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Added in API level 1


Interfaces and classes providing a framework for locking and waiting for conditions that is distinct from built-in synchronization and monitors. The framework permits much greater flexibility in the use of locks and conditions, at the expense of more awkward syntax.

The Lock interface supports locking disciplines that differ in semantics (reentrant, fair, etc), and that can be used in non-block-structured contexts including hand-over-hand and lock reordering algorithms. The main implementation is ReentrantLock.

The ReadWriteLock interface similarly defines locks that may be shared among readers but are exclusive to writers. Only a single implementation, ReentrantReadWriteLock, is provided, since it covers most standard usage contexts. But programmers may create their own implementations to cover nonstandard requirements.

The Condition interface describes condition variables that may be associated with Locks. These are similar in usage to the implicit monitors accessed using Object.wait, but offer extended capabilities. In particular, multiple Condition objects may be associated with a single Lock. To avoid compatibility issues, the names of Condition methods are different from the corresponding Object versions.

The AbstractQueuedSynchronizer class serves as a useful superclass for defining locks and other synchronizers that rely on queuing blocked threads. The AbstractQueuedLongSynchronizer class provides the same functionality but extends support to 64 bits of synchronization state. Both extend class AbstractOwnableSynchronizer, a simple class that helps record the thread currently holding exclusive synchronization. The LockSupport class provides lower-level blocking and unblocking support that is useful for those developers implementing their own customized lock classes.


Condition Condition factors out the Object monitor methods (wait, notify and notifyAll) into distinct objects to give the effect of having multiple wait-sets per object, by combining them with the use of arbitrary Lock implementations. 
Lock Lock implementations provide more extensive locking operations than can be obtained using synchronized methods and statements. 
ReadWriteLock A ReadWriteLock maintains a pair of associated locks, one for read-only operations and one for writing. 


AbstractOwnableSynchronizer A synchronizer that may be exclusively owned by a thread. 
AbstractQueuedLongSynchronizer A version of AbstractQueuedSynchronizer in which synchronization state is maintained as a long
AbstractQueuedLongSynchronizer.ConditionObject Condition implementation for a AbstractQueuedLongSynchronizer serving as the basis of a Lock implementation. 
AbstractQueuedSynchronizer Provides a framework for implementing blocking locks and related synchronizers (semaphores, events, etc) that rely on first-in-first-out (FIFO) wait queues. 
AbstractQueuedSynchronizer.ConditionObject Condition implementation for a AbstractQueuedSynchronizer serving as the basis of a Lock implementation. 
LockSupport Basic thread blocking primitives for creating locks and other synchronization classes. 
ReentrantLock A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities. 
ReentrantReadWriteLock An implementation of ReadWriteLock supporting similar semantics to ReentrantLock
ReentrantReadWriteLock.ReadLock The lock returned by method readLock()
ReentrantReadWriteLock.WriteLock The lock returned by method writeLock()
StampedLock A capability-based lock with three modes for controlling read/write access. 
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