- All Implemented Interfaces:
Serializable,Lock
Lock with the same basic
behavior and semantics as the implicit monitor lock accessed using
synchronized methods and statements, but with extended
capabilities.
A ReentrantLock is owned by the thread last
successfully locking, but not yet unlocking it. A thread invoking
lock will return, successfully acquiring the lock, when
the lock is not owned by another thread. The method will return
immediately if the current thread already owns the lock. This can
be checked using methods isHeldByCurrentThread(), and getHoldCount().
The constructor for this class accepts an optional
fairness parameter. When set true, under
contention, locks favor granting access to the longest-waiting
thread. Otherwise this lock does not guarantee any particular
access order. Programs using fair locks accessed by many threads
may display lower overall throughput (i.e., are slower; often much
slower) than those using the default setting, but have smaller
variances in times to obtain locks and guarantee lack of
starvation. Note however, that fairness of locks does not guarantee
fairness of thread scheduling. Thus, one of many threads using a
fair lock may obtain it multiple times in succession while other
active threads are not progressing and not currently holding the
lock.
Also note that the untimed tryLock() method does not
honor the fairness setting. It will succeed if the lock
is available even if other threads are waiting.
It is recommended practice to always immediately
follow a call to lock with a try block, most
typically in a before/after construction such as:
class X {
private final ReentrantLock lock = new ReentrantLock();
// ...
public void m() {
lock.lock(); // block until condition holds
try {
// ... method body
} finally {
lock.unlock();
}
}
}
In addition to implementing the Lock interface, this
class defines a number of public and protected
methods for inspecting the state of the lock. Some of these
methods are only useful for instrumentation and monitoring.
Serialization of this class behaves in the same way as built-in locks: a deserialized lock is in the unlocked state, regardless of its state when serialized.
This lock supports a maximum of 2147483647 recursive locks by
the same thread. Attempts to exceed this limit result in
Error throws from locking methods.
- Since:
- 1.5
- See Also:
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Constructor Summary
ConstructorsConstructorDescriptionCreates an instance ofReentrantLock.ReentrantLock(boolean fair) Creates an instance ofReentrantLockwith the given fairness policy. -
Method Summary
Modifier and TypeMethodDescriptionintQueries the number of holds on this lock by the current thread.protected ThreadgetOwner()Returns the thread that currently owns this lock, ornullif not owned.protected Collection<Thread> Returns a collection containing threads that may be waiting to acquire this lock.final intReturns an estimate of the number of threads waiting to acquire this lock.protected Collection<Thread> getWaitingThreads(Condition condition) Returns a collection containing those threads that may be waiting on the given condition associated with this lock.intgetWaitQueueLength(Condition condition) Returns an estimate of the number of threads waiting on the given condition associated with this lock.final booleanhasQueuedThread(Thread thread) Queries whether the given thread is waiting to acquire this lock.final booleanQueries whether any threads are waiting to acquire this lock.booleanhasWaiters(Condition condition) Queries whether any threads are waiting on the given condition associated with this lock.final booleanisFair()Returnstrueif this lock has fairness set true.booleanQueries if this lock is held by the current thread.booleanisLocked()Queries if this lock is held by any thread.voidlock()Acquires the lock.voidAcquires the lock unless the current thread is interrupted.toString()Returns a string identifying this lock, as well as its lock state.booleantryLock()Acquires the lock only if it is not held by another thread at the time of invocation.booleanAcquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted.voidunlock()Attempts to release this lock.
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Constructor Details
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ReentrantLock
public ReentrantLock()Creates an instance ofReentrantLock. This is equivalent to usingReentrantLock(false). -
ReentrantLock
public ReentrantLock(boolean fair) Creates an instance ofReentrantLockwith the given fairness policy.- Parameters:
fair-trueif this lock should use a fair ordering policy
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Method Details
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lock
public void lock()Acquires the lock.Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one.
If the current thread already holds the lock then the hold count is incremented by one and the method returns immediately.
If the lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until the lock has been acquired, at which time the lock hold count is set to one.
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lockInterruptibly
Acquires the lock unless the current thread is interrupted.Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one.
If the current thread already holds this lock then the hold count is incremented by one and the method returns immediately.
