content: /*
   * 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 &quot;in-order&quot; 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();
  }
  