public interface ReadWriteLock
locks, one for read-only operations and one for writing. The
read lockmay be held simultaneously by multiple reader threads, so long as there are no writers. The
write lockis exclusive.
All ReadWriteLock implementations must guarantee that
the memory synchronization effects of writeLock operations
(as specified in the
Lock interface) also hold with respect
to the associated readLock. That is, a thread successfully
acquiring the read lock will see all updates made upon previous
release of the write lock.
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 writer thread) can modify the shared data, in many cases any number of threads can concurrently read the data (hence reader 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.
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.
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: