1 /* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7 package java.util.concurrent.locks; 8 9 /** 10 * A <tt>ReadWriteLock</tt> maintains a pair of associated {@link 11 * Lock locks}, one for read-only operations and one for writing. 12 * The {@link #readLock read lock} may be held simultaneously by 13 * multiple reader threads, so long as there are no writers. The 14 * {@link #writeLock write lock} is exclusive. 15 * 16 * <p>All <tt>ReadWriteLock</tt> implementations must guarantee that 17 * the memory synchronization effects of <tt>writeLock</tt> operations 18 * (as specified in the {@link Lock} interface) also hold with respect 19 * to the associated <tt>readLock</tt>. That is, a thread successfully 20 * acquiring the read lock will see all updates made upon previous 21 * release of the write lock. 22 * 23 * <p>A read-write lock allows for a greater level of concurrency in 24 * accessing shared data than that permitted by a mutual exclusion lock. 25 * It exploits the fact that while only a single thread at a time (a 26 * <em>writer</em> thread) can modify the shared data, in many cases any 27 * number of threads can concurrently read the data (hence <em>reader</em> 28 * threads). 29 * In theory, the increase in concurrency permitted by the use of a read-write 30 * lock will lead to performance improvements over the use of a mutual 31 * exclusion lock. In practice this increase in concurrency will only be fully 32 * realized on a multi-processor, and then only if the access patterns for 33 * the shared data are suitable. 34 * 35 * <p>Whether or not a read-write lock will improve performance over the use 36 * of a mutual exclusion lock depends on the frequency that the data is 37 * read compared to being modified, the duration of the read and write 38 * operations, and the contention for the data - that is, the number of 39 * threads that will try to read or write the data at the same time. 40 * For example, a collection that is initially populated with data and 41 * thereafter infrequently modified, while being frequently searched 42 * (such as a directory of some kind) is an ideal candidate for the use of 43 * a read-write lock. However, if updates become frequent then the data 44 * spends most of its time being exclusively locked and there is little, if any 45 * increase in concurrency. Further, if the read operations are too short 46 * the overhead of the read-write lock implementation (which is inherently 47 * more complex than a mutual exclusion lock) can dominate the execution 48 * cost, particularly as many read-write lock implementations still serialize 49 * all threads through a small section of code. Ultimately, only profiling 50 * and measurement will establish whether the use of a read-write lock is 51 * suitable for your application. 52 * 53 * 54 * <p>Although the basic operation of a read-write lock is straight-forward, 55 * there are many policy decisions that an implementation must make, which 56 * may affect the effectiveness of the read-write lock in a given application. 57 * Examples of these policies include: 58 * <ul> 59 * <li>Determining whether to grant the read lock or the write lock, when 60 * both readers and writers are waiting, at the time that a writer releases 61 * the write lock. Writer preference is common, as writes are expected to be 62 * short and infrequent. Reader preference is less common as it can lead to 63 * lengthy delays for a write if the readers are frequent and long-lived as 64 * expected. Fair, or "in-order" implementations are also possible. 65 * 66 * <li>Determining whether readers that request the read lock while a 67 * reader is active and a writer is waiting, are granted the read lock. 68 * Preference to the reader can delay the writer indefinitely, while 69 * preference to the writer can reduce the potential for concurrency. 70 * 71 * <li>Determining whether the locks are reentrant: can a thread with the 72 * write lock reacquire it? Can it acquire a read lock while holding the 73 * write lock? Is the read lock itself reentrant? 74 * 75 * <li>Can the write lock be downgraded to a read lock without allowing 76 * an intervening writer? Can a read lock be upgraded to a write lock, 77 * in preference to other waiting readers or writers? 78 * 79 * </ul> 80 * You should consider all of these things when evaluating the suitability 81 * of a given implementation for your application. 82 * 83 * @see ReentrantReadWriteLock 84 * @see Lock 85 * @see ReentrantLock 86 * 87 * @since 1.5 88 * @author Doug Lea 89 */ 90 public interface ReadWriteLock { 91 /** 92 * Returns the lock used for reading. 93 * 94 * @return the lock used for reading. 95 */ 96 Lock readLock(); 97 98 /** 99 * Returns the lock used for writing. 100 * 101 * @return the lock used for writing. 102 */ 103 Lock writeLock(); 104 } 105
