Resettable CountdownLatch

user_87197 0
Viewed 17298

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

cyclicbarrier countdownlatch concurrency css-selectors java
9 Answers

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.

user_44660120

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());
    }
user_78067990

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.
user_12063010 
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.

user_123419930

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 
}
user_23663970
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