author | ohair |
Tue, 25 May 2010 15:58:33 -0700 | |
changeset 5506 | 202f599c92aa |
parent 5202 | 20198b5f3653 |
child 7518 | 0282db800fe1 |
permissions | -rw-r--r-- |
2 | 1 |
/* |
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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/licenses/publicdomain |
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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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final static class NonfairSync extends Sync { |
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private static final long serialVersionUID = -2694183684443567898L; |
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229 |
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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final static 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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297 |
* one of two things happens: |
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298 |
* <ul> |
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299 |
* <li>Some other thread invokes the {@link #release} method for this |
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300 |
* semaphore and the current thread is next to be assigned a permit; or |
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301 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
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* the current thread. |
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* </ul> |
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304 |
* |
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305 |
* <p>If the current thread: |
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306 |
* <ul> |
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307 |
* <li>has its interrupted status set on entry to this method; or |
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308 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
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* for a permit, |
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310 |
* </ul> |
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311 |
* then {@link InterruptedException} is thrown and the current thread's |
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* interrupted status is cleared. |
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* |
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314 |
* @throws InterruptedException if the current thread is interrupted |
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315 |
*/ |
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316 |
public void acquire() throws InterruptedException { |
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317 |
sync.acquireSharedInterruptibly(1); |
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318 |
} |
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319 |
||
320 |
/** |
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321 |
* Acquires a permit from this semaphore, blocking until one is |
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322 |
* available. |
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323 |
* |
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324 |
* <p>Acquires a permit, if one is available and returns immediately, |
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325 |
* reducing the number of available permits by one. |
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326 |
* |
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327 |
* <p>If no permit is available then the current thread becomes |
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328 |
* disabled for thread scheduling purposes and lies dormant until |
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329 |
* some other thread invokes the {@link #release} method for this |
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330 |
* semaphore and the current thread is next to be assigned a permit. |
|
331 |
* |
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332 |
* <p>If the current thread is {@linkplain Thread#interrupt interrupted} |
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333 |
* while waiting for a permit then it will continue to wait, but the |
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334 |
* time at which the thread is assigned a permit may change compared to |
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335 |
* the time it would have received the permit had no interruption |
