java/11 : java.base/java/util/concurrent/Semaphore.java

Semaphore
https://openjdk.java.net/
GPLv2 + Classpath Exception
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.Collection;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each acquire blocks if necessary until a permit is available, and then takes it. Each release adds a permit, potentially releasing a blocking acquirer. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly. 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:
 
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;
  }
 }
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 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. 
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 binary
semaphore, 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 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. 
The constructor for this class optionally accepts a
fairness parameter. When set false, this class makes no
guarantees about the order in which threads acquire permits. In
particular, barging is permitted, that is, a thread invoking 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 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 acquire before another, but reach the ordering point after the other, and similarly upon return from the method. Also note that the untimed tryAcquire methods do not honor the fairness setting, but will take any permits that are available. 
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.
This class also provides convenience methods to acquire and release multiple permits at a time. These methods are generally more efficient and effective than loops. However, they do not establish any preference order. For example, if thread A invokes s.acquire(3) and thread B invokes s.acquire(2), and two permits become available, then there is no guarantee that thread B will obtain them unless its acquire came first and Semaphore s is in fair mode. 
Memory consistency effects: Actions in a thread prior to calling a "release" method such as release() happen-before actions following a successful "acquire" method such as acquire() in another thread. 
Author: Doug Lea
Since: 1.5/**
 * 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> {@code
 * 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 java.util.concurrent.locks.Lock}
 * implementations), that the &quot;lock&quot; 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. These methods are generally more efficient and
 * effective than loops. However, they do not establish any preference
 * order. For example, if thread A invokes {@code s.acquire(3}) and
 * thread B invokes {@code s.acquire(2)}, and two permits become
 * available, then there is no guarantee that thread B will obtain
 * them unless its acquire came first and Semaphore {@code s} is in
 * fair mode.
 *
 * <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 /** 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.
/**
     * 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 current = getState();
                int next = current + releases;
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
                if (compareAndSetState(current, next))
                    return true;
            }
        }

        final void reducePermits(int reductions) {
            for (;;) {
                int current = getState();
                int next = current - reductions;
                if (next > current) // underflow
                    throw new Error("Permit count underflow");
                if (compareAndSetState(current, next))
                    return;
            }
        }

        final int drainPermits() {
            for (;;) {
                int current = getState();
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }
    }

    NonFair version
/**
     * NonFair version
     */
    static final 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
/**
     * Fair version
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = 2014338818796000944L;

        FairSync(int permits) {
            super(permits);
        }

        protected int tryAcquireShared(int acquires) {
            for (;;) {
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
    }

    Creates a Semaphore with the given number of permits and nonfair fairness setting. 
Params: permits – the initial number of permits available.
       This value may be negative, in which case releases
       must occur before any acquires will be granted./**
     * 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 Semaphore with the given number of permits and the given fairness setting. 
Params: permits – the initial number of permits available.
       This value may be negative, in which case releases
       must occur before any acquires will be granted.
fair – true if this semaphore will guarantee first-in first-out granting of permits under contention, else false/**
     * 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 interrupted. Acquires a permit, if one is available and returns immediately,
reducing the number of available permits by one.
If no permit is available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
one of two things happens:

Some other thread invokes the release method for this semaphore and the current thread is next to be assigned a permit; or 
Some other thread interrupts the current thread. 
If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while waiting for a permit, 
 then InterruptedException is thrown and the current thread's interrupted status is cleared. 
Throws: InterruptedException – if the current thread is interrupted/**
     * 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.
Acquires a permit, if one is available and returns immediately,
reducing the number of available permits by one.
If no permit is available then the current thread becomes disabled for thread scheduling purposes and lies dormant until some other thread invokes the release method for this semaphore and the current thread is next to be assigned a permit. 
If the current thread is 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. /**
     * 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.
Acquires a permit, if one is available and returns immediately, with the value true, reducing the number of available permits by one. 
If no permit is available then this method will return immediately with the value false. 
Even when this semaphore has been set to use a fair ordering policy, a call to tryAcquire() will 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 tryAcquire(0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption). 
Returns: true if a permit was acquired and false otherwise/**
     * 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 &quot;barging&quot; 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 interrupted. Acquires a permit, if one is available and returns immediately, with the value true, reducing the number of available permits by one. 
If no permit is available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
one of three things happens:

Some other thread invokes the release method for this semaphore and the current thread is next to be assigned a permit; or 
Some other thread interrupts the current thread; or 
The specified waiting time elapses.

