Resettable CountdownLatch

Question

I need something which is directly equivalent to CountDownLatch, but is resettable (remaining thread-safe!). I can't use classic synchronisation constructs as they simply don't work in this situation (complex locking issues). At the moment, I'm creating many CountDownLatch objects, each replacing the previous one. I believe this is doing in the young generation in the GC (due to the sheer number of objects). You can see the code which uses the latches below (it's part of the java.net mock for a ns-3 network simulator interface).

Some ideas might be to try CyclicBarrier (JDK5+) or Phaser (JDK7)

I can test code and get back to anyone that finds a solution to this problem, since I'm the only one who can insert it into the running system to see what happens :)

/**
 *
 */
package kokunet;

import java.io.IOException;
import java.nio.channels.ClosedSelectorException;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import kokuks.IConnectionSocket;
import kokuks.KKSAddress;
import kokuks.KKSSocket;
import kokuks.KKSSocketListener;

/**
 * KSelector
 * @version 1.0
 * @author Chris Dennett
 */
public class KSelector extends SelectorImpl {
    // True if this Selector has been closed
    private volatile boolean closed = false;

    // Lock for close and cleanup
    final class CloseLock {}
    private final Object closeLock = new CloseLock();

    private volatile boolean selecting = false;
    private volatile boolean wakeup = false;

    class SocketListener implements KKSSocketListener {
        protected volatile CountDownLatch latch = null;

        /**
         *
         */
        public SocketListener() {
            newLatch();
        }

        protected synchronized CountDownLatch newLatch() {
            return this.latch = new CountDownLatch(1);
        }

        protected synchronized void refreshReady(KKSSocket socket) {
            if (!selecting) return;

            synchronized (socketToChannel) {
                SelChImpl ch = socketToChannel.get(socket);
                if (ch == null) {
                    System.out.println("ks sendCB: channel not found for socket: " + socket);
                    return;
                }
                synchronized (channelToKey) {
                    SelectionKeyImpl sk = channelToKey.get(ch);
                    if (sk != null) {
                        if (handleSelect(sk)) {
                            latch.countDown();
                        }
                    }
                }
            }
        }
        @Override
        public void connectionSucceeded(KKSSocket socket) {
            refreshReady(socket);
        }
        @Override
        public void connectionFailed(KKSSocket socket) {
            refreshReady(socket);
        }
        @Override
        public void dataSent(KKSSocket socket, long bytesSent) {
            refreshReady(socket);
        }
        @Override
        public void sendCB(KKSSocket socket, long bytesAvailable) {
            refreshReady(socket);
        }
        @Override
        public void onRecv(KKSSocket socket) {
            refreshReady(socket);
        }
        @Override
        public void newConnectionCreated(KKSSocket socket, KKSSocket newSocket, KKSAddress remoteaddress) {
            refreshReady(socket);
        }
        @Override
        public void normalClose(KKSSocket socket) {
            wakeup();
        }
        @Override
        public void errorClose(KKSSocket socket) {
            wakeup();
        }
    }

    protected final Map<KKSSocket, SelChImpl>        socketToChannel = new HashMap<KKSSocket, SelChImpl>();
    protected final Map<SelChImpl, SelectionKeyImpl> channelToKey    = new HashMap<SelChImpl, SelectionKeyImpl>();
    protected final SocketListener currListener = new SocketListener();
    protected Thread selectingThread = null;

    SelChImpl getChannelForSocket(KKSSocket s) {
        synchronized (socketToChannel) {
            return socketToChannel.get(s);
        }
    }

