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/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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* This file is available under and governed by the GNU General Public
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* License version 2 only, as published by the Free Software Foundation.
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* However, the following notice accompanied the original version of this
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* file:
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*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/publicdomain/zero/1.0/
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*/
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package java.util.concurrent;
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import java.util.*;
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import java.util.concurrent.locks.*;
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import java.util.concurrent.atomic.*;
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/**
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* A counting semaphore. Conceptually, a semaphore maintains a set of
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* permits. Each {@link #acquire} blocks if necessary until a permit is
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* available, and then takes it. Each {@link #release} adds a permit,
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* potentially releasing a blocking acquirer.
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* However, no actual permit objects are used; the {@code Semaphore} just
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* keeps a count of the number available and acts accordingly.
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*
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* <p>Semaphores are often used to restrict the number of threads than can
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* access some (physical or logical) resource. For example, here is
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* a class that uses a semaphore to control access to a pool of items:
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* <pre>
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* class Pool {
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* private static final int MAX_AVAILABLE = 100;
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* private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
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*
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* public Object getItem() throws InterruptedException {
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* available.acquire();
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* return getNextAvailableItem();
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* }
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*
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* public void putItem(Object x) {
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* if (markAsUnused(x))
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* available.release();
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* }
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*
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* // Not a particularly efficient data structure; just for demo
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*
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* protected Object[] items = ... whatever kinds of items being managed
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* protected boolean[] used = new boolean[MAX_AVAILABLE];
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*
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* protected synchronized Object getNextAvailableItem() {
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* for (int i = 0; i < MAX_AVAILABLE; ++i) {
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* if (!used[i]) {
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* used[i] = true;
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* return items[i];
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* }
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* }
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* return null; // not reached
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* }
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*
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* protected synchronized boolean markAsUnused(Object item) {
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* for (int i = 0; i < MAX_AVAILABLE; ++i) {
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* if (item == items[i]) {
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* if (used[i]) {
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* used[i] = false;
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* return true;
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* } else
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* return false;
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* }
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* }
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* return false;
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* }
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*
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* }
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* </pre>
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*
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* <p>Before obtaining an item each thread must acquire a permit from
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* the semaphore, guaranteeing that an item is available for use. When
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* the thread has finished with the item it is returned back to the
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* pool and a permit is returned to the semaphore, allowing another
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* thread to acquire that item. Note that no synchronization lock is
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* held when {@link #acquire} is called as that would prevent an item
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* from being returned to the pool. The semaphore encapsulates the
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* synchronization needed to restrict access to the pool, separately
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* from any synchronization needed to maintain the consistency of the
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* pool itself.
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*
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* <p>A semaphore initialized to one, and which is used such that it
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* only has at most one permit available, can serve as a mutual
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* exclusion lock. This is more commonly known as a <em>binary
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* semaphore</em>, because it only has two states: one permit
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* available, or zero permits available. When used in this way, the
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* binary semaphore has the property (unlike many {@link Lock}
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* implementations), that the "lock" can be released by a
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* thread other than the owner (as semaphores have no notion of
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* ownership). This can be useful in some specialized contexts, such
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* as deadlock recovery.
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*
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* <p> The constructor for this class optionally accepts a
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* <em>fairness</em> parameter. When set false, this class makes no
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* guarantees about the order in which threads acquire permits. In
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* particular, <em>barging</em> is permitted, that is, a thread
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* invoking {@link #acquire} can be allocated a permit ahead of a
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* thread that has been waiting - logically the new thread places itself at
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* the head of the queue of waiting threads. When fairness is set true, the
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* semaphore guarantees that threads invoking any of the {@link
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* #acquire() acquire} methods are selected to obtain permits in the order in
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* which their invocation of those methods was processed
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* (first-in-first-out; FIFO). Note that FIFO ordering necessarily
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* applies to specific internal points of execution within these
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* methods. So, it is possible for one thread to invoke
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* {@code acquire} before another, but reach the ordering point after
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* the other, and similarly upon return from the method.
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* Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
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* honor the fairness setting, but will take any permits that are
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* available.
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*
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* <p>Generally, semaphores used to control resource access should be
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* initialized as fair, to ensure that no thread is starved out from
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* accessing a resource. When using semaphores for other kinds of
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* synchronization control, the throughput advantages of non-fair
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* ordering often outweigh fairness considerations.
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*
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* <p>This class also provides convenience methods to {@link
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* #acquire(int) acquire} and {@link #release(int) release} multiple
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* permits at a time. Beware of the increased risk of indefinite
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* postponement when these methods are used without fairness set true.
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*
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* <p>Memory consistency effects: Actions in a thread prior to calling
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* a "release" method such as {@code release()}
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
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* actions following a successful "acquire" method such as {@code acquire()}
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* in another thread.
