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/*

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/licenses/publicdomain

 */

package java.util.concurrent;

import java.util.*;

import java.util.concurrent.locks.*;

import java.util.concurrent.atomic.*;

/**

 * A counting semaphore.  Conceptually, a semaphore maintains a set of

 * permits.  Each {@link #acquire} blocks if necessary until a permit is

 * available, and then takes it.  Each {@link #release} adds a permit,

 * potentially releasing a blocking acquirer.

 * However, no actual permit objects are used; the {@code Semaphore} just

 * keeps a count of the number available and acts accordingly.

 *

 * <p>Semaphores are often used to restrict the number of threads than can

 * access some (physical or logical) resource. For example, here is

 * a class that uses a semaphore to control access to a pool of items:

 * <pre>

 * class Pool {

 *   private static final int MAX_AVAILABLE = 100;

 *   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);

 *

 *   public Object getItem() throws InterruptedException {

 *     available.acquire();

 *     return getNextAvailableItem();

 *   }

 *

 *   public void putItem(Object x) {

 *     if (markAsUnused(x))

 *       available.release();

 *   }

 *

 *   // Not a particularly efficient data structure; just for demo

 *

 *   protected Object[] items = ... whatever kinds of items being managed

 *   protected boolean[] used = new boolean[MAX_AVAILABLE];

 *

 *   protected synchronized Object getNextAvailableItem() {

 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {

 *       if (!used[i]) {

 *          used[i] = true;

 *          return items[i];

 *       }

 *     }

 *     return null; // not reached

 *   }

 *

 *   protected synchronized boolean markAsUnused(Object item) {

 *     for (int i = 0; i < MAX_AVAILABLE; ++i) {

 *       if (item == items[i]) {

 *          if (used[i]) {

 *            used[i] = false;

 *            return true;

 *          } else

 *            return false;

 *       }

 *     }

 *     return false;

 *   }

 *

 * }

 * </pre>

 *

 * <p>Before obtaining an item each thread must acquire a permit from

 * the semaphore, guaranteeing that an item is available for use. When

 * the thread has finished with the item it is returned back to the

 * pool and a permit is returned to the semaphore, allowing another

 * thread to acquire that item.  Note that no synchronization lock is

 * held when {@link #acquire} is called as that would prevent an item

 * from being returned to the pool.  The semaphore encapsulates the

 * synchronization needed to restrict access to the pool, separately

 * from any synchronization needed to maintain the consistency of the

 * pool itself.

 *

 * <p>A semaphore initialized to one, and which is used such that it

 * only has at most one permit available, can serve as a mutual

 * exclusion lock.  This is more commonly known as a <em>binary

 * semaphore</em>, because it only has two states: one permit

 * available, or zero permits available.  When used in this way, the

 * binary semaphore has the property (unlike many {@link Lock}

 * implementations), that the "lock" can be released by a

 * thread other than the owner (as semaphores have no notion of

 * ownership).  This can be useful in some specialized contexts, such

 * as deadlock recovery.

 *

 * <p> The constructor for this class optionally accepts a

 * <em>fairness</em> parameter. When set false, this class makes no

 * guarantees about the order in which threads acquire permits. In

 * particular, <em>barging</em> is permitted, that is, a thread

 * invoking {@link #acquire} can be allocated a permit ahead of a

 * thread that has been waiting - logically the new thread places itself at

 * the head of the queue of waiting threads. When fairness is set true, the

 * semaphore guarantees that threads invoking any of the {@link

 * #acquire() acquire} methods are selected to obtain permits in the order in

 * which their invocation of those methods was processed

 * (first-in-first-out; FIFO). Note that FIFO ordering necessarily

 * applies to specific internal points of execution within these

 * methods.  So, it is possible for one thread to invoke

 * {@code acquire} before another, but reach the ordering point after

 * the other, and similarly upon return from the method.

 * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not

 * honor the fairness setting, but will take any permits that are

 * available.

 *

 * <p>Generally, semaphores used to control resource access should be

 * initialized as fair, to ensure that no thread is starved out from

 * accessing a resource. When using semaphores for other kinds of

 * synchronization control, the throughput advantages of non-fair

 * ordering often outweigh fairness considerations.

