java并发包concurrent翻译及源码分析之：ReadWriteLock

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/*
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 * 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 <tt>ReadWriteLock</tt> maintains a pair of associated {@link
 * Lock locks}, one for read-only operations and one for writing.
 * The {@link #readLock read lock} may be held simultaneously by
 * multiple reader threads, so long as there are no writers.  The
 * {@link #writeLock write lock} is exclusive.
 * 一个ReadWriteLock维护一对相关的锁，一个仅用于读操作的读锁，和另外一个仅用于写操作的写锁。
 * 读锁可能被多个读线程同时拥有，只要没有写入者。写锁则是唯一的。
 *
 * <p>All <tt>ReadWriteLock</tt> implementations must guarantee that
 * the memory synchronization effects of <tt>writeLock</tt> operations
 * (as specified in the {@link Lock} interface) also hold with respect
 * to the associated <tt>readLock</tt>. That is, a thread successfully
 * acquiring the read lock will see all updates made upon previous
 * release of the write lock.
 * 所有的ReadWriteLock实现者必须保证写锁操作对内存同步的影响也会被相关读锁所持有。
 * 也就是说，一个成功获取读锁的线程将会看到先前版本写锁所做的所有更新。
 *
 * <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).
 * 在访问共享数据时，一个读写锁比互斥锁允许更大程度的并发。
 * 它利用了一个事实，即当某一时刻仅有一个线程（一个写线程）能修改共享数据，在许多情况下，任何数目的线程（读线程）可以读取共享数据。
 *
 * 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.
 * 在理论上，通过读写锁增加的并发会比使用互斥锁导致性能的改善。
 * 在实践中，并发性的增加只能在多处理器，访问共享数据的模式与之相配的情况下才能得到充分的体现。
 *
 * <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.
     * 返回用于读的锁
     * @return the lock used for reading.
     */
    Lock readLock();

    /**
     * Returns the lock used for writing.
     * 返回用于写的锁
     * @return the lock used for writing.
     */
    Lock writeLock();
}