jaroslav@1890: /*
jaroslav@1890:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@1890:  *
jaroslav@1890:  * This code is free software; you can redistribute it and/or modify it
jaroslav@1890:  * under the terms of the GNU General Public License version 2 only, as
jaroslav@1890:  * published by the Free Software Foundation.  Oracle designates this
jaroslav@1890:  * particular file as subject to the "Classpath" exception as provided
jaroslav@1890:  * by Oracle in the LICENSE file that accompanied this code.
jaroslav@1890:  *
jaroslav@1890:  * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@1890:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@1890:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
jaroslav@1890:  * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@1890:  * accompanied this code).
jaroslav@1890:  *
jaroslav@1890:  * You should have received a copy of the GNU General Public License version
jaroslav@1890:  * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@1890:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@1890:  *
jaroslav@1890:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@1890:  * or visit www.oracle.com if you need additional information or have any
jaroslav@1890:  * questions.
jaroslav@1890:  */
jaroslav@1890: 
jaroslav@1890: /*
jaroslav@1890:  * This file is available under and governed by the GNU General Public
jaroslav@1890:  * License version 2 only, as published by the Free Software Foundation.
jaroslav@1890:  * However, the following notice accompanied the original version of this
jaroslav@1890:  * file:
jaroslav@1890:  *
jaroslav@1890:  * Written by Doug Lea with assistance from members of JCP JSR-166
jaroslav@1890:  * Expert Group and released to the public domain, as explained at
jaroslav@1890:  * http://creativecommons.org/publicdomain/zero/1.0/
jaroslav@1890:  */
jaroslav@1890: 
jaroslav@1890: package java.util.concurrent;
jaroslav@1890: import java.util.*;
jaroslav@1890: import java.util.concurrent.locks.*;
jaroslav@1890: import java.util.concurrent.atomic.*;
jaroslav@1890: 
jaroslav@1890: /**
jaroslav@1890:  * A counting semaphore.  Conceptually, a semaphore maintains a set of
jaroslav@1890:  * permits.  Each {@link #acquire} blocks if necessary until a permit is
jaroslav@1890:  * available, and then takes it.  Each {@link #release} adds a permit,
jaroslav@1890:  * potentially releasing a blocking acquirer.
jaroslav@1890:  * However, no actual permit objects are used; the {@code Semaphore} just
jaroslav@1890:  * keeps a count of the number available and acts accordingly.
jaroslav@1890:  *
jaroslav@1890:  * <p>Semaphores are often used to restrict the number of threads than can
jaroslav@1890:  * access some (physical or logical) resource. For example, here is
jaroslav@1890:  * a class that uses a semaphore to control access to a pool of items:
jaroslav@1890:  * <pre>
jaroslav@1890:  * class Pool {
jaroslav@1890:  *   private static final int MAX_AVAILABLE = 100;
jaroslav@1890:  *   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
jaroslav@1890:  *
jaroslav@1890:  *   public Object getItem() throws InterruptedException {
jaroslav@1890:  *     available.acquire();
jaroslav@1890:  *     return getNextAvailableItem();
jaroslav@1890:  *   }
jaroslav@1890:  *
jaroslav@1890:  *   public void putItem(Object x) {
jaroslav@1890:  *     if (markAsUnused(x))
jaroslav@1890:  *       available.release();
jaroslav@1890:  *   }
jaroslav@1890:  *
jaroslav@1890:  *   // Not a particularly efficient data structure; just for demo
jaroslav@1890:  *
jaroslav@1890:  *   protected Object[] items = ... whatever kinds of items being managed
jaroslav@1890:  *   protected boolean[] used = new boolean[MAX_AVAILABLE];
jaroslav@1890:  *
jaroslav@1890:  *   protected synchronized Object getNextAvailableItem() {
jaroslav@1890:  *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
jaroslav@1890:  *       if (!used[i]) {
jaroslav@1890:  *          used[i] = true;
jaroslav@1890:  *          return items[i];
jaroslav@1890:  *       }
jaroslav@1890:  *     }
jaroslav@1890:  *     return null; // not reached
jaroslav@1890:  *   }
jaroslav@1890:  *
jaroslav@1890:  *   protected synchronized boolean markAsUnused(Object item) {
jaroslav@1890:  *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
jaroslav@1890:  *       if (item == items[i]) {
jaroslav@1890:  *          if (used[i]) {
jaroslav@1890:  *            used[i] = false;
jaroslav@1890:  *            return true;
jaroslav@1890:  *          } else
jaroslav@1890:  *            return false;
jaroslav@1890:  *       }
jaroslav@1890:  *     }
jaroslav@1890:  *     return false;
jaroslav@1890:  *   }
jaroslav@1890:  *
jaroslav@1890:  * }
jaroslav@1890:  * </pre>
jaroslav@1890:  *
jaroslav@1890:  * <p>Before obtaining an item each thread must acquire a permit from
jaroslav@1890:  * the semaphore, guaranteeing that an item is available for use. When
jaroslav@1890:  * the thread has finished with the item it is returned back to the
jaroslav@1890:  * pool and a permit is returned to the semaphore, allowing another
jaroslav@1890:  * thread to acquire that item.  Note that no synchronization lock is
jaroslav@1890:  * held when {@link #acquire} is called as that would prevent an item
jaroslav@1890:  * from being returned to the pool.  The semaphore encapsulates the
jaroslav@1890:  * synchronization needed to restrict access to the pool, separately
jaroslav@1890:  * from any synchronization needed to maintain the consistency of the
jaroslav@1890:  * pool itself.