 *

 * <p>This class also provides convenience methods to {@link

 * #acquire(int) acquire} and {@link #release(int) release} multiple

 * permits at a time.  Beware of the increased risk of indefinite

 * postponement when these methods are used without fairness set true.

 *

 * <p>Memory consistency effects: Actions in a thread prior to calling

 * a "release" method such as {@code release()}

 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

 * actions following a successful "acquire" method such as {@code acquire()}

 * in another thread.

 *

 * @since 1.5

 * @author Doug Lea

 *

 */

public class Semaphore implements java.io.Serializable {

    private static final long serialVersionUID = -3222578661600680210L;

    /** All mechanics via AbstractQueuedSynchronizer subclass */

    private final Sync sync;

    /**

     * Synchronization implementation for semaphore.  Uses AQS state

     * to represent permits. Subclassed into fair and nonfair

     * versions.

     */

    abstract static class Sync extends AbstractQueuedSynchronizer {

        private static final long serialVersionUID = 1192457210091910933L;

        Sync(int permits) {

            setState(permits);

        }

        final int getPermits() {

            return getState();

        }

        final int nonfairTryAcquireShared(int acquires) {

            for (;;) {

                int available = getState();

                int remaining = available - acquires;

                if (remaining < 0 ||

                    compareAndSetState(available, remaining))

                    return remaining;

            }

        }

        protected final boolean tryReleaseShared(int releases) {

            for (;;) {

                int p = getState();

                if (compareAndSetState(p, p + releases))

                    return true;

            }

        }

        final void reducePermits(int reductions) {

            for (;;) {

                int current = getState();

                int next = current - reductions;

                if (compareAndSetState(current, next))

                    return;

            }

        }

        final int drainPermits() {

            for (;;) {

                int current = getState();

                if (current == 0 || compareAndSetState(current, 0))

                    return current;

            }

        }

    }

    /**

     * NonFair version

     */

    final static class NonfairSync extends Sync {

        private static final long serialVersionUID = -2694183684443567898L;

        NonfairSync(int permits) {

            super(permits);

        }

        protected int tryAcquireShared(int acquires) {

            return nonfairTryAcquireShared(acquires);

        }

    }

    /**

     * Fair version

     */

    final static class FairSync extends Sync {

        private static final long serialVersionUID = 2014338818796000944L;

        FairSync(int permits) {

            super(permits);

        }

        protected int tryAcquireShared(int acquires) {

            Thread current = Thread.currentThread();

            for (;;) {

                Thread first = getFirstQueuedThread();

                if (first != null && first != current)

                    return -1;

                int available = getState();

                int remaining = available - acquires;

                if (remaining < 0 ||

                    compareAndSetState(available, remaining))

                    return remaining;

            }

        }

    }

    /**

     * Creates a {@code Semaphore} with the given number of

     * permits and nonfair fairness setting.

     *

     * @param permits the initial number of permits available.

     *        This value may be negative, in which case releases

     *        must occur before any acquires will be granted.

     */

    public Semaphore(int permits) {

        sync = new NonfairSync(permits);

    }

    /**

     * Creates a {@code Semaphore} with the given number of

     * permits and the given fairness setting.

     *

     * @param permits the initial number of permits available.

     *        This value may be negative, in which case releases

     *        must occur before any acquires will be granted.

     * @param fair {@code true} if this semaphore will guarantee

     *        first-in first-out granting of permits under contention,

     *        else {@code false}

     */

    public Semaphore(int permits, boolean fair) {

        sync = (fair)? new FairSync(permits) : new NonfairSync(permits);

    }

    /**

     * Acquires a permit from this semaphore, blocking until one is

     * available, or the thread is {@linkplain Thread#interrupt interrupted}.

     *

     * <p>Acquires a permit, if one is available and returns immediately,

     * reducing the number of available permits by one.

     *

     * <p>If no permit is available then the current thread becomes

     * disabled for thread scheduling purposes and lies dormant until

     * one of two things happens:

     * <ul>

     * <li>Some other thread invokes the {@link #release} method for this

     * semaphore and the current thread is next to be assigned a permit; 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

     * for a permit,

     * </ul>

     * then {@link InterruptedException} is thrown and the current thread's

     * interrupted status is cleared.

