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Resettable CountdownLatch

由 偶尔善良 提交于 2019-12-03 10:07:56

I copied CountDownLatch and implemented a reset() method that resets the internal Sync class to its initial state (starting count) :) Appears to work fine. No more unnecessary object creation \o/ It was not possible to subclass because sync was private. Boo.

import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.TimeUnit;
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>
 * 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>
 *
 * <p>Memory consistency effects: 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 ResettableCountDownLatch {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count.
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        public final int startCount;

        Sync(int count) {
            this.startCount = count;
            setState(startCount);
        }

        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;
            }
        }

        public void reset() {
             setState(startCount);
        }
    }

    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 ResettableCountDownLatch(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);
    }

    public void reset() {
        sync.reset();
    }

    /**
     * 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() + "]";
    }
}

Based on @Fidel -s answer, I made a drop-in replacement for ResettableCountDownLatch. The changes I made

  • mLatch is private volatile
  • mInitialCount is private final
  • the return type of the simple await() has changed to void.

Otherwise, the original code is cool too. So, this is the full, enhanced code:

public class ResettableCountDownLatch {

    private final int initialCount;
    private volatile CountDownLatch latch;

    public ResettableCountDownLatch(int  count) {
        initialCount = count;
        latch = new CountDownLatch(count);
    }

    public void reset() {
        latch = new CountDownLatch(initialCount);
    }

    public void countDown() {
        latch.countDown();
    }

    public void await() throws InterruptedException {
        latch.await();
    }

    public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
        return latch.await(timeout, unit);
    }
}

Update

Based on @Systemplanet-s comment, here is a safer version of reset():

    // An atomic reference is required because reset() is not that atomic anymore, not even with `volatile`.
    private final AtomicReference<CountDownLatch> latchHolder = new AtomicReference<>();

    public void reset() {
        // obtaining a local reference for modifying the required latch
        final CountDownLatch oldLatch = latchHolder.getAndSet(null);
        if (oldLatch != null) {
            // checking the count each time to prevent unnecessary countdowns due to parallel countdowns
            while (0L < oldLatch.getCount()) {
                oldLatch.countDown();
            }
        }
    }

Basically, it's a choice between simplicity and safety. I.e. if you are willing to move the responsibility to the client of your code, then it's enough to set the reference null in reset().

On the other hand, if you want to make it easy for the users of this code, then you need to use a little more tricks.

I'm not sure if this is fatally flawed but I recently had the same problem and solved it by simply instantiating a new CountDownLatch object each time I wanted to reset. Something like this:

Waiter:

bla();
latch.await();
//now the latch has counted down to 0
blabla();

CountDowner

foo();
latch.countDown();
//now the latch has counted down to 0
latch = new CountDownLatch(1);
Waiter.receiveReferenceToNewLatch(latch);
bar();

Obviously this is a heavy abstraction but thus far it has worked for me and doesn't require you to tinker with any class definitions.

Phaser has more options, we can implement resettable countdownLatch using that.

Please read below basic concepts from the following sites

https://examples.javacodegeeks.com/core-java/util/concurrent/phaser/java-util-concurrent-phaser-example/

http://netjs.blogspot.in/2016/01/phaser-in-java-concurrency.html

import java.util.concurrent.Phaser;
/**
 * Resettable countdownLatch using phaser
 */
public class PhaserExample {
    public static void main(String[] args) throws InterruptedException {
        Phaser phaser = new Phaser(3); // you can use constructor hint or
                                        // register() or mixture of both
        // register self... so parties are incremented to 4 (3+1) now
        phaser.register();
        //register is one time call for all the phases.
        //means no need to register for every phase             


        int phasecount = phaser.getPhase();
        System.out.println("Phasecount is " + phasecount);
        new PhaserExample().testPhaser(phaser, 2000);
        new PhaserExample().testPhaser(phaser, 4000);
        new PhaserExample().testPhaser(phaser, 6000);

        // similar to await() in countDownLatch/CyclicBarrier
        // parties are decremented to 3 (4+1) now
        phaser.arriveAndAwaitAdvance(); 
        // once all the thread arrived at same level, barrier opens
        System.out.println("Barrier has broken.");
        phasecount = phaser.getPhase();
        System.out.println("Phasecount is " + phasecount);

        //second phase
        new PhaserExample().testPhaser(phaser, 2000);
        new PhaserExample().testPhaser(phaser, 4000);
        new PhaserExample().testPhaser(phaser, 6000);
        phaser.arriveAndAwaitAdvance(); 
        // once all the thread arrived at same level, barrier opens
        System.out.println("Barrier has broken.");
        phasecount = phaser.getPhase();
        System.out.println("Phasecount is " + phasecount);

    }

    private void testPhaser(final Phaser phaser, final int sleepTime) {
        // phaser.register(); //Already constructor hint is given so not
        // required
        new Thread() {
            @Override
            public void run() {
                try {
                    Thread.sleep(sleepTime);
                    System.out.println(Thread.currentThread().getName() + " arrived");
                    // phaser.arrive(); //similar to CountDownLatch#countDown()
                    phaser.arriveAndAwaitAdvance();// thread will wait till Barrier opens
                    // arriveAndAwaitAdvance is similar to CyclicBarrier#await()
                }
                catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + " after passing barrier");
            }
        }.start();
    }
}

Another drop-in replacement

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class ResettableCountDownLatch {
    int mInitialCount;
    CountDownLatch mLatch;

    public ResettableCountDownLatch(int  count) {
        mInitialCount = count;
        mLatch = new CountDownLatch(count);
    }

    public void reset() {
        mLatch = new CountDownLatch(mInitialCount);
    }

    public void countDown() {
        mLatch.countDown();
    }

    public boolean await() throws InterruptedException {
        boolean result = mLatch.await();
        return result;
    }

    public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
        boolean result = mLatch.await(timeout, unit);
        return result;
    }
}

Looks like you want to turn asynchronous I/O to synchronous. The whole idea of using asynchronous I/O is to avoid threads, but CountDownLatch requres using threads. This is an obvious contradiction in your question. So, you can:

  • keep using threads and employ synchronous I/O instead of Selectors and the suff. This will be much more simple and reliable
  • keep using asynchronous I/0 and give up CountDownLatch. Then you need an asynchronous library - look at RxJava, Akka, or df4j.
  • continue to develop your project for fun. Then you can try to use java.util.Semaphore instead of CountDownLatch, or program your own synchronization class using synchronized/wait/notify.
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java
concurrency
css-selectors
countdownlatch
cyclicbarrier
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