jaroslav@1890:  *
jaroslav@1890:  * <p>A semaphore initialized to one, and which is used such that it
jaroslav@1890:  * only has at most one permit available, can serve as a mutual
jaroslav@1890:  * exclusion lock.  This is more commonly known as a <em>binary
jaroslav@1890:  * semaphore</em>, because it only has two states: one permit
jaroslav@1890:  * available, or zero permits available.  When used in this way, the
jaroslav@1890:  * binary semaphore has the property (unlike many {@link Lock}
jaroslav@1890:  * implementations), that the &quot;lock&quot; can be released by a
jaroslav@1890:  * thread other than the owner (as semaphores have no notion of
jaroslav@1890:  * ownership).  This can be useful in some specialized contexts, such
jaroslav@1890:  * as deadlock recovery.
jaroslav@1890:  *
jaroslav@1890:  * <p> The constructor for this class optionally accepts a
jaroslav@1890:  * <em>fairness</em> parameter. When set false, this class makes no
jaroslav@1890:  * guarantees about the order in which threads acquire permits. In
jaroslav@1890:  * particular, <em>barging</em> is permitted, that is, a thread
jaroslav@1890:  * invoking {@link #acquire} can be allocated a permit ahead of a
jaroslav@1890:  * thread that has been waiting - logically the new thread places itself at
jaroslav@1890:  * the head of the queue of waiting threads. When fairness is set true, the
jaroslav@1890:  * semaphore guarantees that threads invoking any of the {@link
jaroslav@1890:  * #acquire() acquire} methods are selected to obtain permits in the order in
jaroslav@1890:  * which their invocation of those methods was processed
jaroslav@1890:  * (first-in-first-out; FIFO). Note that FIFO ordering necessarily
jaroslav@1890:  * applies to specific internal points of execution within these
jaroslav@1890:  * methods.  So, it is possible for one thread to invoke
jaroslav@1890:  * {@code acquire} before another, but reach the ordering point after
jaroslav@1890:  * the other, and similarly upon return from the method.
jaroslav@1890:  * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
jaroslav@1890:  * honor the fairness setting, but will take any permits that are
jaroslav@1890:  * available.
jaroslav@1890:  *
jaroslav@1890:  * <p>Generally, semaphores used to control resource access should be
jaroslav@1890:  * initialized as fair, to ensure that no thread is starved out from
jaroslav@1890:  * accessing a resource. When using semaphores for other kinds of
jaroslav@1890:  * synchronization control, the throughput advantages of non-fair
jaroslav@1890:  * ordering often outweigh fairness considerations.
jaroslav@1890:  *
jaroslav@1890:  * <p>This class also provides convenience methods to {@link
jaroslav@1890:  * #acquire(int) acquire} and {@link #release(int) release} multiple
jaroslav@1890:  * permits at a time.  Beware of the increased risk of indefinite
jaroslav@1890:  * postponement when these methods are used without fairness set true.