    SelectionKeyImpl getSelKeyForChannel(KKSSocket s) {
        synchronized (channelToKey) {
            return channelToKey.get(s);
        }
    }

    protected boolean markRead(SelectionKeyImpl impl) {
        synchronized (impl) {
            if (!impl.isValid()) return false;
            impl.nioReadyOps(impl.readyOps() | SelectionKeyImpl.OP_READ);
            return selectedKeys.add(impl);
        }
    }

    protected boolean markWrite(SelectionKeyImpl impl) {
        synchronized (impl) {
            if (!impl.isValid()) return false;
            impl.nioReadyOps(impl.readyOps() | SelectionKeyImpl.OP_WRITE);
            return selectedKeys.add(impl);
        }
    }

    protected boolean markAccept(SelectionKeyImpl impl) {
        synchronized (impl) {
            if (!impl.isValid()) return false;
            impl.nioReadyOps(impl.readyOps() | SelectionKeyImpl.OP_ACCEPT);
            return selectedKeys.add(impl);
        }
    }

    protected boolean markConnect(SelectionKeyImpl impl) {
        synchronized (impl) {
            if (!impl.isValid()) return false;
            impl.nioReadyOps(impl.readyOps() | SelectionKeyImpl.OP_CONNECT);
            return selectedKeys.add(impl);
        }
    }

    /**
     * @param provider
     */
    protected KSelector(SelectorProvider provider) {
        super(provider);
    }

    /* (non-Javadoc)
     * @see kokunet.SelectorImpl#implClose()
     */
    @Override
    protected void implClose() throws IOException {
        provider().getApp().printMessage("implClose: closed: " + closed);
        synchronized (closeLock) {
            if (closed) return;
            closed = true;
            for (SelectionKey sk : keys) {
                provider().getApp().printMessage("dereg1");
                deregister((AbstractSelectionKey)sk);
                provider().getApp().printMessage("dereg2");
                SelectableChannel selch = sk.channel();
                if (!selch.isOpen() && !selch.isRegistered())
                    ((SelChImpl)selch).kill();
            }
            implCloseInterrupt();
        }
    }

    protected void implCloseInterrupt() {
        wakeup();
    }

    private boolean handleSelect(SelectionKey k) {
        synchronized (k) {
            boolean notify = false;

            if (!k.isValid()) {
                k.cancel();
                ((SelectionKeyImpl)k).channel.socket().removeListener(currListener);
                return false;
            }

            SelectionKeyImpl ski = (SelectionKeyImpl)k;

            if ((ski.interestOps() & SelectionKeyImpl.OP_READ) != 0) {
                if (ski.channel.socket().getRxAvailable() > 0) {
                    notify |= markRead(ski);
                }
            }

            if ((ski.interestOps() & SelectionKeyImpl.OP_WRITE) != 0) {
                if (ski.channel.socket().getTxAvailable() > 0) {
                    notify |= markWrite(ski);
                }
            }

            if ((ski.interestOps() & SelectionKeyImpl.OP_CONNECT) != 0) {
                if (!ski.channel.socket().isConnectionless()) {
                    IConnectionSocket cs = (IConnectionSocket)ski.channel.socket();
                    if (!ski.channel.socket().isAccepting() && !cs.isConnecting() && !cs.isConnected()) {
                        notify |= markConnect(ski);
                    }
                }
            }

            if ((ski.interestOps() & SelectionKeyImpl.OP_ACCEPT) != 0) {
                //provider().getApp().printMessage("accept check: ski: " + ski + ", connectionless: " + ski.channel.socket().isConnectionless() + ", listening: " + ski.channel.socket().isListening() + ", hasPendingConn: " + (ski.channel.socket().isConnectionless() ? "nope!" : ((IConnectionSocket)ski.channel.socket()).hasPendingConnections()));
                if (!ski.channel.socket().isConnectionless() && ski.channel.socket().isListening()) {
                    IConnectionSocket cs = (IConnectionSocket)ski.channel.socket();
                    if (cs.hasPendingConnections()) {
                        notify |= markAccept(ski);
                    }
                }
            }
            return notify;
        }
    }

    private boolean handleSelect() {
        boolean notify = false;