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*
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* @since 1.5
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* @author Doug Lea
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*
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*/
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public class Semaphore implements java.io.Serializable {
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private static final long serialVersionUID = -3222578661600680210L;
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/** All mechanics via AbstractQueuedSynchronizer subclass */
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private final Sync sync;
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/**
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* Synchronization implementation for semaphore. Uses AQS state
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* to represent permits. Subclassed into fair and nonfair
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* versions.
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*/
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abstract static class Sync extends AbstractQueuedSynchronizer {
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private static final long serialVersionUID = 1192457210091910933L;
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Sync(int permits) {
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setState(permits);
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}
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final int getPermits() {
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return getState();
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}
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final int nonfairTryAcquireShared(int acquires) {
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for (;;) {
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int available = getState();
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int remaining = available - acquires;
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if (remaining < 0 ||
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compareAndSetState(available, remaining))
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return remaining;
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}
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}
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protected final boolean tryReleaseShared(int releases) {
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for (;;) {
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int current = getState();
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int next = current + releases;
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if (next < current) // overflow
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throw new Error("Maximum permit count exceeded");
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if (compareAndSetState(current, next))
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return true;
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}
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}
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final void reducePermits(int reductions) {
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for (;;) {
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int current = getState();
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int next = current - reductions;
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if (next > current) // underflow
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throw new Error("Permit count underflow");
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if (compareAndSetState(current, next))
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return;
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}
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}
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final int drainPermits() {
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for (;;) {
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int current = getState();
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if (current == 0 || compareAndSetState(current, 0))
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return current;
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}
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}
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}
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/**
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* NonFair version
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*/
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static final class NonfairSync extends Sync {
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private static final long serialVersionUID = -2694183684443567898L;
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NonfairSync(int permits) {
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super(permits);
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}
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protected int tryAcquireShared(int acquires) {
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return nonfairTryAcquireShared(acquires);
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}
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}
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/**
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* Fair version
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*/
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static final class FairSync extends Sync {
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private static final long serialVersionUID = 2014338818796000944L;
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FairSync(int permits) {
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super(permits);
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}
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protected int tryAcquireShared(int acquires) {
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for (;;) {
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if (hasQueuedPredecessors())
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return -1;
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int available = getState();
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int remaining = available - acquires;
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if (remaining < 0 ||
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compareAndSetState(available, remaining))
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return remaining;
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}
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}
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}
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/**
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* Creates a {@code Semaphore} with the given number of
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* permits and nonfair fairness setting.
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*
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* @param permits the initial number of permits available.
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* This value may be negative, in which case releases
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* must occur before any acquires will be granted.
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*/
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public Semaphore(int permits) {
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sync = new NonfairSync(permits);
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}
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/**
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* Creates a {@code Semaphore} with the given number of
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* permits and the given fairness setting.
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*
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* @param permits the initial number of permits available.
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* This value may be negative, in which case releases
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* must occur before any acquires will be granted.
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* @param fair {@code true} if this semaphore will guarantee
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* first-in first-out granting of permits under contention,
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* else {@code false}
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*/
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public Semaphore(int permits, boolean fair) {
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sync = fair ? new FairSync(permits) : new NonfairSync(permits);
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}
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/**
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* Acquires a permit from this semaphore, blocking until one is
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* available, or the thread is {@linkplain Thread#interrupt interrupted}.
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*
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* <p>Acquires a permit, if one is available and returns immediately,
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* reducing the number of available permits by one.
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*
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* <p>If no permit is available then the current thread becomes
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* disabled for thread scheduling purposes and lies dormant until
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* one of two things happens:
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* <ul>
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* <li>Some other thread invokes the {@link #release} method for this
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* semaphore and the current thread is next to be assigned a permit; or
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread.
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* </ul>
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*
|
jaroslav@1890
|
305 |
* <p>If the current thread:
|
jaroslav@1890
|
306 |
* <ul>
|
jaroslav@1890
|
307 |
* <li>has its interrupted status set on entry to this method; or
|
jaroslav@1890
|
308 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
jaroslav@1890
|
309 |
* for a permit,
|
jaroslav@1890
|
310 |
* </ul>
|
jaroslav@1890
|
311 |
* then {@link InterruptedException} is thrown and the current thread's
|
jaroslav@1890
|
312 |
* interrupted status is cleared.
|
jaroslav@1890
|
313 |
*
|
jaroslav@1890
|
314 |
* @throws InterruptedException if the current thread is interrupted
|
jaroslav@1890
|
315 |
*/
|
jaroslav@1890
|
316 |
public void acquire() throws InterruptedException {
|
jaroslav@1890
|
317 |
sync.acquireSharedInterruptibly(1);
|
jaroslav@1890
|
318 |
}
|
jaroslav@1890
|
319 |
|
jaroslav@1890
|
320 |
/**
|
jaroslav@1890
|
321 |
* Acquires a permit from this semaphore, blocking until one is
|
jaroslav@1890
|
322 |
* available.
