Сбрасываемый обратный отсчет

Я скопировал CountDownLatch и реализовал reset() метод, который сбрасывает внутренний класс Sync в его начальное состояние (начальный счетчик) :) Кажется, работает нормально. Больше нет ненужного создания объекта \ o / Невозможно создать подкласс, потому что sync был частным. Бу.

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() + "]";
    }
}

Основываясь на ответе @Fidel -s, я произвел замену ResettableCountDownLatch. Изменения, которые я сделал

• mLatch is private volatile
• mInitialCount is private final
• тип возврата простого await() изменен на void.

В остальном исходный код тоже крутой. Итак, это полный расширенный код:

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);
    }
}

Обновлять

На основании комментария @Systemplanet -s, вот более безопасная версия 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();
            }
        }
    }

По сути, это выбор между простотой и безопасностью. Т.е. если вы готовы переложить ответственность за свой код на клиента, то достаточно установить ссылку null в reset().

С другой стороны, если вы хотите упростить пользователям этот код, вам нужно использовать еще немного уловок.

Я не уверен, что это фатально ошибочно, но недавно у меня была такая же проблема, и я решил ее, просто создавая экземпляр нового объекта CountDownLatch каждый раз, когда я хотел выполнить сброс. Что-то вроде этого:

Официант:

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();

Очевидно, это тяжелая абстракция, но до сих пор она у меня работала и не требует от вас возиться с определениями классов.

Phaser имеет больше возможностей, мы может реализовать сбрасываемый countdownLatch, используя это.

Прочтите ниже основные концепции со следующих сайтов

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();
    }
}

Еще одна простая замена

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;
    }
}

Похоже, вы хотите превратить асинхронный ввод-вывод в синхронный. Вся идея использования асинхронного ввода-вывода состоит в том, чтобы избежать потоков, но CountDownLatch требует использования потоков. Это очевидное противоречие в вашем вопросе. Так что вы можете:

• продолжайте использовать потоки и используйте синхронный ввод-вывод вместо селекторов и суффиксов. Это будет намного проще и надежнее
• продолжайте использовать асинхронный ввод / вывод и откажитесь от CountDownLatch. Тогда вам понадобится асинхронная библиотека - посмотрите RxJava, Akka или df4j.
• продолжайте развивать свой проект в свое удовольствие. Затем вы можете попробовать использовать java.util.Semaphore вместо CountDownLatch или запрограммировать свой собственный класс синхронизации с помощью synchronized / wait / notify.

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);
}
}

Это сработало для меня.

Из того, что я смог понять из объяснения OP и исходного кода, сбрасываемый CountDownLatch не совсем адекватная концепция для проблемы, которую он собирался решить. Документация самой CountDownLatch упоминает вариант использования OP как простые ворота, инициализируемые счетчиком до единицы:

CountDownLatch, инициализированный счетчиком, равным единице, служит простой защелкой включения / выключения или вентилем: все потоки, вызывающие await, ждут у шлюза, пока он не будет открыт потоком, вызывающим countDown.

, но реализация CountDownLatch не идет дальше в этом направлении.

Итак, у меня возникла проблема, аналогичная проблеме OP, я решил ввести класс SimpleGate со следующими свойствами:

• Количество разрешений - одно, что означает, что оно может находиться в состоянии On или Off;

• Существует специальный поток, называемый Gate Keeper, который разрешен только shut off или open up воротам;

• Право на охрану ворот передается;

• открытие ворот немедленно позволяет потокам, которые пытались come through сделать это (эта очень логичная особенность была упущена из виду в других ответах);

• поскольку ожидается высокая конкуренция потоков, справедливость поддерживается в качестве опции, это позволяет уменьшить эффект перебоев потоков.

    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 
  }