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

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();
    }
}