|
jaroslav@1890
|
323 |
*
|
jaroslav@1890
|
324 |
* <p>Acquires a permit, if one is available and returns immediately,
|
jaroslav@1890
|
325 |
* reducing the number of available permits by one.
|
jaroslav@1890
|
326 |
*
|
jaroslav@1890
|
327 |
* <p>If no permit is available then the current thread becomes
|
jaroslav@1890
|
328 |
* disabled for thread scheduling purposes and lies dormant until
|
jaroslav@1890
|
329 |
* some other thread invokes the {@link #release} method for this
|
jaroslav@1890
|
330 |
* semaphore and the current thread is next to be assigned a permit.
|
jaroslav@1890
|
331 |
*
|
jaroslav@1890
|
332 |
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
|
jaroslav@1890
|
333 |
* while waiting for a permit then it will continue to wait, but the
|
jaroslav@1890
|
334 |
* time at which the thread is assigned a permit may change compared to
|
jaroslav@1890
|
335 |
* the time it would have received the permit had no interruption
|
jaroslav@1890
|
336 |
* occurred. When the thread does return from this method its interrupt
|
jaroslav@1890
|
337 |
* status will be set.
|
jaroslav@1890
|
338 |
*/
|
jaroslav@1890
|
339 |
public void acquireUninterruptibly() {
|
jaroslav@1890
|
340 |
sync.acquireShared(1);
|
jaroslav@1890
|
341 |
}
|
jaroslav@1890
|
342 |
|
jaroslav@1890
|
343 |
/**
|
jaroslav@1890
|
344 |
* Acquires a permit from this semaphore, only if one is available at the
|
jaroslav@1890
|
345 |
* time of invocation.
|
jaroslav@1890
|
346 |
*
|
jaroslav@1890
|
347 |
* <p>Acquires a permit, if one is available and returns immediately,
|
jaroslav@1890
|
348 |
* with the value {@code true},
|
jaroslav@1890
|
349 |
* reducing the number of available permits by one.
|
jaroslav@1890
|
350 |
*
|
jaroslav@1890
|
351 |
* <p>If no permit is available then this method will return
|
jaroslav@1890
|
352 |
* immediately with the value {@code false}.
|
jaroslav@1890
|
353 |
*
|
jaroslav@1890
|
354 |
* <p>Even when this semaphore has been set to use a
|
jaroslav@1890
|
355 |
* fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
|
jaroslav@1890
|
356 |
* immediately acquire a permit if one is available, whether or not
|
jaroslav@1890
|
357 |
* other threads are currently waiting.
|
jaroslav@1890
|
358 |
* This "barging" behavior can be useful in certain
|
jaroslav@1890
|
359 |
* circumstances, even though it breaks fairness. If you want to honor
|
jaroslav@1890
|
360 |
* the fairness setting, then use
|
jaroslav@1890
|
361 |
* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
|
jaroslav@1890
|
362 |
* which is almost equivalent (it also detects interruption).
|
jaroslav@1890
|
363 |
*
|
jaroslav@1890
|
364 |
* @return {@code true} if a permit was acquired and {@code false}
|
jaroslav@1890
|
365 |
* otherwise
|
jaroslav@1890
|
366 |
*/
|
jaroslav@1890
|
367 |
public boolean tryAcquire() {
|
jaroslav@1890
|
368 |
return sync.nonfairTryAcquireShared(1) >= 0;
|
jaroslav@1890
|
369 |
}
|
jaroslav@1890
|
370 |
|
jaroslav@1890
|
371 |
/**
|
jaroslav@1890
|
372 |
* Acquires a permit from this semaphore, if one becomes available
|
jaroslav@1890
|
373 |
* within the given waiting time and the current thread has not
|
jaroslav@1890
|
374 |
* been {@linkplain Thread#interrupt interrupted}.
|
jaroslav@1890
|
375 |
*
|
jaroslav@1890
|
376 |
* <p>Acquires a permit, if one is available and returns immediately,
|
jaroslav@1890
|
377 |
* with the value {@code true},
|
jaroslav@1890
|
378 |
* reducing the number of available permits by one.
|
jaroslav@1890
|
379 |
*
|
jaroslav@1890
|
380 |
* <p>If no permit is available then the current thread becomes
|
jaroslav@1890
|
381 |
* disabled for thread scheduling purposes and lies dormant until
|
jaroslav@1890
|
382 |
* one of three things happens:
|
jaroslav@1890
|
383 |
* <ul>
|
jaroslav@1890
|
384 |
* <li>Some other thread invokes the {@link #release} method for this
|
jaroslav@1890
|
385 |
* semaphore and the current thread is next to be assigned a permit; or
|
jaroslav@1890
|
386 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
|
jaroslav@1890
|
387 |
* the current thread; or
|
jaroslav@1890
|
388 |
* <li>The specified waiting time elapses.