jaroslav@1890:  *
jaroslav@1890:  * <p>Memory consistency effects: Actions in a thread prior to calling
jaroslav@1890:  * a "release" method such as {@code release()}
jaroslav@1890:  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
jaroslav@1890:  * actions following a successful "acquire" method such as {@code acquire()}
jaroslav@1890:  * in another thread.
jaroslav@1890:  *
jaroslav@1890:  * @since 1.5
jaroslav@1890:  * @author Doug Lea
jaroslav@1890:  *
jaroslav@1890:  */
jaroslav@1890: 
jaroslav@1890: public class Semaphore implements java.io.Serializable {
jaroslav@1890:     private static final long serialVersionUID = -3222578661600680210L;
jaroslav@1890:     /** All mechanics via AbstractQueuedSynchronizer subclass */
jaroslav@1890:     private final Sync sync;
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Synchronization implementation for semaphore.  Uses AQS state
jaroslav@1890:      * to represent permits. Subclassed into fair and nonfair
jaroslav@1890:      * versions.
jaroslav@1890:      */
jaroslav@1890:     abstract static class Sync extends AbstractQueuedSynchronizer {
jaroslav@1890:         private static final long serialVersionUID = 1192457210091910933L;
jaroslav@1890: 
jaroslav@1890:         Sync(int permits) {
jaroslav@1890:             setState(permits);
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         final int getPermits() {
jaroslav@1890:             return getState();
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         final int nonfairTryAcquireShared(int acquires) {
jaroslav@1890:             for (;;) {
jaroslav@1890:                 int available = getState();
jaroslav@1890:                 int remaining = available - acquires;
jaroslav@1890:                 if (remaining < 0 ||
jaroslav@1890:                     compareAndSetState(available, remaining))
jaroslav@1890:                     return remaining;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         protected final boolean tryReleaseShared(int releases) {
jaroslav@1890:             for (;;) {
jaroslav@1890:                 int current = getState();
jaroslav@1890:                 int next = current + releases;
jaroslav@1890:                 if (next < current) // overflow
jaroslav@1890:                     throw new Error("Maximum permit count exceeded");
jaroslav@1890:                 if (compareAndSetState(current, next))
jaroslav@1890:                     return true;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         final void reducePermits(int reductions) {
jaroslav@1890:             for (;;) {
jaroslav@1890:                 int current = getState();
jaroslav@1890:                 int next = current - reductions;
jaroslav@1890:                 if (next > current) // underflow
jaroslav@1890:                     throw new Error("Permit count underflow");
jaroslav@1890:                 if (compareAndSetState(current, next))
jaroslav@1890:                     return;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         final int drainPermits() {
jaroslav@1890:             for (;;) {
jaroslav@1890:                 int current = getState();
jaroslav@1890:                 if (current == 0 || compareAndSetState(current, 0))
jaroslav@1890:                     return current;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * NonFair version
jaroslav@1890:      */
jaroslav@1890:     static final class NonfairSync extends Sync {
jaroslav@1890:         private static final long serialVersionUID = -2694183684443567898L;
jaroslav@1890: 
jaroslav@1890:         NonfairSync(int permits) {
jaroslav@1890:             super(permits);
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         protected int tryAcquireShared(int acquires) {
jaroslav@1890:             return nonfairTryAcquireShared(acquires);
jaroslav@1890:         }
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Fair version
jaroslav@1890:      */
jaroslav@1890:     static final class FairSync extends Sync {
jaroslav@1890:         private static final long serialVersionUID = 2014338818796000944L;
jaroslav@1890: 
jaroslav@1890:         FairSync(int permits) {
jaroslav@1890:             super(permits);
jaroslav@1890:         }
jaroslav@1890: 
jaroslav@1890:         protected int tryAcquireShared(int acquires) {
jaroslav@1890:             for (;;) {
jaroslav@1890:                 if (hasQueuedPredecessors())
jaroslav@1890:                     return -1;
jaroslav@1890:                 int available = getState();
jaroslav@1890:                 int remaining = available - acquires;
jaroslav@1890:                 if (remaining < 0 ||
jaroslav@1890:                     compareAndSetState(available, remaining))
jaroslav@1890:                     return remaining;
jaroslav@1890:             }
jaroslav@1890:         }
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Creates a {@code Semaphore} with the given number of
jaroslav@1890:      * permits and nonfair fairness setting.