If the lock is held by another thread 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.
If the lock is acquired by the current thread then the lock hold count is set to one.
If the current thread:
- has its interrupted status set on entry to this method; or
- is interrupted while acquiring the lock,
InterruptedExceptionis thrown and the current thread's interrupted status is cleared.In this implementation, as this method is an explicit interruption point, preference is given to responding to the interrupt over normal or reentrant acquisition of the lock.
- Specified by:
lockInterruptiblyin interfaceLock- Throws:
InterruptedException- if the current thread is interrupted
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tryLock
public boolean tryLock()Acquires the lock only if it is not held by another thread at the time of invocation.Acquires the lock if it is not held by another thread and returns immediately with the value
true, setting the lock hold count to one. Even when this lock has been set to use a fair ordering policy, a call totryLock()will immediately acquire the lock if it is available, whether or not other threads are currently waiting for the lock. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness. If you want to honor the fairness setting for this lock, then usetryLock(0, TimeUnit.SECONDS)which is almost equivalent (it also detects interruption).If the current thread already holds this lock then the hold count is incremented by one and the method returns
true.If the lock is held by another thread then this method will return immediately with the value
false. -
tryLock
Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted.Acquires the lock if it is not held by another thread and returns immediately with the value
true, setting the lock hold count to one. If this lock has been set to use a fair ordering policy then an available lock will not be acquired if any other threads are waiting for the lock. This is in contrast to thetryLock()method. If you want a timedtryLockthat does permit barging on a fair lock then combine the timed and un-timed forms together:if (lock.tryLock() || lock.tryLock(timeout, unit)) { ... }If the current thread already holds this lock then the hold count is incremented by one and the method returns
true.If the lock is held by another thread 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; or
- The specified waiting time elapses
If the lock is acquired then the value
trueis returned and the lock hold count is set to one.If the current thread:
- has its interrupted status set on entry to this method; or
- is interrupted while acquiring the lock,
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.In this implementation, as this method is an explicit interruption point, preference is given to responding to the interrupt over normal or reentrant acquisition of the lock, and over reporting the elapse of the waiting time.
- Specified by:
tryLockin interfaceLock- Parameters:
timeout- the time to wait for the lockunit- the time unit of the timeout argument- Returns:
trueif the lock was free and was acquired by the current thread, or the lock was already held by the current thread; andfalseif the waiting time elapsed before the lock could be acquired- Throws:
InterruptedException- if the current thread is interruptedNullPointerException- if the time unit is null
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unlock
public void unlock()Attempts to release this lock.If the current thread is the holder of this lock then the hold count is decremented. If the hold count is now zero then the lock is released. If the current thread is not the holder of this lock then
IllegalMonitorStateExceptionis thrown.- Specified by:
unlockin interfaceLock- Throws:
IllegalMonitorStateException- if the current thread does not hold this lock
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newCondition
Returns aConditioninstance for use with thisLockinstance.The returned
Conditioninstance supports the same usages as do theObjectmonitor methods (wait,notify, andnotifyAll) when used with the built-in monitor lock.- If this lock is not held when any of the
Conditionwaiting or signalling methods are called, then anIllegalMonitorStateExceptionis thrown. - When the condition waiting methods are called the lock is released and, before they return, the lock is reacquired and the lock hold count restored to what it was when the method was called.
- If a thread is interrupted
while waiting then the wait will terminate, an
InterruptedExceptionwill be thrown, and the thread's interrupted status will be cleared. - Waiting threads are signalled in FIFO order.
- The ordering of lock reacquisition for threads returning from waiting methods is the same as for threads initially acquiring the lock, which is in the default case not specified, but for fair locks favors those threads that have been waiting the longest.
- Specified by:
newConditionin interfaceLock- Returns:
- the Condition object
- If this lock is not held when any of the
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getHoldCount
public int getHoldCount()Queries the number of holds on this lock by the current thread.A thread has a hold on a lock for each lock action that is not matched by an unlock action.