|
jaroslav@1890
|
389 |
* </ul>
|
jaroslav@1890
|
390 |
*
|
jaroslav@1890
|
391 |
* <p>If a permit is acquired then the value {@code true} is returned.
|
jaroslav@1890
|
392 |
*
|
jaroslav@1890
|
393 |
* <p>If the current thread:
|
jaroslav@1890
|
394 |
* <ul>
|
jaroslav@1890
|
395 |
* <li>has its interrupted status set on entry to this method; or
|
jaroslav@1890
|
396 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
jaroslav@1890
|
397 |
* to acquire a permit,
|
jaroslav@1890
|
398 |
* </ul>
|
jaroslav@1890
|
399 |
* then {@link InterruptedException} is thrown and the current thread's
|
jaroslav@1890
|
400 |
* interrupted status is cleared.
|
jaroslav@1890
|
401 |
*
|
jaroslav@1890
|
402 |
* <p>If the specified waiting time elapses then the value {@code false}
|
jaroslav@1890
|
403 |
* is returned. If the time is less than or equal to zero, the method
|
jaroslav@1890
|
404 |
* will not wait at all.
|
jaroslav@1890
|
405 |
*
|
jaroslav@1890
|
406 |
* @param timeout the maximum time to wait for a permit
|
jaroslav@1890
|
407 |
* @param unit the time unit of the {@code timeout} argument
|
jaroslav@1890
|
408 |
* @return {@code true} if a permit was acquired and {@code false}
|
jaroslav@1890
|
409 |
* if the waiting time elapsed before a permit was acquired
|
jaroslav@1890
|
410 |
* @throws InterruptedException if the current thread is interrupted
|
jaroslav@1890
|
411 |
*/
|
jaroslav@1890
|
412 |
public boolean tryAcquire(long timeout, TimeUnit unit)
|
jaroslav@1890
|
413 |
throws InterruptedException {
|
jaroslav@1890
|
414 |
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
|
jaroslav@1890
|
415 |
}
|
jaroslav@1890
|
416 |
|
jaroslav@1890
|
417 |
/**
|
jaroslav@1890
|
418 |
* Releases a permit, returning it to the semaphore.
|
jaroslav@1890
|
419 |
*
|
jaroslav@1890
|
420 |
* <p>Releases a permit, increasing the number of available permits by
|
jaroslav@1890
|
421 |
* one. If any threads are trying to acquire a permit, then one is
|
jaroslav@1890
|
422 |
* selected and given the permit that was just released. That thread
|
jaroslav@1890
|
423 |
* is (re)enabled for thread scheduling purposes.
|
jaroslav@1890
|
424 |
*
|
jaroslav@1890
|
425 |
* <p>There is no requirement that a thread that releases a permit must
|
jaroslav@1890
|
426 |
* have acquired that permit by calling {@link #acquire}.
|
jaroslav@1890
|
427 |
* Correct usage of a semaphore is established by programming convention
|
jaroslav@1890
|
428 |
* in the application.
|
jaroslav@1890
|
429 |
*/
|
jaroslav@1890
|
430 |
public void release() {
|
jaroslav@1890
|
431 |
sync.releaseShared(1);
|
jaroslav@1890
|
432 |
}
|
jaroslav@1890
|
433 |
|
jaroslav@1890
|
434 |
/**
|
jaroslav@1890
|
435 |
* Acquires the given number of permits from this semaphore,
|
jaroslav@1890
|
436 |
* blocking until all are available,
|
jaroslav@1890
|
437 |
* or the thread is {@linkplain Thread#interrupt interrupted}.
|
jaroslav@1890
|
438 |
*
|
jaroslav@1890
|
439 |
* <p>Acquires the given number of permits, if they are available,
|
jaroslav@1890
|
440 |
* and returns immediately, reducing the number of available permits
|
jaroslav@1890
|
441 |
* by the given amount.
|
jaroslav@1890
|
442 |
*
|
jaroslav@1890
|
443 |
* <p>If insufficient permits are available then the current thread becomes
|
jaroslav@1890
|
444 |
* disabled for thread scheduling purposes and lies dormant until
|
jaroslav@1890
|
445 |
* one of two things happens:
|
jaroslav@1890
|
446 |
* <ul>
|
jaroslav@1890
|
447 |
* <li>Some other thread invokes one of the {@link #release() release}
|
jaroslav@1890
|
448 |
* methods for this semaphore, the current thread is next to be assigned
|
jaroslav@1890
|
449 |
* permits and the number of available permits satisfies this request; or
|
jaroslav@1890
|
450 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
|
jaroslav@1890
|
451 |
* the current thread.