|
336 |
* occurred. When the thread does return from this method its interrupt |
|
337 |
* status will be set. |
|
338 |
*/ |
|
339 |
public void acquireUninterruptibly() { |
|
340 |
sync.acquireShared(1); |
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341 |
} |
|
342 |
||
343 |
/** |
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344 |
* Acquires a permit from this semaphore, only if one is available at the |
|
345 |
* time of invocation. |
|
346 |
* |
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347 |
* <p>Acquires a permit, if one is available and returns immediately, |
|
348 |
* with the value {@code true}, |
|
349 |
* reducing the number of available permits by one. |
|
350 |
* |
|
351 |
* <p>If no permit is available then this method will return |
|
352 |
* immediately with the value {@code false}. |
|
353 |
* |
|
354 |
* <p>Even when this semaphore has been set to use a |
|
355 |
* fair ordering policy, a call to {@code tryAcquire()} <em>will</em> |
|
356 |
* immediately acquire a permit if one is available, whether or not |
|
357 |
* other threads are currently waiting. |
|
358 |
* This "barging" behavior can be useful in certain |
|
359 |
* circumstances, even though it breaks fairness. If you want to honor |
|
360 |
* the fairness setting, then use |
|
361 |
* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) } |
|
362 |
* which is almost equivalent (it also detects interruption). |
|
363 |
* |
|
364 |
* @return {@code true} if a permit was acquired and {@code false} |
|
365 |
* otherwise |
|
366 |
*/ |
|
367 |
public boolean tryAcquire() { |
|
368 |
return sync.nonfairTryAcquireShared(1) >= 0; |
|
369 |
} |
|
370 |
||
371 |
/** |
|
372 |
* Acquires a permit from this semaphore, if one becomes available |
|
373 |
* within the given waiting time and the current thread has not |
|
374 |
* been {@linkplain Thread#interrupt interrupted}. |
|
375 |
* |
|
376 |
* <p>Acquires a permit, if one is available and returns immediately, |
|
377 |
* with the value {@code true}, |
|
378 |
* reducing the number of available permits by one. |
|
379 |
* |
|
380 |
* <p>If no permit is available then the current thread becomes |
|
381 |
* disabled for thread scheduling purposes and lies dormant until |
|
382 |
* one of three things happens: |
|
383 |
* <ul> |
|
384 |
* <li>Some other thread invokes the {@link #release} method for this |
|
385 |
* semaphore and the current thread is next to be assigned a permit; or |
|
386 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
|
387 |
* the current thread; or |
|
388 |
* <li>The specified waiting time elapses. |
|
389 |
* </ul> |
|
390 |
* |
|
391 |
* <p>If a permit is acquired then the value {@code true} is returned. |
|
392 |
* |
|
393 |
* <p>If the current thread: |
|
394 |
* <ul> |
|
395 |
* <li>has its interrupted status set on entry to this method; or |
|
396 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
|
397 |
* to acquire a permit, |
|
398 |
* </ul> |
|
399 |
* then {@link InterruptedException} is thrown and the current thread's |
|
400 |
* interrupted status is cleared. |
|
401 |
* |
|
402 |
* <p>If the specified waiting time elapses then the value {@code false} |
|
403 |
* is returned. If the time is less than or equal to zero, the method |
|
404 |
* will not wait at all. |
|
405 |
* |
|
406 |
* @param timeout the maximum time to wait for a permit |
|
407 |
* @param unit the time unit of the {@code timeout} argument |
|
408 |
* @return {@code true} if a permit was acquired and {@code false} |
|
409 |
* if the waiting time elapsed before a permit was acquired |
|
410 |
* @throws InterruptedException if the current thread is interrupted |
|
411 |
*/ |
|
412 |
public boolean tryAcquire(long timeout, TimeUnit unit) |
|
413 |
throws InterruptedException { |
|
414 |
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
|
415 |
} |
|
416 |
||
417 |
/** |
|
418 |
* Releases a permit, returning it to the semaphore. |
|
419 |
* |
|
420 |
* <p>Releases a permit, increasing the number of available permits by |
|
421 |
* one. If any threads are trying to acquire a permit, then one is |
|
422 |
* selected and given the permit that was just released. That thread |
|
423 |
* is (re)enabled for thread scheduling purposes. |
|
424 |
* |
|
425 |
* <p>There is no requirement that a thread that releases a permit must |
|
426 |
* have acquired that permit by calling {@link #acquire}. |
|
427 |
* Correct usage of a semaphore is established by programming convention |
|
428 |