        // get initial status
        for (SelectionKey k : keys) {
            notify |= handleSelect(k);
        }

        return notify;
    }

    /* (non-Javadoc)
     * @see kokunet.SelectorImpl#doSelect(long)
     */
    @Override
    protected int doSelect(long timeout) throws IOException {
        processDeregisterQueue();

        long timestartedms = System.currentTimeMillis();

        synchronized (selectedKeys) {
            synchronized (currListener) {
                wakeup = false;
                selectingThread = Thread.currentThread();
                selecting = true;
            }
            try {
                handleSelect();

                if (!selectedKeys.isEmpty() || timeout == 0) {
                    return selectedKeys.size();
                }

                //TODO: useless op if we have keys available
                for (SelectionKey key : keys) {
                    ((SelectionKeyImpl)key).channel.socket().addListener(currListener);
                }
                try {
                    while (!wakeup && isOpen() && selectedKeys.isEmpty()) {
                        CountDownLatch latch = null;
                        synchronized (currListener) {
                            if (wakeup || !isOpen() || !selectedKeys.isEmpty()) {
                                break;
                            }
                            latch = currListener.newLatch();
                        }
                        try {
                            if (timeout > 0) {
                                long currtimems = System.currentTimeMillis();
                                long remainingMS = (timestartedms + timeout) - currtimems;

                                if (remainingMS > 0) {
                                    latch.await(remainingMS, TimeUnit.MILLISECONDS);
                                } else {
                                    break;
                                }
                            } else {
                                latch.await();
                            }
                        } catch (InterruptedException e) {

                        }
                    }
                    return selectedKeys.size();
                } finally {
                    for (SelectionKey key : keys) {
                        ((SelectionKeyImpl)key).channel.socket().removeListener(currListener);
                    }
                }
            } finally {
                synchronized (currListener) {
                    selecting = false;
                    selectingThread = null;
                    wakeup = false;
                }
            }
        }
    }

    /* (non-Javadoc)
     * @see kokunet.SelectorImpl#implRegister(kokunet.SelectionKeyImpl)
     */
    @Override
    protected void implRegister(SelectionKeyImpl ski) {
        synchronized (closeLock) {
            if (closed) throw new ClosedSelectorException();
            synchronized (channelToKey) {
                synchronized (socketToChannel) {
                    keys.add(ski);
                    socketToChannel.put(ski.channel.socket(), ski.channel);
                    channelToKey.put(ski.channel, ski);
                }
            }
        }

    }

    /* (non-Javadoc)
     * @see kokunet.SelectorImpl#implDereg(kokunet.SelectionKeyImpl)
     */
    @Override
    protected void implDereg(SelectionKeyImpl ski) throws IOException {
        synchronized (channelToKey) {
            synchronized (socketToChannel) {
                keys.remove(ski);
                socketToChannel.remove(ski.channel.socket());
                channelToKey.remove(ski.channel);

                SelectableChannel selch = ski.channel();

                if (!selch.isOpen() && !selch.isRegistered())
                    ((SelChImpl)selch).kill();
            }
        }
    }

    /* (non-Javadoc)
     * @see kokunet.SelectorImpl#wakeup()
     */
    @Override
    public Selector wakeup() {
        synchronized (currListener) {
            if (selecting) {
                wakeup = true;
                selecting = false;
                selectingThread.interrupt();
                selectingThread = null;
            }
        }
        return this;
    }
}

Cheers,
Chris

Answer 1

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() + "]";
    }
}

Answer 2

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.

Answer 3

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();
    }
}

Answer 4

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.

Answer 5

Use Phaser.

if only one thread should to do work. U can join AtomicBoolean and Phaser

 AtomicBoolean someConditionInProgress = new AtomicBoolean("false"); Phaser onConditionalPhaser = new Phaser(1);


   (some function) if (!someConditionInProgress.compareAndSet(false, true)) {
        try {
           //do something
        } finally {
            someConditionInProgress.set(false);
            //release barier
            onConditionalPhaser.arrive();
        }
    } else {
        onConditionalPhaser.awaitAdvance(onConditionalPhaser.getPhase());
    }

Answer 6

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.