|
jaroslav@1890
|
452 |
* </ul>
|
jaroslav@1890
|
453 |
*
|
jaroslav@1890
|
454 |
* <p>If the current thread:
|
jaroslav@1890
|
455 |
* <ul>
|
jaroslav@1890
|
456 |
* <li>has its interrupted status set on entry to this method; or
|
jaroslav@1890
|
457 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
jaroslav@1890
|
458 |
* for a permit,
|
jaroslav@1890
|
459 |
* </ul>
|
jaroslav@1890
|
460 |
* then {@link InterruptedException} is thrown and the current thread's
|
jaroslav@1890
|
461 |
* interrupted status is cleared.
|
jaroslav@1890
|
462 |
* Any permits that were to be assigned to this thread are instead
|
jaroslav@1890
|
463 |
* assigned to other threads trying to acquire permits, as if
|
jaroslav@1890
|
464 |
* permits had been made available by a call to {@link #release()}.
|
jaroslav@1890
|
465 |
*
|
jaroslav@1890
|
466 |
* @param permits the number of permits to acquire
|
jaroslav@1890
|
467 |
* @throws InterruptedException if the current thread is interrupted
|
jaroslav@1890
|
468 |
* @throws IllegalArgumentException if {@code permits} is negative
|
jaroslav@1890
|
469 |
*/
|
jaroslav@1890
|
470 |
public void acquire(int permits) throws InterruptedException {
|
jaroslav@1890
|
471 |
if (permits < 0) throw new IllegalArgumentException();
|
jaroslav@1890
|
472 |
sync.acquireSharedInterruptibly(permits);
|
jaroslav@1890
|
473 |
}
|
jaroslav@1890
|
474 |
|
jaroslav@1890
|
475 |
/**
|
jaroslav@1890
|
476 |
* Acquires the given number of permits from this semaphore,
|
jaroslav@1890
|
477 |
* blocking until all are available.
|
jaroslav@1890
|
478 |
*
|
jaroslav@1890
|
479 |
* <p>Acquires the given number of permits, if they are available,
|
jaroslav@1890
|
480 |
* and returns immediately, reducing the number of available permits
|
jaroslav@1890
|
481 |
* by the given amount.
|
jaroslav@1890
|
482 |
*
|
jaroslav@1890
|
483 |
* <p>If insufficient permits are available then the current thread becomes
|
jaroslav@1890
|
484 |
* disabled for thread scheduling purposes and lies dormant until
|
jaroslav@1890
|
485 |
* some other thread invokes one of the {@link #release() release}
|
jaroslav@1890
|
486 |
* methods for this semaphore, the current thread is next to be assigned
|
jaroslav@1890
|
487 |
* permits and the number of available permits satisfies this request.
|
jaroslav@1890
|
488 |
*
|
jaroslav@1890
|
489 |
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
|
jaroslav@1890
|
490 |
* while waiting for permits then it will continue to wait and its
|
jaroslav@1890
|
491 |
* position in the queue is not affected. When the thread does return
|
jaroslav@1890
|
492 |
* from this method its interrupt status will be set.
|
jaroslav@1890
|
493 |
*
|
jaroslav@1890
|
494 |
* @param permits the number of permits to acquire
|
jaroslav@1890
|
495 |
* @throws IllegalArgumentException if {@code permits} is negative
|
jaroslav@1890
|
496 |
*
|
jaroslav@1890
|
497 |
*/
|
jaroslav@1890
|
498 |
public void acquireUninterruptibly(int permits) {
|
jaroslav@1890
|
499 |
if (permits < 0) throw new IllegalArgumentException();
|
jaroslav@1890
|
500 |
sync.acquireShared(permits);
|
jaroslav@1890
|
501 |
}
|
jaroslav@1890
|
502 |
|
jaroslav@1890
|
503 |
/**
|
jaroslav@1890
|
504 |
* Acquires the given number of permits from this semaphore, only
|
jaroslav@1890
|
505 |
* if all are available at the time of invocation.
|
jaroslav@1890
|
506 |
*
|
jaroslav@1890
|
507 |
* <p>Acquires the given number of permits, if they are available, and
|
jaroslav@1890
|
508 |
* returns immediately, with the value {@code true},
|
jaroslav@1890
|
509 |
* reducing the number of available permits by the given amount.
|
jaroslav@1890
|
510 |
*
|
jaroslav@1890
|
511 |
* <p>If insufficient permits are available then this method will return
|
jaroslav@1890
|
512 |
* immediately with the value {@code false} and the number of available
|
jaroslav@1890
|
513 |
* permits is unchanged.