* in the application. |
|
429 |
*/ |
|
430 |
public void release() { |
|
431 |
sync.releaseShared(1); |
|
432 |
} |
|
433 |
||
434 |
/** |
|
435 |
* Acquires the given number of permits from this semaphore, |
|
436 |
* blocking until all are available, |
|
437 |
* or the thread is {@linkplain Thread#interrupt interrupted}. |
|
438 |
* |
|
439 |
* <p>Acquires the given number of permits, if they are available, |
|
440 |
* and returns immediately, reducing the number of available permits |
|
441 |
* by the given amount. |
|
442 |
* |
|
443 |
* <p>If insufficient permits are available then the current thread becomes |
|
444 |
* disabled for thread scheduling purposes and lies dormant until |
|
445 |
* one of two things happens: |
|
446 |
* <ul> |
|
447 |
* <li>Some other thread invokes one of the {@link #release() release} |
|
448 |
* methods for this semaphore, the current thread is next to be assigned |
|
449 |
* permits and the number of available permits satisfies this request; or |
|
450 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
|
451 |
* the current thread. |
|
452 |
* </ul> |
|
453 |
* |
|
454 |
* <p>If the current thread: |
|
455 |
* <ul> |
|
456 |
* <li>has its interrupted status set on entry to this method; or |
|
457 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
|
458 |
* for a permit, |
|
459 |
* </ul> |
|
460 |
* then {@link InterruptedException} is thrown and the current thread's |
|
461 |
* interrupted status is cleared. |
|
462 |
* Any permits that were to be assigned to this thread are instead |
|
463 |
* assigned to other threads trying to acquire permits, as if |
|
464 |
* permits had been made available by a call to {@link #release()}. |
|
465 |
* |
|
466 |
* @param permits the number of permits to acquire |
|
467 |
* @throws InterruptedException if the current thread is interrupted |
|
468 |
* @throws IllegalArgumentException if {@code permits} is negative |
|
469 |
*/ |
|
470 |
public void acquire(int permits) throws InterruptedException { |
|
471 |
if (permits < 0) throw new IllegalArgumentException(); |
|
472 |
sync.acquireSharedInterruptibly(permits); |
|
473 |
} |
|
474 |
||
475 |
/** |
|
476 |
* Acquires the given number of permits from this semaphore, |
|
477 |
* blocking until all are available. |
|
478 |
* |
|
479 |
* <p>Acquires the given number of permits, if they are available, |
|
480 |
* and returns immediately, reducing the number of available permits |
|
481 |
* by the given amount. |
|
482 |
* |
|
483 |
* <p>If insufficient permits are available then the current thread becomes |
|
484 |
* disabled for thread scheduling purposes and lies dormant until |
|
485 |
* some other thread invokes one of the {@link #release() release} |
|
486 |
* methods for this semaphore, the current thread is next to be assigned |
|
487 |
* permits and the number of available permits satisfies this request. |
|
488 |
* |
|
489 |
* <p>If the current thread is {@linkplain Thread#interrupt interrupted} |
|
490 |
* while waiting for permits then it will continue to wait and its |
|
491 |
* position in the queue is not affected. When the thread does return |
|
492 |
* from this method its interrupt status will be set. |
|
493 |
* |
|
494 |
* @param permits the number of permits to acquire |
|
495 |
* @throws IllegalArgumentException if {@code permits} is negative |
|
496 |
* |
|
497 |
*/ |
|
498 |
public void acquireUninterruptibly(int permits) { |
|
499 |
if (permits < 0) throw new IllegalArgumentException(); |
|
500 |
sync.acquireShared(permits); |
|
501 |
} |
|
502 |
||
503 |
/** |
|
504 |
* Acquires the given number of permits from this semaphore, only |
|
505 |
* if all are available at the time of invocation. |
|
506 |
* |
|
507 |
* <p>Acquires the given number of permits, if they are available, and |
|
508 |
* returns immediately, with the value {@code true}, |
|
509 |
* reducing the number of available permits by the given amount. |
|
510 |
* |
|
511 |
* <p>If insufficient permits are available then this method will return |
|
512 |
* immediately with the value {@code false} and the number of available |
|
513 |
* permits is unchanged. |
|
514 |
* |
|
515 |
* <p>Even when this semaphore has been set to use a fair ordering |
|
516 |
* policy, a call to {@code tryAcquire} <em>will</em> |
|
517 |
* immediately acquire a permit if one is available, whether or |
|
518 |
* not other threads are currently waiting. This |
|
519 |
* "barging" behavior can be useful in certain |
|
520 |
* circumstances, even though it breaks fairness. If you want to |