The hold count information is typically only used for testing and debugging purposes. For example, if a certain section of code should not be entered with the lock already held then we can assert that fact:
class X { final ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert lock.getHoldCount() == 0; lock.lock(); try { // ... method body } finally { lock.unlock(); } } }- Returns:
- the number of holds on this lock by the current thread, or zero if this lock is not held by the current thread
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isHeldByCurrentThread
public boolean isHeldByCurrentThread()Queries if this lock is held by the current thread.Analogous to the
Thread.holdsLock(Object)method for built-in monitor locks, this method is typically used for debugging and testing. For example, a method that should only be called while a lock is held can assert that this is the case:class X { final ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert lock.isHeldByCurrentThread(); // ... method body } }It can also be used to ensure that a reentrant lock is used in a non-reentrant manner, for example:
class X { final ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert !lock.isHeldByCurrentThread(); lock.lock(); try { // ... method body } finally { lock.unlock(); } } }- Returns:
trueif current thread holds this lock andfalseotherwise
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isLocked
public boolean isLocked()Queries if this lock is held by any thread. This method is designed for use in monitoring of the system state, not for synchronization control.- Returns:
trueif any thread holds this lock andfalseotherwise
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isFair
public final boolean isFair()Returnstrueif this lock has fairness set true.- Returns:
trueif this lock has fairness set true
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getOwner
Returns the thread that currently owns this lock, ornullif not owned. When this method is called by a thread that is not the owner, the return value reflects a best-effort approximation of current lock status. For example, the owner may be momentarilynulleven if there are threads trying to acquire the lock but have not yet done so. This method is designed to facilitate construction of subclasses that provide more extensive lock monitoring facilities.- Returns:
- the owner, or
nullif not owned
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hasQueuedThreads
public final boolean hasQueuedThreads()Queries whether any threads are waiting to acquire this lock. Note that because cancellations may occur at any time, atruereturn does not guarantee that any other thread will ever acquire this lock. This method is designed primarily for use in monitoring of the system state.- Returns:
trueif there may be other threads waiting to acquire the lock
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hasQueuedThread
Queries whether the given thread is waiting to acquire this lock. Note that because cancellations may occur at any time, atruereturn does not guarantee that this thread will ever acquire this lock. This method is designed primarily for use in monitoring of the system state.- Parameters:
thread- the thread- Returns:
trueif the given thread is queued waiting for this lock- Throws:
NullPointerException- if the thread is null
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getQueueLength
public final int getQueueLength()Returns an estimate of the number of threads waiting to acquire this lock. The value is only an estimate because the number of threads may change dynamically while this method traverses internal data structures. This method is designed for use in monitoring system state, not for synchronization control.- Returns:
- the estimated number of threads waiting for this lock
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getQueuedThreads
Returns a collection containing threads that may be waiting to acquire this lock. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive monitoring facilities.- Returns:
- the collection of threads
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hasWaiters
Queries whether any threads are waiting on the given condition associated with this lock. Note that because timeouts and interrupts may occur at any time, atruereturn does not guarantee that a futuresignalwill awaken any threads. This method is designed primarily for use in monitoring of the system state.- Parameters:
condition- the condition- Returns:
trueif there are any waiting threads- Throws:
IllegalMonitorStateException- if this lock is not heldIllegalArgumentException- if the given condition is not associated with this lockNullPointerException- if the condition is null
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getWaitQueueLength
Returns an estimate of the number of threads waiting on the given condition associated with this lock. Note that because timeouts and interrupts may occur at any time, the estimate serves only as an upper bound on the actual number of waiters. This method is designed for use in monitoring of the system state, not for synchronization control.- Parameters:
condition- the condition- Returns:
- the estimated number of waiting threads
- Throws:
IllegalMonitorStateException- if this lock is not heldIllegalArgumentException- if the given condition is not associated with this lockNullPointerException- if the condition is null
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getWaitingThreads
Returns a collection containing those threads that may be waiting on the given condition associated with this lock. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive condition monitoring facilities.- Parameters:
condition- the condition- Returns:
- the collection of threads
- Throws:
IllegalMonitorStateException- if this lock is not heldIllegalArgumentException- if the given condition is not associated with this lockNullPointerException- if the condition is null
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toString
Returns a string identifying this lock, as well as its lock state. The state, in brackets, includes either the String"Unlocked"or the String"Locked by"followed by the name of the owning thread.
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