|
jaroslav@1890
|
514 |
*
|
jaroslav@1890
|
515 |
* <p>Even when this semaphore has been set to use a fair ordering
|
jaroslav@1890
|
516 |
* policy, a call to {@code tryAcquire} <em>will</em>
|
jaroslav@1890
|
517 |
* immediately acquire a permit if one is available, whether or
|
jaroslav@1890
|
518 |
* not other threads are currently waiting. This
|
jaroslav@1890
|
519 |
* "barging" behavior can be useful in certain
|
jaroslav@1890
|
520 |
* circumstances, even though it breaks fairness. If you want to
|
jaroslav@1890
|
521 |
* honor the fairness setting, then use {@link #tryAcquire(int,
|
jaroslav@1890
|
522 |
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
|
jaroslav@1890
|
523 |
* which is almost equivalent (it also detects interruption).
|
jaroslav@1890
|
524 |
*
|
jaroslav@1890
|
525 |
* @param permits the number of permits to acquire
|
jaroslav@1890
|
526 |
* @return {@code true} if the permits were acquired and
|
jaroslav@1890
|
527 |
* {@code false} otherwise
|
jaroslav@1890
|
528 |
* @throws IllegalArgumentException if {@code permits} is negative
|
jaroslav@1890
|
529 |
*/
|
jaroslav@1890
|
530 |
public boolean tryAcquire(int permits) {
|
jaroslav@1890
|
531 |
if (permits < 0) throw new IllegalArgumentException();
|
jaroslav@1890
|
532 |
return sync.nonfairTryAcquireShared(permits) >= 0;
|
jaroslav@1890
|
533 |
}
|
jaroslav@1890
|
534 |
|
jaroslav@1890
|
535 |
/**
|
jaroslav@1890
|
536 |
* Acquires the given number of permits from this semaphore, if all
|
jaroslav@1890
|
537 |
* become available within the given waiting time and the current
|
jaroslav@1890
|
538 |
* thread has not been {@linkplain Thread#interrupt interrupted}.
|
jaroslav@1890
|
539 |
*
|
jaroslav@1890
|
540 |
* <p>Acquires the given number of permits, if they are available and
|
jaroslav@1890
|
541 |
* returns immediately, with the value {@code true},
|
jaroslav@1890
|
542 |
* reducing the number of available permits by the given amount.
|
jaroslav@1890
|
543 |
*
|
jaroslav@1890
|
544 |
* <p>If insufficient permits are available then
|
jaroslav@1890
|
545 |
* the current thread becomes disabled for thread scheduling
|
jaroslav@1890
|
546 |
* purposes and lies dormant until one of three things happens:
|
jaroslav@1890
|
547 |
* <ul>
|
jaroslav@1890
|
548 |
* <li>Some other thread invokes one of the {@link #release() release}
|
jaroslav@1890
|
549 |
* methods for this semaphore, the current thread is next to be assigned
|
jaroslav@1890
|
550 |
* permits and the number of available permits satisfies this request; or
|
jaroslav@1890
|
551 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
|
jaroslav@1890
|
552 |
* the current thread; or
|
jaroslav@1890
|
553 |
* <li>The specified waiting time elapses.
|
jaroslav@1890
|
554 |
* </ul>
|
jaroslav@1890
|
555 |
*
|
jaroslav@1890
|
556 |
* <p>If the permits are acquired then the value {@code true} is returned.
|
jaroslav@1890
|
557 |
*
|
jaroslav@1890
|
558 |
* <p>If the current thread:
|
jaroslav@1890
|
559 |
* <ul>
|
jaroslav@1890
|
560 |
* <li>has its interrupted status set on entry to this method; or
|
jaroslav@1890
|
561 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
jaroslav@1890
|
562 |
* to acquire the permits,
|
jaroslav@1890
|
563 |
* </ul>
|
jaroslav@1890
|
564 |
* then {@link InterruptedException} is thrown and the current thread's
|
jaroslav@1890
|
565 |
* interrupted status is cleared.
|
jaroslav@1890
|
566 |
* Any permits that were to be assigned to this thread, are instead
|
jaroslav@1890
|
567 |
* assigned to other threads trying to acquire permits, as if
|
jaroslav@1890
|
568 |
* the permits had been made available by a call to {@link #release()}.
|
jaroslav@1890
|
569 |
*
|
jaroslav@1890
|
570 |
* <p>If the specified waiting time elapses then the value {@code false}
|
jaroslav@1890
|
571 |
* is returned. If the time is less than or equal to zero, the method
|
jaroslav@1890
|
572 |
* will not wait at all. Any permits that were to be assigned to this
|
jaroslav@1890
|
573 |
* thread, are instead assigned to other threads trying to acquire
|
jaroslav@1890
|
574 |
* permits, as if the permits had been made available by a call to
|
jaroslav@1890
|
575 |
* {@link #release()}.