|
521 |
* honor the fairness setting, then use {@link #tryAcquire(int, |
|
522 |
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) } |
|
523 |
* which is almost equivalent (it also detects interruption). |
|
524 |
* |
|
525 |
* @param permits the number of permits to acquire |
|
526 |
* @return {@code true} if the permits were acquired and |
|
527 |
* {@code false} otherwise |
|
528 |
* @throws IllegalArgumentException if {@code permits} is negative |
|
529 |
*/ |
|
530 |
public boolean tryAcquire(int permits) { |
|
531 |
if (permits < 0) throw new IllegalArgumentException(); |
|
532 |
return sync.nonfairTryAcquireShared(permits) >= 0; |
|
533 |
} |
|
534 |
||
535 |
/** |
|
536 |
* Acquires the given number of permits from this semaphore, if all |
|
537 |
* become available within the given waiting time and the current |
|
538 |
* thread has not been {@linkplain Thread#interrupt interrupted}. |
|
539 |
* |
|
540 |
* <p>Acquires the given number of permits, if they are available and |
|
541 |
* returns immediately, with the value {@code true}, |
|
542 |
* reducing the number of available permits by the given amount. |
|
543 |
* |
|
544 |
* <p>If insufficient permits are available then |
|
545 |
* the current thread becomes disabled for thread scheduling |
|
546 |
* purposes and lies dormant until one of three things happens: |
|
547 |
* <ul> |
|
548 |
* <li>Some other thread invokes one of the {@link #release() release} |
|
549 |
* methods for this semaphore, the current thread is next to be assigned |
|
550 |
* permits and the number of available permits satisfies this request; or |
|
551 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
|
552 |
* the current thread; or |
|
553 |
* <li>The specified waiting time elapses. |
|
554 |
* </ul> |
|
555 |
* |
|
556 |
* <p>If the permits are acquired then the value {@code true} is returned. |
|
557 |
* |
|
558 |
* <p>If the current thread: |
|
559 |
* <ul> |
|
560 |
* <li>has its interrupted status set on entry to this method; or |
|
561 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
|
562 |
* to acquire the permits, |
|
563 |
* </ul> |
|
564 |
* then {@link InterruptedException} is thrown and the current thread's |
|
565 |
* interrupted status is cleared. |
|
566 |
* Any permits that were to be assigned to this thread, are instead |
|
567 |
* assigned to other threads trying to acquire permits, as if |
|
568 |
* the permits had been made available by a call to {@link #release()}. |
|
569 |
* |
|
570 |
* <p>If the specified waiting time elapses then the value {@code false} |
|
571 |
* is returned. If the time is less than or equal to zero, the method |
|
572 |
* will not wait at all. Any permits that were to be assigned to this |
|
573 |
* thread, are instead assigned to other threads trying to acquire |
|
574 |
* permits, as if the permits had been made available by a call to |
|
575 |
* {@link #release()}. |
|
576 |
* |
|
577 |
* @param permits the number of permits to acquire |
|
578 |
* @param timeout the maximum time to wait for the permits |
|
579 |
* @param unit the time unit of the {@code timeout} argument |
|
580 |
* @return {@code true} if all permits were acquired and {@code false} |
|
581 |
* if the waiting time elapsed before all permits were acquired |
|
582 |
* @throws InterruptedException if the current thread is interrupted |
|
583 |
* @throws IllegalArgumentException if {@code permits} is negative |
|
584 |
*/ |
|
585 |
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) |
|
586 |
throws InterruptedException { |
|
587 |
if (permits < 0) throw new IllegalArgumentException(); |
|
588 |
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); |
|
589 |
} |
|
590 |
||
591 |
/** |
|
592 |
* Releases the given number of permits, returning them to the semaphore. |
|
593 |
* |
|
594 |
* <p>Releases the given number of permits, increasing the number of |
|
595 |
* available permits by that amount. |
|
596 |
* If any threads are trying to acquire permits, then one |
|
597 |
* is selected and given the permits that were just released. |
|
598 |
* If the number of available permits satisfies that thread's request |
|
599 |
* then that thread is (re)enabled for thread scheduling purposes; |
|
600 |
* otherwise the thread will wait until sufficient permits are available. |
|
601 |
* If there are still permits available |
|
602 |
* after this thread's request has been satisfied, then those permits |
|
603 |
* are assigned in turn to other threads trying to acquire permits. |
|
604 |
* |
|
