Java tutorial
/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent.locks; /** * A {@code ReadWriteLock} maintains a pair of associated {@link * Lock locks}, one for read-only operations and one for writing. * The {@linkplain #readLock read lock} may be held simultaneously * by multiple reader threads, so long as there are no writers. * The {@linkplain #writeLock write lock} is exclusive. * * <p>All {@code ReadWriteLock} implementations must guarantee that * the memory synchronization effects of {@code writeLock} operations * (as specified in the {@link Lock} interface) also hold with respect * to the associated {@code readLock}. That is, a thread successfully * acquiring the read lock will see all updates made upon previous * release of the write lock. * * <p>A read-write lock allows for a greater level of concurrency in * accessing shared data than that permitted by a mutual exclusion lock. * It exploits the fact that while only a single thread at a time (a * <em>writer</em> thread) can modify the shared data, in many cases any * number of threads can concurrently read the data (hence <em>reader</em> * threads). * In theory, the increase in concurrency permitted by the use of a read-write * lock will lead to performance improvements over the use of a mutual * exclusion lock. In practice this increase in concurrency will only be fully * realized on a multi-processor, and then only if the access patterns for * the shared data are suitable. * * <p>Whether or not a read-write lock will improve performance over the use * of a mutual exclusion lock depends on the frequency that the data is * read compared to being modified, the duration of the read and write * operations, and the contention for the data - that is, the number of * threads that will try to read or write the data at the same time. * For example, a collection that is initially populated with data and * thereafter infrequently modified, while being frequently searched * (such as a directory of some kind) is an ideal candidate for the use of * a read-write lock. However, if updates become frequent then the data * spends most of its time being exclusively locked and there is little, if any * increase in concurrency. Further, if the read operations are too short * the overhead of the read-write lock implementation (which is inherently * more complex than a mutual exclusion lock) can dominate the execution * cost, particularly as many read-write lock implementations still serialize * all threads through a small section of code. Ultimately, only profiling * and measurement will establish whether the use of a read-write lock is * suitable for your application. * * <p>Although the basic operation of a read-write lock is straight-forward, * there are many policy decisions that an implementation must make, which * may affect the effectiveness of the read-write lock in a given application. * Examples of these policies include: * <ul> * <li>Determining whether to grant the read lock or the write lock, when * both readers and writers are waiting, at the time that a writer releases * the write lock. Writer preference is common, as writes are expected to be * short and infrequent. Reader preference is less common as it can lead to * lengthy delays for a write if the readers are frequent and long-lived as * expected. Fair, or "in-order" implementations are also possible. * * <li>Determining whether readers that request the read lock while a * reader is active and a writer is waiting, are granted the read lock. * Preference to the reader can delay the writer indefinitely, while * preference to the writer can reduce the potential for concurrency. * * <li>Determining whether the locks are reentrant: can a thread with the * write lock reacquire it? Can it acquire a read lock while holding the * write lock? Is the read lock itself reentrant? * * <li>Can the write lock be downgraded to a read lock without allowing * an intervening writer? Can a read lock be upgraded to a write lock, * in preference to other waiting readers or writers? * * </ul> * You should consider all of these things when evaluating the suitability * of a given implementation for your application. * * @see ReentrantReadWriteLock * @see Lock * @see ReentrantLock * * @since 1.5 * @author Doug Lea */ public interface ReadWriteLock { /** * Returns the lock used for reading. * * @return the lock used for reading */ Lock readLock(); /** * Returns the lock used for writing. * * @return the lock used for writing */ Lock writeLock(); }