jaroslav@1890:      *
jaroslav@1890:      * @param permits the initial number of permits available.
jaroslav@1890:      *        This value may be negative, in which case releases
jaroslav@1890:      *        must occur before any acquires will be granted.
jaroslav@1890:      */
jaroslav@1890:     public Semaphore(int permits) {
jaroslav@1890:         sync = new NonfairSync(permits);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Creates a {@code Semaphore} with the given number of
jaroslav@1890:      * permits and the given fairness setting.
jaroslav@1890:      *
jaroslav@1890:      * @param permits the initial number of permits available.
jaroslav@1890:      *        This value may be negative, in which case releases
jaroslav@1890:      *        must occur before any acquires will be granted.
jaroslav@1890:      * @param fair {@code true} if this semaphore will guarantee
jaroslav@1890:      *        first-in first-out granting of permits under contention,
jaroslav@1890:      *        else {@code false}
jaroslav@1890:      */
jaroslav@1890:     public Semaphore(int permits, boolean fair) {
jaroslav@1890:         sync = fair ? new FairSync(permits) : new NonfairSync(permits);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires a permit from this semaphore, blocking until one is
jaroslav@1890:      * available, or the thread is {@linkplain Thread#interrupt interrupted}.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires a permit, if one is available and returns immediately,
jaroslav@1890:      * reducing the number of available permits by one.
jaroslav@1890:      *
jaroslav@1890:      * <p>If no permit is available then the current thread becomes
jaroslav@1890:      * disabled for thread scheduling purposes and lies dormant until
jaroslav@1890:      * one of two things happens:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>Some other thread invokes the {@link #release} method for this
jaroslav@1890:      * semaphore and the current thread is next to be assigned a permit; or
jaroslav@1890:      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
jaroslav@1890:      * the current thread.
jaroslav@1890:      * </ul>
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>has its interrupted status set on entry to this method; or
jaroslav@1890:      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
jaroslav@1890:      * for a permit,
jaroslav@1890:      * </ul>
jaroslav@1890:      * then {@link InterruptedException} is thrown and the current thread's
jaroslav@1890:      * interrupted status is cleared.
jaroslav@1890:      *
jaroslav@1890:      * @throws InterruptedException if the current thread is interrupted
jaroslav@1890:      */
jaroslav@1890:     public void acquire() throws InterruptedException {
jaroslav@1890:         sync.acquireSharedInterruptibly(1);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires a permit from this semaphore, blocking until one is
jaroslav@1890:      * available.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires a permit, if one is available and returns immediately,
jaroslav@1890:      * reducing the number of available permits by one.
jaroslav@1890:      *
jaroslav@1890:      * <p>If no permit is available then the current thread becomes
jaroslav@1890:      * disabled for thread scheduling purposes and lies dormant until
jaroslav@1890:      * some other thread invokes the {@link #release} method for this
jaroslav@1890:      * semaphore and the current thread is next to be assigned a permit.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
jaroslav@1890:      * while waiting for a permit then it will continue to wait, but the
jaroslav@1890:      * time at which the thread is assigned a permit may change compared to
jaroslav@1890:      * the time it would have received the permit had no interruption
jaroslav@1890:      * occurred.  When the thread does return from this method its interrupt
jaroslav@1890:      * status will be set.
jaroslav@1890:      */
jaroslav@1890:     public void acquireUninterruptibly() {
jaroslav@1890:         sync.acquireShared(1);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires a permit from this semaphore, only if one is available at the
jaroslav@1890:      * time of invocation.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires a permit, if one is available and returns immediately,
jaroslav@1890:      * with the value {@code true},
jaroslav@1890:      * reducing the number of available permits by one.
jaroslav@1890:      *
jaroslav@1890:      * <p>If no permit is available then this method will return
jaroslav@1890:      * immediately with the value {@code false}.
jaroslav@1890:      *
jaroslav@1890:      * <p>Even when this semaphore has been set to use a
jaroslav@1890:      * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
jaroslav@1890:      * immediately acquire a permit if one is available, whether or not
jaroslav@1890:      * other threads are currently waiting.
jaroslav@1890:      * This &quot;barging&quot; behavior can be useful in certain
jaroslav@1890:      * circumstances, even though it breaks fairness. If you want to honor
jaroslav@1890:      * the fairness setting, then use
jaroslav@1890:      * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
jaroslav@1890:      * which is almost equivalent (it also detects interruption).