If a permit is acquired then the value true is returned. 
If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while waiting to acquire a permit, 
 then InterruptedException is thrown and the current thread's interrupted status is cleared. If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all. 
Params: timeout – the maximum time to wait for a permit
unit – the time unit of the timeout argument
Throws: InterruptedException – if the current thread is interrupted
Returns: true if a permit was acquired and false if the waiting time elapsed before a permit was acquired/**
     * 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.
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.
There is no requirement that a thread that releases a permit must have acquired that permit by calling acquire. Correct usage of a semaphore is established by programming convention in the application. /**
     * 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 interrupted. Acquires the given number of permits, if they are available, and returns immediately, reducing the number of available permits by the given amount. This method has the same effect as the loop for (int i = 0; i < permits; ++i) acquire(); except that it atomically acquires the permits all at once: 
If insufficient permits are available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
one of two things happens:

Some other thread invokes one of the release methods for this semaphore and the current thread is next to be assigned permits and the number of available permits satisfies this request; or 
Some other thread interrupts the current thread. 
If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while waiting for a permit, 
 then 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 release(). 
Params: permits – the number of permits to acquire
Throws: InterruptedException – if the current thread is interrupted
IllegalArgumentException – if permits is negative/**
     * 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. This method has the same effect as the
     * loop {@code for (int i = 0; i < permits; ++i) acquire();} except
     * that it atomically acquires the permits all at once:
     *
     * <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 and 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.
Acquires the given number of permits, if they are available, and returns immediately, reducing the number of available permits by the given amount. This method has the same effect as the loop for (int i = 0; i < permits; ++i) acquireUninterruptibly(); except that it atomically acquires the permits all at once: 
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 release methods for this semaphore and the current thread is next to be assigned permits and the number of available permits satisfies this request. 
If the current thread is 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. 
Params: permits – the number of permits to acquire
Throws: IllegalArgumentException – if permits is negative/**
     * 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. This method has the same effect as the
     * loop {@code for (int i = 0; i < permits; ++i) acquireUninterruptibly();}
     * except that it atomically acquires the permits all at once:
     *
     * <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 and 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.
Acquires the given number of permits, if they are available, and returns immediately, with the value true, reducing the number of available permits by the given amount. 
If insufficient permits are available then this method will return immediately with the value false and the number of available permits is unchanged. 
Even when this semaphore has been set to use a fair ordering policy, a call to tryAcquire will 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 tryAcquire(permits, 0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption). 
Params: permits – the number of permits to acquire
Throws: IllegalArgumentException – if permits is negative
Returns: true if the permits were acquired and false otherwise/**
     * 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
     * &quot;barging&quot; 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 interrupted. Acquires the given number of permits, if they are available and returns immediately, with the value true, reducing the number of available permits by the given amount. 
If insufficient permits are available then
the current thread becomes disabled for thread scheduling
purposes and lies dormant until one of three things happens:

Some other thread invokes one of the release methods for this semaphore and the current thread is next to be assigned permits and the number of available permits satisfies this request; or 
Some other thread interrupts the current thread; or 
The specified waiting time elapses.

If the permits are acquired then the value true is returned. 
If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while waiting to acquire the permits, 
 then 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 release(). If the specified waiting time elapses then the value 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 release(). 
Params: permits – the number of permits to acquire
timeout – the maximum time to wait for the permits
unit – the time unit of the timeout argument
Throws: InterruptedException – if the current thread is interrupted
IllegalArgumentException – if permits is negative
Returns: true if all permits were acquired and false if the waiting time elapsed before all permits were acquired/**
     * 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 and 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.
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 thread
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.
There is no requirement that a thread that releases a permit must have acquired that permit by calling acquire. Correct usage of a semaphore is established by programming convention in the application. 
Params: permits – the number of permits to release
Throws: IllegalArgumentException – if permits is negative/**
     * 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 thread
     * 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.
This method is typically used for debugging and testing purposes.
Returns: the number of permits available in this semaphore/**
     * 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, or if negative permits are available, releases them.
Upon return, zero permits are available.
Returns: the number of permits acquired or, if negative, the
number released/**
     * Acquires and returns all permits that are immediately
     * available, or if negative permits are available, releases them.
     * Upon return, zero permits are available.
     *
     * @return the number of permits acquired or, if negative, the
     * number released
     */
    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 acquire in that it does not block waiting for permits to become available. 
Params: reduction – the number of permits to remove
Throws: IllegalArgumentException – if reduction is negative/**
     * 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 true if this semaphore has fairness set true. 
Returns: true if this semaphore has fairness set true/**
     * 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 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. 
Returns: true if there may be other threads waiting to acquire the lock/**
     * 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
system state, not for synchronization control.
Returns: the estimated number of threads waiting for this lock/**
     * 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
     * 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.
Returns: the collection of threads/**
     * 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 "Permits =" followed by the number of permits. 
Returns: a string identifying this semaphore, as well as its state/**
     * 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() + "]";
    }
}
Params:	timeout – the maximum time to wait for a permit unit – the time unit of the `timeout` argument
Throws:	InterruptedException – if the current thread is interrupted
Returns:	`true` if a permit was acquired and `false` if the waiting time elapsed before a permit was acquired
Params:	permits – the number of permits to acquire
Throws:	InterruptedException – if the current thread is interrupted IllegalArgumentException – if `permits` is negative
Params:	permits – the number of permits to release
Throws:	IllegalArgumentException – if `permits` is negative
Params:	reduction – the number of permits to remove
Throws:	IllegalArgumentException – if `reduction` is negative
/

java/ 11/ java.base/java/util/concurrent/Semaphore.java