605 |
* <p>There is no requirement that a thread that releases a permit must |
|
606 |
* have acquired that permit by calling {@link Semaphore#acquire acquire}. |
|
607 |
* Correct usage of a semaphore is established by programming convention |
|
608 |
* in the application. |
|
609 |
* |
|
610 |
* @param permits the number of permits to release |
|
611 |
* @throws IllegalArgumentException if {@code permits} is negative |
|
612 |
*/ |
|
613 |
public void release(int permits) { |
|
614 |
if (permits < 0) throw new IllegalArgumentException(); |
|
615 |
sync.releaseShared(permits); |
|
616 |
} |
|
617 |
||
618 |
/** |
|
619 |
* Returns the current number of permits available in this semaphore. |
|
620 |
* |
|
621 |
* <p>This method is typically used for debugging and testing purposes. |
|
622 |
* |
|
623 |
* @return the number of permits available in this semaphore |
|
624 |
*/ |
|
625 |
public int availablePermits() { |
|
626 |
return sync.getPermits(); |
|
627 |
} |
|
628 |
||
629 |
/** |
|
630 |
* Acquires and returns all permits that are immediately available. |
|
631 |
* |
|
632 |
* @return the number of permits acquired |
|
633 |
*/ |
|
634 |
public int drainPermits() { |
|
635 |
return sync.drainPermits(); |
|
636 |
} |
|
637 |
||
638 |
/** |
|
639 |
* Shrinks the number of available permits by the indicated |
|
640 |
* reduction. This method can be useful in subclasses that use |
|
641 |
* semaphores to track resources that become unavailable. This |
|
642 |
* method differs from {@code acquire} in that it does not block |
|
643 |
* waiting for permits to become available. |
|
644 |
* |
|
645 |
* @param reduction the number of permits to remove |
|
646 |
* @throws IllegalArgumentException if {@code reduction} is negative |
|
647 |
*/ |
|
648 |
protected void reducePermits(int reduction) { |
|
649 |
if (reduction < 0) throw new IllegalArgumentException(); |
|
650 |
sync.reducePermits(reduction); |
|
651 |
} |
|
652 |
||
653 |
/** |
|
654 |
* Returns {@code true} if this semaphore has fairness set true. |
|
655 |
* |
|
656 |
* @return {@code true} if this semaphore has fairness set true |
|
657 |
*/ |
|
658 |
public boolean isFair() { |
|
659 |
return sync instanceof FairSync; |
|
660 |
} |
|
661 |
||
662 |
/** |
|
663 |
* Queries whether any threads are waiting to acquire. Note that |
|
664 |
* because cancellations may occur at any time, a {@code true} |
|
665 |
* return does not guarantee that any other thread will ever |
|
666 |
* acquire. This method is designed primarily for use in |
|
667 |
* monitoring of the system state. |
|
668 |
* |
|
669 |
* @return {@code true} if there may be other threads waiting to |
|
670 |
* acquire the lock |
|
671 |
*/ |
|
672 |
public final boolean hasQueuedThreads() { |
|
673 |
return sync.hasQueuedThreads(); |
|
674 |
} |
|
675 |
||
676 |
/** |
|
677 |
* Returns an estimate of the number of threads waiting to acquire. |
|
678 |
* The value is only an estimate because the number of threads may |
|
679 |
* change dynamically while this method traverses internal data |
|
680 |
* structures. This method is designed for use in monitoring of the |
|
681 |
* system state, not for synchronization control. |
|
682 |
* |
|
683 |
* @return the estimated number of threads waiting for this lock |
|
684 |
*/ |
|
685 |
public final int getQueueLength() { |
|
686 |
return sync.getQueueLength(); |
|
687 |
} |
|
688 |
||
689 |
/** |
|
690 |
* Returns a collection containing threads that may be waiting to acquire. |
|
691 |
* Because the actual set of threads may change dynamically while |
|
692 |
* constructing this result, the returned collection is only a best-effort |
|
693 |
* estimate. The elements of the returned collection are in no particular |
|
694 |
* order. This method is designed to facilitate construction of |
|
695 |
* subclasses that provide more extensive monitoring facilities. |
|
696 |
* |
|
697 |
* @return the collection of threads |
|
698 |
*/ |
|
699 |
protected Collection<Thread> getQueuedThreads() { |
|
700 |
return sync.getQueuedThreads(); |
|
701 |
} |
|
702 |
||
703 |
/** |
|
704 |
* Returns a string identifying this semaphore, as well as its state. |
|
705 |
* The state, in brackets, includes the String {@code "Permits ="} |
|
706 |
* followed by the number of permits. |
|
707 |
* |
|
708 |
* @return a string identifying this semaphore, as well as its state |
|
709 |
*/ |
|
710 |
public String toString() { |
|
711 |
return super.toString() + "[Permits = " + sync.getPermits() + "]"; |
|
712 |
} |
|
713 |
} |