Source for java.util.concurrent.locks.ReadWriteLock

   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/licenses/publicdomain
   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 &quot;in-order&quot; 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: }