|
jaroslav@1890
|
576 |
*
|
jaroslav@1890
|
577 |
* @param permits the number of permits to acquire
|
jaroslav@1890
|
578 |
* @param timeout the maximum time to wait for the permits
|
jaroslav@1890
|
579 |
* @param unit the time unit of the {@code timeout} argument
|
jaroslav@1890
|
580 |
* @return {@code true} if all permits were acquired and {@code false}
|
jaroslav@1890
|
581 |
* if the waiting time elapsed before all permits were acquired
|
jaroslav@1890
|
582 |
* @throws InterruptedException if the current thread is interrupted
|
jaroslav@1890
|
583 |
* @throws IllegalArgumentException if {@code permits} is negative
|
jaroslav@1890
|
584 |
*/
|
jaroslav@1890
|
585 |
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
|
jaroslav@1890
|
586 |
throws InterruptedException {
|
jaroslav@1890
|
587 |
if (permits < 0) throw new IllegalArgumentException();
|
jaroslav@1890
|
588 |
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
|
jaroslav@1890
|
589 |
}
|
jaroslav@1890
|
590 |
|
jaroslav@1890
|
591 |
/**
|
jaroslav@1890
|
592 |
* Releases the given number of permits, returning them to the semaphore.
|
jaroslav@1890
|
593 |
*
|
jaroslav@1890
|
594 |
* <p>Releases the given number of permits, increasing the number of
|
jaroslav@1890
|
595 |
* available permits by that amount.
|
jaroslav@1890
|
596 |
* If any threads are trying to acquire permits, then one
|
jaroslav@1890
|
597 |
* is selected and given the permits that were just released.
|
jaroslav@1890
|
598 |
* If the number of available permits satisfies that thread's request
|
jaroslav@1890
|
599 |
* then that thread is (re)enabled for thread scheduling purposes;
|
jaroslav@1890
|
600 |
* otherwise the thread will wait until sufficient permits are available.
|
jaroslav@1890
|
601 |
* If there are still permits available
|
jaroslav@1890
|
602 |
* after this thread's request has been satisfied, then those permits
|
jaroslav@1890
|
603 |
* are assigned in turn to other threads trying to acquire permits.
|
jaroslav@1890
|
604 |
*
|
jaroslav@1890
|
605 |
* <p>There is no requirement that a thread that releases a permit must
|
jaroslav@1890
|
606 |
* have acquired that permit by calling {@link Semaphore#acquire acquire}.
|
jaroslav@1890
|
607 |
* Correct usage of a semaphore is established by programming convention
|
jaroslav@1890
|
608 |
* in the application.
|
jaroslav@1890
|
609 |
*
|
jaroslav@1890
|
610 |
* @param permits the number of permits to release
|
jaroslav@1890
|
611 |
* @throws IllegalArgumentException if {@code permits} is negative
|
jaroslav@1890
|
612 |
*/
|
jaroslav@1890
|
613 |
public void release(int permits) {
|
jaroslav@1890
|
614 |
if (permits < 0) throw new IllegalArgumentException();
|
jaroslav@1890
|
615 |
sync.releaseShared(permits);
|
jaroslav@1890
|
616 |
}
|
jaroslav@1890
|
617 |
|
jaroslav@1890
|
618 |
/**
|
jaroslav@1890
|
619 |
* Returns the current number of permits available in this semaphore.
|
jaroslav@1890
|
620 |
*
|
jaroslav@1890
|
621 |
* <p>This method is typically used for debugging and testing purposes.
|
jaroslav@1890
|
622 |
*
|
jaroslav@1890
|
623 |
* @return the number of permits available in this semaphore
|
jaroslav@1890
|
624 |
*/
|
jaroslav@1890
|
625 |
public int availablePermits() {
|
jaroslav@1890
|
626 |
return sync.getPermits();
|
jaroslav@1890
|
627 |
}
|
jaroslav@1890
|
628 |
|
jaroslav@1890
|
629 |
/**
|
jaroslav@1890
|
630 |
* Acquires and returns all permits that are immediately available.
|
jaroslav@1890
|
631 |
*
|
jaroslav@1890
|
632 |
* @return the number of permits acquired
|
jaroslav@1890
|
633 |
*/
|
jaroslav@1890
|
634 |
public int drainPermits() {
|
jaroslav@1890
|
635 |
return sync.drainPermits();
|
jaroslav@1890
|
636 |
}
|
jaroslav@1890
|
637 |
|
jaroslav@1890
|
638 |
/**
|
jaroslav@1890
|
639 |
* Shrinks the number of available permits by the indicated
|
jaroslav@1890
|
640 |
* reduction. This method can be useful in subclasses that use
|
jaroslav@1890
|
641 |
* semaphores to track resources that become unavailable. This
|
jaroslav@1890
|
642 |
* method differs from {@code acquire} in that it does not block
|
jaroslav@1890
|
643 |
* waiting for permits to become available.
