Interface Lock
Lock implementations provide more extensive locking operations than can be obtained using synchronized methods and statements. They allow more flexible structuring, may have quite different properties, and may support multiple associated Condition objects.
A lock is a tool for controlling access to a shared resource by multiple threads. Commonly, a lock provides exclusive access to a shared resource: only one thread at a time can acquire the lock and all access to the shared resource requires that the lock be acquired first. However, some locks may allow concurrent access to a shared resource, such as the read lock of a ReadWriteLock .
The use of synchronized methods or statements provides access to the implicit monitor lock associated with every object, but forces all lock acquisition and release to occur in a block-structured way: when multiple locks are acquired they must be released in the opposite order, and all locks must be released in the same lexical scope in which they were acquired.
While the scoping mechanism for synchronized methods and statements makes it much easier to program with monitor locks, and helps avoid many common programming errors involving locks, there are occasions where you need to work with locks in a more flexible way. For example, some algorithms for traversing concurrently accessed data structures require the use of «hand-over-hand» or «chain locking»: you acquire the lock of node A, then node B, then release A and acquire C, then release B and acquire D and so on. Implementations of the Lock interface enable the use of such techniques by allowing a lock to be acquired and released in different scopes, and allowing multiple locks to be acquired and released in any order.
With this increased flexibility comes additional responsibility. The absence of block-structured locking removes the automatic release of locks that occurs with synchronized methods and statements. In most cases, the following idiom should be used:
Lock l = . ; l.lock(); try < // access the resource protected by this lock >finally
When locking and unlocking occur in different scopes, care must be taken to ensure that all code that is executed while the lock is held is protected by try-finally or try-catch to ensure that the lock is released when necessary.
Lock implementations provide additional functionality over the use of synchronized methods and statements by providing a non-blocking attempt to acquire a lock ( tryLock() ), an attempt to acquire the lock that can be interrupted ( lockInterruptibly() , and an attempt to acquire the lock that can timeout ( tryLock(long, TimeUnit) ).
A Lock class can also provide behavior and semantics that is quite different from that of the implicit monitor lock, such as guaranteed ordering, non-reentrant usage, or deadlock detection. If an implementation provides such specialized semantics then the implementation must document those semantics.
Note that Lock instances are just normal objects and can themselves be used as the target in a synchronized statement. Acquiring the monitor lock of a Lock instance has no specified relationship with invoking any of the lock() methods of that instance. It is recommended that to avoid confusion you never use Lock instances in this way, except within their own implementation.
Except where noted, passing a null value for any parameter will result in a NullPointerException being thrown.
Memory Synchronization
- A successful lock operation has the same memory synchronization effects as a successful Lock action.
- A successful unlock operation has the same memory synchronization effects as a successful Unlock action.
Implementation Considerations
The three forms of lock acquisition (interruptible, non-interruptible, and timed) may differ in their performance characteristics, ordering guarantees, or other implementation qualities. Further, the ability to interrupt the ongoing acquisition of a lock may not be available in a given Lock class. Consequently, an implementation is not required to define exactly the same guarantees or semantics for all three forms of lock acquisition, nor is it required to support interruption of an ongoing lock acquisition. An implementation is required to clearly document the semantics and guarantees provided by each of the locking methods. It must also obey the interruption semantics as defined in this interface, to the extent that interruption of lock acquisition is supported: which is either totally, or only on method entry.
As interruption generally implies cancellation, and checks for interruption are often infrequent, an implementation can favor responding to an interrupt over normal method return. This is true even if it can be shown that the interrupt occurred after another action may have unblocked the thread. An implementation should document this behavior.
Package java.util.concurrent.locks
Interfaces and classes providing a framework for locking and waiting for conditions that is distinct from built-in synchronization and monitors.
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 implementations provide more extensive locking operations than can be obtained using synchronized methods and statements.
A ReadWriteLock maintains a pair of associated locks , one for read-only operations and one for writing.
Provides a framework for implementing blocking locks and related synchronizers (semaphores, events, etc) that rely on first-in-first-out (FIFO) wait queues.
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.
Package java.util.concurrent.locks Description
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.
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For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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Package java.util.concurrent.locks
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.
Package java.util.concurrent.locks
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.