jaroslav@1890:      *
jaroslav@1890:      * @return {@code true} if a permit was acquired and {@code false}
jaroslav@1890:      *         otherwise
jaroslav@1890:      */
jaroslav@1890:     public boolean tryAcquire() {
jaroslav@1890:         return sync.nonfairTryAcquireShared(1) >= 0;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires a permit from this semaphore, if one becomes available
jaroslav@1890:      * within the given waiting time and the current thread has not
jaroslav@1890:      * been {@linkplain Thread#interrupt interrupted}.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires a permit, if one is available and returns immediately,
jaroslav@1890:      * with the value {@code true},
jaroslav@1890:      * reducing the number of available permits by one.
jaroslav@1890:      *
jaroslav@1890:      * <p>If no permit is available then the current thread becomes
jaroslav@1890:      * disabled for thread scheduling purposes and lies dormant until
jaroslav@1890:      * one of three things happens:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>Some other thread invokes the {@link #release} method for this
jaroslav@1890:      * semaphore and the current thread is next to be assigned a permit; or
jaroslav@1890:      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
jaroslav@1890:      * the current thread; or
jaroslav@1890:      * <li>The specified waiting time elapses.
jaroslav@1890:      * </ul>
jaroslav@1890:      *
jaroslav@1890:      * <p>If a permit is acquired then the value {@code true} is returned.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>has its interrupted status set on entry to this method; or
jaroslav@1890:      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
jaroslav@1890:      * to acquire a permit,
jaroslav@1890:      * </ul>
jaroslav@1890:      * then {@link InterruptedException} is thrown and the current thread's
jaroslav@1890:      * interrupted status is cleared.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the specified waiting time elapses then the value {@code false}
jaroslav@1890:      * is returned.  If the time is less than or equal to zero, the method
jaroslav@1890:      * will not wait at all.
jaroslav@1890:      *
jaroslav@1890:      * @param timeout the maximum time to wait for a permit
jaroslav@1890:      * @param unit the time unit of the {@code timeout} argument
jaroslav@1890:      * @return {@code true} if a permit was acquired and {@code false}
jaroslav@1890:      *         if the waiting time elapsed before a permit was acquired
jaroslav@1890:      * @throws InterruptedException if the current thread is interrupted
jaroslav@1890:      */
jaroslav@1890:     public boolean tryAcquire(long timeout, TimeUnit unit)
jaroslav@1890:         throws InterruptedException {
jaroslav@1890:         return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Releases a permit, returning it to the semaphore.
jaroslav@1890:      *
jaroslav@1890:      * <p>Releases a permit, increasing the number of available permits by
jaroslav@1890:      * one.  If any threads are trying to acquire a permit, then one is
jaroslav@1890:      * selected and given the permit that was just released.  That thread
jaroslav@1890:      * is (re)enabled for thread scheduling purposes.
jaroslav@1890:      *
jaroslav@1890:      * <p>There is no requirement that a thread that releases a permit must
jaroslav@1890:      * have acquired that permit by calling {@link #acquire}.
jaroslav@1890:      * Correct usage of a semaphore is established by programming convention
jaroslav@1890:      * in the application.
jaroslav@1890:      */
jaroslav@1890:     public void release() {
jaroslav@1890:         sync.releaseShared(1);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires the given number of permits from this semaphore,
jaroslav@1890:      * blocking until all are available,
jaroslav@1890:      * or the thread is {@linkplain Thread#interrupt interrupted}.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires the given number of permits, if they are available,
jaroslav@1890:      * and returns immediately, reducing the number of available permits
jaroslav@1890:      * by the given amount.
jaroslav@1890:      *
jaroslav@1890:      * <p>If insufficient permits are available then the current thread becomes
jaroslav@1890:      * disabled for thread scheduling purposes and lies dormant until
jaroslav@1890:      * one of two things happens:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>Some other thread invokes one of the {@link #release() release}
jaroslav@1890:      * methods for this semaphore, the current thread is next to be assigned
jaroslav@1890:      * permits and the number of available permits satisfies this request; or
jaroslav@1890:      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
jaroslav@1890:      * the current thread.