     *

     * @throws InterruptedException if the current thread is interrupted

     */

    public void acquire() throws InterruptedException {

        sync.acquireSharedInterruptibly(1);

    }

    /**

     * Acquires a permit from this semaphore, blocking until one is

     * available.

     *

     * <p>Acquires a permit, if one is available and returns immediately,

     * reducing the number of available permits by one.

     *

     * <p>If no permit is available then the current thread becomes

     * disabled for thread scheduling purposes and lies dormant until

     * some other thread invokes the {@link #release} method for this

     * semaphore and the current thread is next to be assigned a permit.

     *

     * <p>If the current thread is {@linkplain Thread#interrupt interrupted}

     * while waiting for a permit then it will continue to wait, but the

     * time at which the thread is assigned a permit may change compared to

     * the time it would have received the permit had no interruption

     * occurred.  When the thread does return from this method its interrupt

     * status will be set.

     */

    public void acquireUninterruptibly() {

        sync.acquireShared(1);

    }

    /**

     * Acquires a permit from this semaphore, only if one is available at the

     * time of invocation.

     *

     * <p>Acquires a permit, if one is available and returns immediately,

     * with the value {@code true},

     * reducing the number of available permits by one.

     *

     * <p>If no permit is available then this method will return

     * immediately with the value {@code false}.

     *

     * <p>Even when this semaphore has been set to use a

     * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>

     * immediately acquire a permit if one is available, whether or not

     * other threads are currently waiting.

     * This "barging" behavior can be useful in certain

     * circumstances, even though it breaks fairness. If you want to honor

     * the fairness setting, then use

     * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }

     * which is almost equivalent (it also detects interruption).

     *

     * @return {@code true} if a permit was acquired and {@code false}

     *         otherwise

     */

    public boolean tryAcquire() {

        return sync.nonfairTryAcquireShared(1) >= 0;

    }

    /**

     * Acquires a permit from this semaphore, if one becomes available

     * within the given waiting time and the current thread has not

     * been {@linkplain Thread#interrupt interrupted}.

     *

     * <p>Acquires a permit, if one is available and returns immediately,

     * with the value {@code true},

     * reducing the number of available permits by one.

     *

     * <p>If no permit is available then the current thread becomes

     * disabled for thread scheduling purposes and lies dormant until

     * one of three things happens:

     * <ul>

     * <li>Some other thread invokes the {@link #release} method for this

     * semaphore and the current thread is next to be assigned a permit; or

     * <li>Some other thread {@linkplain Thread#interrupt interrupts}

     * the current thread; or

     * <li>The specified waiting time elapses.

     * </ul>

     *

     * <p>If a permit is acquired then the value {@code true} is returned.

     *

     * <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

     * to acquire a permit,

     * </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 for a permit

     * @param unit the time unit of the {@code timeout} argument

     * @return {@code true} if a permit was acquired and {@code false}

     *         if the waiting time elapsed before a permit was acquired

     * @throws InterruptedException if the current thread is interrupted

     */

    public boolean tryAcquire(long timeout, TimeUnit unit)

        throws InterruptedException {

        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));

    }

    /**

     * Releases a permit, returning it to the semaphore.

     *

     * <p>Releases a permit, increasing the number of available permits by

     * one.  If any threads are trying to acquire a permit, then one is

     * selected and given the permit that was just released.  That thread

     * is (re)enabled for thread scheduling purposes.

     *

     * <p>There is no requirement that a thread that releases a permit must

     * have acquired that permit by calling {@link #acquire}.

     * Correct usage of a semaphore is established by programming convention

     * in the application.

     */

    public void release() {

        sync.releaseShared(1);

    }

    /**

     * Acquires the given number of permits from this semaphore,

     * blocking until all are available,

     * or the thread is {@linkplain Thread#interrupt interrupted}.

     *

     * <p>Acquires the given number of permits, if they are available,

     * and returns immediately, reducing the number of available permits

     * by the given amount.

     *

     * <p>If insufficient permits are available then the current thread becomes

     * disabled for thread scheduling purposes and lies dormant until

     * one of two things happens:

     * <ul>

     * <li>Some other thread invokes one of the {@link #release() release}

     * methods for this semaphore, the current thread is next to be assigned

     * permits and the number of available permits satisfies this request; 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

     * for a permit,

     * </ul>

     * then {@link InterruptedException} is thrown and the current thread's

     * interrupted status is cleared.

