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- All Known Implementing Classes:
ReentrantLock,ReentrantReadWriteLock.ReadLock,ReentrantReadWriteLock.WriteLock
public interface LockLockimplementations provide more extensive locking operations than can be obtained usingsynchronizedmethods and statements. They allow more flexible structuring, may have quite different properties, and may support multiple associatedConditionobjects.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
synchronizedmethods 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
synchronizedmethods 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 theLockinterface 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
synchronizedmethods and statements. In most cases, the following idiom should be used:
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 l = ...; l.lock(); try { // access the resource protected by this lock } finally { l.unlock(); }Lockimplementations provide additional functionality over the use ofsynchronizedmethods 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
Lockclass 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
Lockinstances are just normal objects and can themselves be used as the target in asynchronizedstatement. Acquiring the monitor lock of aLockinstance has no specified relationship with invoking any of thelock()methods of that instance. It is recommended that to avoid confusion you never useLockinstances in this way, except within their own implementation.Except where noted, passing a
nullvalue for any parameter will result in aNullPointerExceptionbeing thrown.Memory Synchronization
All
Lockimplementations must enforce the same memory synchronization semantics as provided by the built-in monitor lock, as described in Chapter 17 of The Java™ Language Specification:- A successful
lockoperation has the same memory synchronization effects as a successful Lock action. - A successful
unlockoperation 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
Lockclass. 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.
- Since:
- 1.5
- See Also:
ReentrantLock,Condition,ReadWriteLock
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Method Summary
All Methods Instance Methods Abstract Methods Modifier and Type Method Description voidlock()Acquires the lock.voidlockInterruptibly()Acquires the lock unless the current thread is interrupted.ConditionnewCondition()Returns a newConditioninstance that is bound to thisLockinstance.booleantryLock()Acquires the lock only if it is free at the time of invocation.booleantryLock(long time, TimeUnit unit)Acquires the lock if it is free within the given waiting time and the current thread has not been interrupted.voidunlock()Releases the lock.
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Method Detail
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lock
void lock()
Acquires the lock.If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until the lock has been acquired.
Implementation Considerations
A
Lockimplementation may be able to detect erroneous use of the lock, such as an invocation that would cause deadlock, and may throw an (unchecked) exception in such circumstances. The circumstances and the exception type must be documented by thatLockimplementation.
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lockInterruptibly
void lockInterruptibly() throws InterruptedExceptionAcquires the lock unless the current thread is interrupted.Acquires the lock if it is available and returns immediately.
If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:
- The lock is acquired by the current thread; or
- Some other thread interrupts the current thread, and interruption of lock acquisition is supported.
If the current thread:
- has its interrupted status set on entry to this method; or
- is interrupted while acquiring the lock, and interruption of lock acquisition is supported,
InterruptedExceptionis thrown and the current thread's interrupted status is cleared.Implementation Considerations
The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation. The programmer should be aware that this may be the case. An implementation should document when this is the case.
An implementation can favor responding to an interrupt over normal method return.
A
Lockimplementation may be able to detect erroneous use of the lock, such as an invocation that would cause deadlock, and may throw an (unchecked) exception in such circumstances. The circumstances and the exception type must be documented by thatLockimplementation.- Throws:
InterruptedException- if the current thread is interrupted while acquiring the lock (and interruption of lock acquisition is supported)
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tryLock
boolean tryLock()
Acquires the lock only if it is free at the time of invocation.Acquires the lock if it is available and returns immediately with the value
true. If the lock is not available then this method will return immediately with the valuefalse.A typical usage idiom for this method would be:
This usage ensures that the lock is unlocked if it was acquired, and doesn't try to unlock if the lock was not acquired.Lock lock = ...; if (lock.tryLock()) { try { // manipulate protected state } finally { lock.unlock(); } } else { // perform alternative actions }- Returns:
trueif the lock was acquired andfalseotherwise
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tryLock
boolean tryLock(long time, TimeUnit unit) throws InterruptedExceptionAcquires the lock if it is free within the given waiting time and the current thread has not been interrupted.If the lock is available this method returns immediately with the value
true. If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:- The lock is acquired by the current thread; or
- Some other thread interrupts the current thread, and interruption of lock acquisition is supported; or
- The specified waiting time elapses
If the lock is acquired then the value
trueis returned.If the current thread:
- has its interrupted status set on entry to this method; or
- is interrupted while acquiring the lock, and interruption of lock acquisition is supported,
InterruptedExceptionis thrown and the current thread's interrupted status is cleared.If the specified waiting time elapses then the value
falseis returned. If the time is less than or equal to zero, the method will not wait at all.Implementation Considerations
The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation. The programmer should be aware that this may be the case. An implementation should document when this is the case.
An implementation can favor responding to an interrupt over normal method return, or reporting a timeout.
A
Lockimplementation may be able to detect erroneous use of the lock, such as an invocation that would cause deadlock, and may throw an (unchecked) exception in such circumstances. The circumstances and the exception type must be documented by thatLockimplementation.- Parameters:
time- the maximum time to wait for the lockunit- the time unit of thetimeargument- Returns:
trueif the lock was acquired andfalseif the waiting time elapsed before the lock was acquired- Throws:
InterruptedException- if the current thread is interrupted while acquiring the lock (and interruption of lock acquisition is supported)
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unlock
void unlock()
Releases the lock.Implementation Considerations
A
Lockimplementation will usually impose restrictions on which thread can release a lock (typically only the holder of the lock can release it) and may throw an (unchecked) exception if the restriction is violated. Any restrictions and the exception type must be documented by thatLockimplementation.
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newCondition
Condition newCondition()
Returns a newConditioninstance that is bound to thisLockinstance.Before waiting on the condition the lock must be held by the current thread. A call to
Condition.await()will atomically release the lock before waiting and re-acquire the lock before the wait returns.Implementation Considerations
The exact operation of the
Conditioninstance depends on theLockimplementation and must be documented by that implementation.- Returns:
- A new
Conditioninstance for thisLockinstance - Throws:
UnsupportedOperationException- if thisLockimplementation does not support conditions
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