jaroslav@1890:      * </ul>
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>has its interrupted status set on entry to this method; or
jaroslav@1890:      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
jaroslav@1890:      * for a permit,
jaroslav@1890:      * </ul>
jaroslav@1890:      * then {@link InterruptedException} is thrown and the current thread's
jaroslav@1890:      * interrupted status is cleared.
jaroslav@1890:      * Any permits that were to be assigned to this thread are instead
jaroslav@1890:      * assigned to other threads trying to acquire permits, as if
jaroslav@1890:      * permits had been made available by a call to {@link #release()}.
jaroslav@1890:      *
jaroslav@1890:      * @param permits the number of permits to acquire
jaroslav@1890:      * @throws InterruptedException if the current thread is interrupted
jaroslav@1890:      * @throws IllegalArgumentException if {@code permits} is negative
jaroslav@1890:      */
jaroslav@1890:     public void acquire(int permits) throws InterruptedException {
jaroslav@1890:         if (permits < 0) throw new IllegalArgumentException();
jaroslav@1890:         sync.acquireSharedInterruptibly(permits);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires the given number of permits from this semaphore,
jaroslav@1890:      * blocking until all are available.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires the given number of permits, if they are available,
jaroslav@1890:      * and returns immediately, reducing the number of available permits
jaroslav@1890:      * by the given amount.
jaroslav@1890:      *
jaroslav@1890:      * <p>If insufficient permits are available then the current thread becomes
jaroslav@1890:      * disabled for thread scheduling purposes and lies dormant until
jaroslav@1890:      * some other thread invokes one of the {@link #release() release}
jaroslav@1890:      * methods for this semaphore, the current thread is next to be assigned
jaroslav@1890:      * permits and the number of available permits satisfies this request.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
jaroslav@1890:      * while waiting for permits then it will continue to wait and its
jaroslav@1890:      * position in the queue is not affected.  When the thread does return
jaroslav@1890:      * from this method its interrupt status will be set.
jaroslav@1890:      *
jaroslav@1890:      * @param permits the number of permits to acquire
jaroslav@1890:      * @throws IllegalArgumentException if {@code permits} is negative
jaroslav@1890:      *
jaroslav@1890:      */
jaroslav@1890:     public void acquireUninterruptibly(int permits) {
jaroslav@1890:         if (permits < 0) throw new IllegalArgumentException();
jaroslav@1890:         sync.acquireShared(permits);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires the given number of permits from this semaphore, only
jaroslav@1890:      * if all are available at the time of invocation.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires the given number of permits, if they are available, and
jaroslav@1890:      * returns immediately, with the value {@code true},
jaroslav@1890:      * reducing the number of available permits by the given amount.
jaroslav@1890:      *
jaroslav@1890:      * <p>If insufficient permits are available then this method will return
jaroslav@1890:      * immediately with the value {@code false} and the number of available
jaroslav@1890:      * permits is unchanged.
jaroslav@1890:      *
jaroslav@1890:      * <p>Even when this semaphore has been set to use a fair ordering
jaroslav@1890:      * policy, a call to {@code tryAcquire} <em>will</em>
jaroslav@1890:      * immediately acquire a permit if one is available, whether or
jaroslav@1890:      * not other threads are currently waiting.  This
jaroslav@1890:      * &quot;barging&quot; behavior can be useful in certain
jaroslav@1890:      * circumstances, even though it breaks fairness. If you want to
jaroslav@1890:      * honor the fairness setting, then use {@link #tryAcquire(int,
jaroslav@1890:      * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
jaroslav@1890:      * which is almost equivalent (it also detects interruption).
jaroslav@1890:      *
jaroslav@1890:      * @param permits the number of permits to acquire
jaroslav@1890:      * @return {@code true} if the permits were acquired and
jaroslav@1890:      *         {@code false} otherwise
jaroslav@1890:      * @throws IllegalArgumentException if {@code permits} is negative
jaroslav@1890:      */
jaroslav@1890:     public boolean tryAcquire(int permits) {
jaroslav@1890:         if (permits < 0) throw new IllegalArgumentException();
jaroslav@1890:         return sync.nonfairTryAcquireShared(permits) >= 0;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires the given number of permits from this semaphore, if all
jaroslav@1890:      * become available within the given waiting time and the current
jaroslav@1890:      * thread has not been {@linkplain Thread#interrupt interrupted}.