     * Any permits that were to be assigned to this thread are instead

     * assigned to other threads trying to acquire permits, as if

     * permits had been made available by a call to {@link #release()}.

     *

     * @param permits the number of permits to acquire

     * @throws InterruptedException if the current thread is interrupted

     * @throws IllegalArgumentException if {@code permits} is negative

     */

    public void acquire(int permits) throws InterruptedException {

        if (permits < 0) throw new IllegalArgumentException();

        sync.acquireSharedInterruptibly(permits);

    }

    /**

     * Acquires the given number of permits from this semaphore,

     * blocking until all are available.

     *

     * <p>Acquires the given number of permits, if they are available,

     * and returns immediately, reducing the number of available permits

     * by the given amount.

     *

     * <p>If insufficient permits are available then the current thread becomes

     * disabled for thread scheduling purposes and lies dormant until

     * some other thread invokes one of the {@link #release() release}

     * methods for this semaphore, the current thread is next to be assigned

     * permits and the number of available permits satisfies this request.

     *

     * <p>If the current thread is {@linkplain Thread#interrupt interrupted}

     * while waiting for permits then it will continue to wait and its

     * position in the queue is not affected.  When the thread does return

     * from this method its interrupt status will be set.

     *

     * @param permits the number of permits to acquire

     * @throws IllegalArgumentException if {@code permits} is negative

     *

     */

    public void acquireUninterruptibly(int permits) {

        if (permits < 0) throw new IllegalArgumentException();

        sync.acquireShared(permits);

    }

    /**

     * Acquires the given number of permits from this semaphore, only

     * if all are available at the time of invocation.

     *

     * <p>Acquires the given number of permits, if they are available, and

     * returns immediately, with the value {@code true},

     * reducing the number of available permits by the given amount.

     *

     * <p>If insufficient permits are available then this method will return

     * immediately with the value {@code false} and the number of available

     * permits is unchanged.

     *

     * <p>Even when this semaphore has been set to use a fair ordering

     * policy, a call to {@code tryAcquire} <em>will</em>

     * immediately acquire a permit if one is available, whether or

     * not other threads are currently waiting.  This

     * "barging" behavior can be useful in certain

     * circumstances, even though it breaks fairness. If you want to

     * honor the fairness setting, then use {@link #tryAcquire(int,

     * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }

     * which is almost equivalent (it also detects interruption).

     *

     * @param permits the number of permits to acquire

     * @return {@code true} if the permits were acquired and

     *         {@code false} otherwise

     * @throws IllegalArgumentException if {@code permits} is negative

     */

    public boolean tryAcquire(int permits) {

        if (permits < 0) throw new IllegalArgumentException();

        return sync.nonfairTryAcquireShared(permits) >= 0;

    }

    /**

     * Acquires the given number of permits from this semaphore, if all

     * become available within the given waiting time and the current

     * thread has not been {@linkplain Thread#interrupt interrupted}.

     *

     * <p>Acquires the given number of permits, if they are available and

     * returns immediately, with the value {@code true},

     * reducing the number of available permits by the given amount.

     *

     * <p>If insufficient permits are available then

     * the current thread becomes disabled for thread scheduling

     * purposes and lies dormant until one of three things happens:

     * <ul>

     * <li>Some other thread invokes one of the {@link #release() release}

     * methods for this semaphore, the current thread is next to be assigned

     * permits and the number of available permits satisfies this request; or

     * <li>Some other thread {@linkplain Thread#interrupt interrupts}

     * the current thread; or

     * <li>The specified waiting time elapses.

     * </ul>

     *

     * <p>If the permits are acquired then the value {@code true} is returned.

     *

     * <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

     * to acquire the permits,

     * </ul>

     * then {@link InterruptedException} is thrown and the current thread's

     * interrupted status is cleared.

     * Any permits that were to be assigned to this thread, are instead

     * assigned to other threads trying to acquire permits, as if

     * the permits had been made available by a call to {@link #release()}.

     *

     * <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.  Any permits that were to be assigned to this

     * thread, are instead assigned to other threads trying to acquire

     * permits, as if the permits had been made available by a call to

     * {@link #release()}.

