1 /* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25 /* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent.locks; 37 38 /** 39 * A {@code ReadWriteLock} maintains a pair of associated {@link 40 * Lock locks}, one for read-only operations and one for writing. 41 * The {@linkplain #readLock read lock} may be held simultaneously 42 * by multiple reader threads, so long as there are no writers. 43 * The {@linkplain #writeLock write lock} is exclusive. 44 * 45 * <p>All {@code ReadWriteLock} implementations must guarantee that 46 * the memory synchronization effects of {@code writeLock} operations 47 * (as specified in the {@link Lock} interface) also hold with respect 48 * to the associated {@code readLock}. That is, a thread successfully 49 * acquiring the read lock will see all updates made upon previous 50 * release of the write lock. 51 * 52 * <p>A read-write lock allows for a greater level of concurrency in 53 * accessing shared data than that permitted by a mutual exclusion lock. 54 * It exploits the fact that while only a single thread at a time (a 55 * <em>writer</em> thread) can modify the shared data, in many cases any 56 * number of threads can concurrently read the data (hence <em>reader</em> 57 * threads). 58 * In theory, the increase in concurrency permitted by the use of a read-write 59 * lock will lead to performance improvements over the use of a mutual 60 * exclusion lock. In practice this increase in concurrency will only be fully 61 * realized on a multi-processor, and then only if the access patterns for 62 * the shared data are suitable. 63 * 64 * <p>Whether or not a read-write lock will improve performance over the use 65 * of a mutual exclusion lock depends on the frequency that the data is 66 * read compared to being modified, the duration of the read and write 67 * operations, and the contention for the data - that is, the number of 68 * threads that will try to read or write the data at the same time. 69 * For example, a collection that is initially populated with data and 70 * thereafter infrequently modified, while being frequently searched 71 * (such as a directory of some kind) is an ideal candidate for the use of 72 * a read-write lock. However, if updates become frequent then the data 73 * spends most of its time being exclusively locked and there is little, if any 74 * increase in concurrency. Further, if the read operations are too short 75 * the overhead of the read-write lock implementation (which is inherently 76 * more complex than a mutual exclusion lock) can dominate the execution 77 * cost, particularly as many read-write lock implementations still serialize 78 * all threads through a small section of code. Ultimately, only profiling 79 * and measurement will establish whether the use of a read-write lock is 80 * suitable for your application. 81 * 82 * 83 * <p>Although the basic operation of a read-write lock is straight-forward, 84 * there are many policy decisions that an implementation must make, which 85 * may affect the effectiveness of the read-write lock in a given application. 86 * Examples of these policies include: 87 * <ul> 88 * <li>Determining whether to grant the read lock or the write lock, when 89 * both readers and writers are waiting, at the time that a writer releases 90 * the write lock. Writer preference is common, as writes are expected to be 91 * short and infrequent. Reader preference is less common as it can lead to 92 * lengthy delays for a write if the readers are frequent and long-lived as 93 * expected. Fair, or "in-order" implementations are also possible. 94 * 95 * <li>Determining whether readers that request the read lock while a 96 * reader is active and a writer is waiting, are granted the read lock. 97 * Preference to the reader can delay the writer indefinitely, while 98 * preference to the writer can reduce the potential for concurrency. 99 * 100 * <li>Determining whether the locks are reentrant: can a thread with the 101 * write lock reacquire it? Can it acquire a read lock while holding the 102 * write lock? Is the read lock itself reentrant? 103 * 104 * <li>Can the write lock be downgraded to a read lock without allowing 105 * an intervening writer? Can a read lock be upgraded to a write lock, 106 * in preference to other waiting readers or writers? 107 * 108 * </ul> 109 * You should consider all of these things when evaluating the suitability 110 * of a given implementation for your application. 111 * 112 * @see ReentrantReadWriteLock 113 * @see Lock 114 * @see ReentrantLock 115 * 116 * @since 1.5 117 * @author Doug Lea 118 */ 119 public interface ReadWriteLock { 120 /** 121 * Returns the lock used for reading. 122 * 123 * @return the lock used for reading 124 */ readLock()125 Lock readLock(); 126 127 /** 128 * Returns the lock used for writing. 129 * 130 * @return the lock used for writing 131 */ writeLock()132 Lock writeLock(); 133 } 134