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1 files changed, 104 insertions, 0 deletions

diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java
new file mode 100644
index 000000000..484f68d15
--- /dev/null
+++ b/libjava/classpath/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java
@@ -0,0 +1,104 @@
+/*
+ * 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/licenses/publicdomain
+ */
+
+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.
+ *
+ * <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.
+ *
+ * <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();
+}