     *

     * @param permits the number of permits to acquire

     * @param timeout the maximum time to wait for the permits

     * @param unit the time unit of the {@code timeout} argument

     * @return {@code true} if all permits were acquired and {@code false}

     *         if the waiting time elapsed before all permits were acquired

     * @throws InterruptedException if the current thread is interrupted

     * @throws IllegalArgumentException if {@code permits} is negative

     */

    public boolean tryAcquire(int permits, long timeout, TimeUnit unit)

        throws InterruptedException {

        if (permits < 0) throw new IllegalArgumentException();

        return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));

    }

    /**

     * Releases the given number of permits, returning them to the semaphore.

     *

     * <p>Releases the given number of permits, increasing the number of

     * available permits by that amount.

     * If any threads are trying to acquire permits, then one

     * is selected and given the permits that were just released.

     * If the number of available permits satisfies that thread's request

     * then that thread is (re)enabled for thread scheduling purposes;

     * otherwise the thread will wait until sufficient permits are available.

     * If there are still permits available

     * after this thread's request has been satisfied, then those permits

     * are assigned in turn to other threads trying to acquire permits.

     *

     * <p>There is no requirement that a thread that releases a permit must

     * have acquired that permit by calling {@link Semaphore#acquire acquire}.

     * Correct usage of a semaphore is established by programming convention

     * in the application.

     *

     * @param permits the number of permits to release

     * @throws IllegalArgumentException if {@code permits} is negative

     */

    public void release(int permits) {

        if (permits < 0) throw new IllegalArgumentException();

        sync.releaseShared(permits);

    }

    /**

     * Returns the current number of permits available in this semaphore.

     *

     * <p>This method is typically used for debugging and testing purposes.

     *

     * @return the number of permits available in this semaphore

     */

    public int availablePermits() {

        return sync.getPermits();

    }

    /**

     * Acquires and returns all permits that are immediately available.

     *

     * @return the number of permits acquired

     */

    public int drainPermits() {

        return sync.drainPermits();

    }

    /**

     * Shrinks the number of available permits by the indicated

     * reduction. This method can be useful in subclasses that use

     * semaphores to track resources that become unavailable. This

     * method differs from {@code acquire} in that it does not block

     * waiting for permits to become available.

     *

     * @param reduction the number of permits to remove

     * @throws IllegalArgumentException if {@code reduction} is negative

     */

    protected void reducePermits(int reduction) {

        if (reduction < 0) throw new IllegalArgumentException();

        sync.reducePermits(reduction);

    }

    /**

     * Returns {@code true} if this semaphore has fairness set true.

     *

     * @return {@code true} if this semaphore has fairness set true

     */

    public boolean isFair() {

        return sync instanceof FairSync;

    }

    /**

     * Queries whether any threads are waiting to acquire. Note that

     * because cancellations may occur at any time, a {@code true}

     * return does not guarantee that any other thread will ever

     * acquire.  This method is designed primarily for use in

     * monitoring of the system state.

     *

     * @return {@code true} if there may be other threads waiting to

     *         acquire the lock

     */

    public final boolean hasQueuedThreads() {

        return sync.hasQueuedThreads();

    }

    /**

     * Returns an estimate of the number of threads waiting to acquire.

     * The value is only an estimate because the number of threads may

     * change dynamically while this method traverses internal data

     * structures.  This method is designed for use in monitoring of the

     * system state, not for synchronization control.

     *

     * @return the estimated number of threads waiting for this lock

     */

    public final int getQueueLength() {

        return sync.getQueueLength();

    }

    /**

     * Returns a collection containing threads that may be waiting to acquire.

     * Because the actual set of threads may change dynamically while

     * constructing this result, the returned collection is only a best-effort

     * estimate.  The elements of the returned collection are in no particular

     * order.  This method is designed to facilitate construction of

     * subclasses that provide more extensive monitoring facilities.

     *

     * @return the collection of threads

     */

    protected Collection<Thread> getQueuedThreads() {

        return sync.getQueuedThreads();

    }

    /**

     * Returns a string identifying this semaphore, as well as its state.

     * The state, in brackets, includes the String {@code "Permits ="}

     * followed by the number of permits.

     *

     * @return a string identifying this semaphore, as well as its state

     */

    public String toString() {

        return super.toString() + "[Permits = " + sync.getPermits() + "]";

    }

}