Answer 7

public class ResettableLatch {
private static final class Sync extends AbstractQueuedSynchronizer {

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

    int getCount() {
        return getState();
    }

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

    public void reset(int count) {
        setState(count);
    }

    protected boolean tryReleaseShared(int releases) {
        for (;;) {
            int c = getState();
            if (c == 0)
                return false;
            int nextc = c - 1;
            if (compareAndSetState(c, nextc))
                return nextc == 0;
        }
    }
}

private final Sync sync;

public ResettableLatch(int count) {
    if (count < 0)
        throw new IllegalArgumentException("count < 0");
    this.sync = new Sync(count);
}

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}

public void countDown() {
    sync.releaseShared(1);
}

public long getCount() {
    return sync.getCount();
}

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

This worked for me.

Answer 8

From what I was able to understand from the OP explanation and source code, the resettable CountDownLatch is not quite adequate concept for the problem he was going to solve. The documentation of the CountDownLatch itself mentions the OP's use case as simple gate initialized with a count of one:

CountDownLatch initialized with a count of one serves as a simple on/off latch, or gate: all threads invoking await wait at the gate until it is opened by a thread invoking countDown.

, but CountDownLatch implementation does not go any further in this direction.

So, myself having a problem similar to that of OP's I decided to introduce a SimpleGate class with the following properties:

• Number of permits is one, which means it can be either in On or Off state;

• There is a dedicated thread, called Gate Keeper that is only allowed to shut off or open up the Gate;

• The right of gate keeping is transferable;

• the opening up the Gate immediately allows the threads, that tried to come through the Gate, to do it (this very logical feature has been overlooked in the other answers);

• as the thread contention is expected to be high, fairness is supported as an option, this allows to decrease an effect of thread barging.

    public class SimpleGate {
  
        private static class Sync extends AbstractQueuedSynchronizer {
  
           // State
           private static final int SHUT = 1;
           private static final int OPEN = 0;
  
           private boolean fair;
  
           public void setFair(boolean fair) {
              this.fair = fair;
           }
  
           public void shutOff() {
              super.setState(SHUT);
           }
  
           @Override
           protected int tryAcquireShared(int arg) {
              if (fair && super.hasQueuedPredecessors())
                 return -1;
              return super.getState() == OPEN ? 1 : -1;
           }
  
           @Override
           protected boolean tryReleaseShared(int arg) {
              super.setState(OPEN); 
              return true;
           }
  
      }
  
      private Sync sync = new Sync();
      private volatile Thread gateKeeper = Thread.currentThread();
  
      public SimpleGate(){
         this(true);
      }
  
      public SimpleGate(boolean shutOff){
         this(shutOff, false);
      }
  
      public SimpleGate(boolean shutOff, boolean fair){
         if (shutOff)
            sync.shutOff();
         sync.setFair(fair);
      }
  
      public void comeThrough(){
         if (Thread.currentThread() == gateKeeper)
            throw new IllegalStateException("Gate Keeper thread is not supposed to come through the gate");
         sync.acquireShared(0);
      }
  
      public void shutOff(){
         if (Thread.currentThread() != gateKeeper)
            throw new IllegalStateException("Only a Gate Keeper thread is allowed to shut off");
          sync.shutOff();
      }
  
      public void openUp(){
         if (Thread.currentThread() != gateKeeper)
            throw new IllegalStateException("Only a Gate Keeper thread is allowed to open up");
         sync.releaseShared(0);
      }
  
      public void transferOwnership(Thread newGateKeeper){
         this.gateKeeper  = newGateKeeper;
      }
  
      // an addition of waiting interruptibly and waiting for specified amount of time, 
      //if they are needed, is trivial 
  }
  

Answer 9

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;
    }
}