jaroslav@1890:      *
jaroslav@1890:      * <p>Acquires the given number of permits, if they are available and
jaroslav@1890:      * returns immediately, with the value {@code true},
jaroslav@1890:      * reducing the number of available permits by the given amount.
jaroslav@1890:      *
jaroslav@1890:      * <p>If insufficient permits are available then
jaroslav@1890:      * the current thread becomes disabled for thread scheduling
jaroslav@1890:      * purposes and lies dormant until one of three things happens:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>Some other thread invokes one of the {@link #release() release}
jaroslav@1890:      * methods for this semaphore, the current thread is next to be assigned
jaroslav@1890:      * permits and the number of available permits satisfies this request; or
jaroslav@1890:      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
jaroslav@1890:      * the current thread; or
jaroslav@1890:      * <li>The specified waiting time elapses.
jaroslav@1890:      * </ul>
jaroslav@1890:      *
jaroslav@1890:      * <p>If the permits are acquired then the value {@code true} is returned.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the current thread:
jaroslav@1890:      * <ul>
jaroslav@1890:      * <li>has its interrupted status set on entry to this method; or
jaroslav@1890:      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
jaroslav@1890:      * to acquire the permits,
jaroslav@1890:      * </ul>
jaroslav@1890:      * then {@link InterruptedException} is thrown and the current thread's
jaroslav@1890:      * interrupted status is cleared.
jaroslav@1890:      * Any permits that were to be assigned to this thread, are instead
jaroslav@1890:      * assigned to other threads trying to acquire permits, as if
jaroslav@1890:      * the permits had been made available by a call to {@link #release()}.
jaroslav@1890:      *
jaroslav@1890:      * <p>If the specified waiting time elapses then the value {@code false}
jaroslav@1890:      * is returned.  If the time is less than or equal to zero, the method
jaroslav@1890:      * will not wait at all.  Any permits that were to be assigned to this
jaroslav@1890:      * thread, are instead assigned to other threads trying to acquire
jaroslav@1890:      * permits, as if the permits had been made available by a call to
jaroslav@1890:      * {@link #release()}.
jaroslav@1890:      *
jaroslav@1890:      * @param permits the number of permits to acquire
jaroslav@1890:      * @param timeout the maximum time to wait for the permits
jaroslav@1890:      * @param unit the time unit of the {@code timeout} argument
jaroslav@1890:      * @return {@code true} if all permits were acquired and {@code false}
jaroslav@1890:      *         if the waiting time elapsed before all permits were acquired
jaroslav@1890:      * @throws InterruptedException if the current thread is interrupted
jaroslav@1890:      * @throws IllegalArgumentException if {@code permits} is negative
jaroslav@1890:      */
jaroslav@1890:     public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
jaroslav@1890:         throws InterruptedException {
jaroslav@1890:         if (permits < 0) throw new IllegalArgumentException();
jaroslav@1890:         return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Releases the given number of permits, returning them to the semaphore.
jaroslav@1890:      *
jaroslav@1890:      * <p>Releases the given number of permits, increasing the number of
jaroslav@1890:      * available permits by that amount.
jaroslav@1890:      * If any threads are trying to acquire permits, then one
jaroslav@1890:      * is selected and given the permits that were just released.
jaroslav@1890:      * If the number of available permits satisfies that thread's request
jaroslav@1890:      * then that thread is (re)enabled for thread scheduling purposes;
jaroslav@1890:      * otherwise the thread will wait until sufficient permits are available.
jaroslav@1890:      * If there are still permits available
jaroslav@1890:      * after this thread's request has been satisfied, then those permits
jaroslav@1890:      * are assigned in turn to other threads trying to acquire permits.
jaroslav@1890:      *
jaroslav@1890:      * <p>There is no requirement that a thread that releases a permit must
jaroslav@1890:      * have acquired that permit by calling {@link Semaphore#acquire acquire}.
jaroslav@1890:      * Correct usage of a semaphore is established by programming convention
jaroslav@1890:      * in the application.
jaroslav@1890:      *
jaroslav@1890:      * @param permits the number of permits to release
jaroslav@1890:      * @throws IllegalArgumentException if {@code permits} is negative
jaroslav@1890:      */
jaroslav@1890:     public void release(int permits) {
jaroslav@1890:         if (permits < 0) throw new IllegalArgumentException();
jaroslav@1890:         sync.releaseShared(permits);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns the current number of permits available in this semaphore.