|
jaroslav@1890
|
644 |
*
|
jaroslav@1890
|
645 |
* @param reduction the number of permits to remove
|
jaroslav@1890
|
646 |
* @throws IllegalArgumentException if {@code reduction} is negative
|
jaroslav@1890
|
647 |
*/
|
jaroslav@1890
|
648 |
protected void reducePermits(int reduction) {
|
jaroslav@1890
|
649 |
if (reduction < 0) throw new IllegalArgumentException();
|
jaroslav@1890
|
650 |
sync.reducePermits(reduction);
|
jaroslav@1890
|
651 |
}
|
jaroslav@1890
|
652 |
|
jaroslav@1890
|
653 |
/**
|
jaroslav@1890
|
654 |
* Returns {@code true} if this semaphore has fairness set true.
|
jaroslav@1890
|
655 |
*
|
jaroslav@1890
|
656 |
* @return {@code true} if this semaphore has fairness set true
|
jaroslav@1890
|
657 |
*/
|
jaroslav@1890
|
658 |
public boolean isFair() {
|
jaroslav@1890
|
659 |
return sync instanceof FairSync;
|
jaroslav@1890
|
660 |
}
|
jaroslav@1890
|
661 |
|
jaroslav@1890
|
662 |
/**
|
jaroslav@1890
|
663 |
* Queries whether any threads are waiting to acquire. Note that
|
jaroslav@1890
|
664 |
* because cancellations may occur at any time, a {@code true}
|
jaroslav@1890
|
665 |
* return does not guarantee that any other thread will ever
|
jaroslav@1890
|
666 |
* acquire. This method is designed primarily for use in
|
jaroslav@1890
|
667 |
* monitoring of the system state.
|
jaroslav@1890
|
668 |
*
|
jaroslav@1890
|
669 |
* @return {@code true} if there may be other threads waiting to
|
jaroslav@1890
|
670 |
* acquire the lock
|
jaroslav@1890
|
671 |
*/
|
jaroslav@1890
|
672 |
public final boolean hasQueuedThreads() {
|
jaroslav@1890
|
673 |
return sync.hasQueuedThreads();
|
jaroslav@1890
|
674 |
}
|
jaroslav@1890
|
675 |
|
jaroslav@1890
|
676 |
/**
|
jaroslav@1890
|
677 |
* Returns an estimate of the number of threads waiting to acquire.
|
jaroslav@1890
|
678 |
* The value is only an estimate because the number of threads may
|
jaroslav@1890
|
679 |
* change dynamically while this method traverses internal data
|
jaroslav@1890
|
680 |
* structures. This method is designed for use in monitoring of the
|
jaroslav@1890
|
681 |
* system state, not for synchronization control.
|
jaroslav@1890
|
682 |
*
|
jaroslav@1890
|
683 |
* @return the estimated number of threads waiting for this lock
|
jaroslav@1890
|
684 |
*/
|
jaroslav@1890
|
685 |
public final int getQueueLength() {
|
jaroslav@1890
|
686 |
return sync.getQueueLength();
|
jaroslav@1890
|
687 |
}
|
jaroslav@1890
|
688 |
|
jaroslav@1890
|
689 |
/**
|
jaroslav@1890
|
690 |
* Returns a collection containing threads that may be waiting to acquire.
|
jaroslav@1890
|
691 |
* Because the actual set of threads may change dynamically while
|
jaroslav@1890
|
692 |
* constructing this result, the returned collection is only a best-effort
|
jaroslav@1890
|
693 |
* estimate. The elements of the returned collection are in no particular
|
jaroslav@1890
|
694 |
* order. This method is designed to facilitate construction of
|
jaroslav@1890
|
695 |
* subclasses that provide more extensive monitoring facilities.
|
jaroslav@1890
|
696 |
*
|
jaroslav@1890
|
697 |
* @return the collection of threads
|
jaroslav@1890
|
698 |
*/
|
jaroslav@1890
|
699 |
protected Collection<Thread> getQueuedThreads() {
|
jaroslav@1890
|
700 |
return sync.getQueuedThreads();
|
jaroslav@1890
|
701 |
}
|
jaroslav@1890
|
702 |
|
jaroslav@1890
|
703 |
/**
|
jaroslav@1890
|
704 |
* Returns a string identifying this semaphore, as well as its state.
|
jaroslav@1890
|
705 |
* The state, in brackets, includes the String {@code "Permits ="}
|
jaroslav@1890
|
706 |
* followed by the number of permits.
|
jaroslav@1890
|
707 |
*
|
jaroslav@1890
|
708 |
* @return a string identifying this semaphore, as well as its state
|
jaroslav@1890
|
709 |
*/
|
jaroslav@1890
|
710 |
public String toString() {
|
jaroslav@1890
|
711 |
return super.toString() + "[Permits = " + sync.getPermits() + "]";
|
jaroslav@1890
|
712 |
}
|
jaroslav@1890
|
713 |
}
|