:
	* <ul>
	* <li>The count reaches zero due to invocations of the
	* {@link #countDown} method; or
	* <li>Some other thread {@linkplain 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 {@code true}.
	*
	* <p>If the current thread:
	* <ul>
	* <li>has its interrupted status set on entry to this method; or
	* <li>is {@linkplain 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 {@code false}
	* 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 {@code timeout} argument
	* @return {@code true} if the count reached zero and {@code false}
	* 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 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 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 {@code "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() + "]";
	}
	}