jaroslav@1890:      *
jaroslav@1890:      * <p>This method is typically used for debugging and testing purposes.
jaroslav@1890:      *
jaroslav@1890:      * @return the number of permits available in this semaphore
jaroslav@1890:      */
jaroslav@1890:     public int availablePermits() {
jaroslav@1890:         return sync.getPermits();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Acquires and returns all permits that are immediately available.
jaroslav@1890:      *
jaroslav@1890:      * @return the number of permits acquired
jaroslav@1890:      */
jaroslav@1890:     public int drainPermits() {
jaroslav@1890:         return sync.drainPermits();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Shrinks the number of available permits by the indicated
jaroslav@1890:      * reduction. This method can be useful in subclasses that use
jaroslav@1890:      * semaphores to track resources that become unavailable. This
jaroslav@1890:      * method differs from {@code acquire} in that it does not block
jaroslav@1890:      * waiting for permits to become available.
jaroslav@1890:      *
jaroslav@1890:      * @param reduction the number of permits to remove
jaroslav@1890:      * @throws IllegalArgumentException if {@code reduction} is negative
jaroslav@1890:      */
jaroslav@1890:     protected void reducePermits(int reduction) {
jaroslav@1890:         if (reduction < 0) throw new IllegalArgumentException();
jaroslav@1890:         sync.reducePermits(reduction);
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns {@code true} if this semaphore has fairness set true.
jaroslav@1890:      *
jaroslav@1890:      * @return {@code true} if this semaphore has fairness set true
jaroslav@1890:      */
jaroslav@1890:     public boolean isFair() {
jaroslav@1890:         return sync instanceof FairSync;
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Queries whether any threads are waiting to acquire. Note that
jaroslav@1890:      * because cancellations may occur at any time, a {@code true}
jaroslav@1890:      * return does not guarantee that any other thread will ever
jaroslav@1890:      * acquire.  This method is designed primarily for use in
jaroslav@1890:      * monitoring of the system state.
jaroslav@1890:      *
jaroslav@1890:      * @return {@code true} if there may be other threads waiting to
jaroslav@1890:      *         acquire the lock
jaroslav@1890:      */
jaroslav@1890:     public final boolean hasQueuedThreads() {
jaroslav@1890:         return sync.hasQueuedThreads();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns an estimate of the number of threads waiting to acquire.
jaroslav@1890:      * The value is only an estimate because the number of threads may
jaroslav@1890:      * change dynamically while this method traverses internal data
jaroslav@1890:      * structures.  This method is designed for use in monitoring of the
jaroslav@1890:      * system state, not for synchronization control.
jaroslav@1890:      *
jaroslav@1890:      * @return the estimated number of threads waiting for this lock
jaroslav@1890:      */
jaroslav@1890:     public final int getQueueLength() {
jaroslav@1890:         return sync.getQueueLength();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns a collection containing threads that may be waiting to acquire.
jaroslav@1890:      * Because the actual set of threads may change dynamically while
jaroslav@1890:      * constructing this result, the returned collection is only a best-effort
jaroslav@1890:      * estimate.  The elements of the returned collection are in no particular
jaroslav@1890:      * order.  This method is designed to facilitate construction of
jaroslav@1890:      * subclasses that provide more extensive monitoring facilities.
jaroslav@1890:      *
jaroslav@1890:      * @return the collection of threads
jaroslav@1890:      */
jaroslav@1890:     protected Collection<Thread> getQueuedThreads() {
jaroslav@1890:         return sync.getQueuedThreads();
jaroslav@1890:     }
jaroslav@1890: 
jaroslav@1890:     /**
jaroslav@1890:      * Returns a string identifying this semaphore, as well as its state.
jaroslav@1890:      * The state, in brackets, includes the String {@code "Permits ="}
jaroslav@1890:      * followed by the number of permits.
jaroslav@1890:      *
jaroslav@1890:      * @return a string identifying this semaphore, as well as its state
jaroslav@1890:      */
jaroslav@1890:     public String toString() {
jaroslav@1890:         return super.toString() + "[Permits = " + sync.getPermits() + "]";
jaroslav@1890:     }
jaroslav@1890: }