- java.lang.Object
-
- java.util.concurrent.locks.ReentrantLock
-
- All Implemented Interfaces:
- Serializable, Lock
public class ReentrantLock extends Object implements Lock, Serializable
A reentrant mutual exclusionLock
with the same basic behavior and semantics as the implicit monitor lock accessed usingsynchronized
methods and statements, but with extended capabilities.A
ReentrantLock
is owned by the thread last successfully locking, but not yet unlocking it. A thread invokinglock
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 methodsisHeldByCurrentThread()
, andgetHoldCount()
.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 untimedtryLock
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 atry
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 methodsisLocked
andgetLockQueueLength
, as well as some associatedprotected
access methods that may be 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:
- Serialized Form
-
-
Constructor Summary
Constructors Constructor and Description ReentrantLock()
Creates an instance ofReentrantLock
.ReentrantLock(boolean fair)
Creates an instance ofReentrantLock
with the given fairness policy.
-
Method Summary
Methods Modifier and Type Method and Description int
getHoldCount()
Queries the number of holds on this lock by the current thread.protected Thread
getOwner()
Returns the thread that currently owns this lock, ornull
if not owned.protected Collection<Thread>
getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire this lock.int
getQueueLength()
Returns 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.int
getWaitQueueLength(Condition condition)
Returns an estimate of the number of threads waiting on the given condition associated with this lock.boolean
hasQueuedThread(Thread thread)
Queries whether the given thread is waiting to acquire this lock.boolean
hasQueuedThreads()
Queries whether any threads are waiting to acquire this lock.boolean
hasWaiters(Condition condition)
Queries whether any threads are waiting on the given condition associated with this lock.boolean
isFair()
Returnstrue
if this lock has fairness set true.boolean
isHeldByCurrentThread()
Queries if this lock is held by the current thread.boolean
isLocked()
Queries if this lock is held by any thread.void
lock()
Acquires the lock.void
lockInterruptibly()
Acquires the lock unless the current thread is interrupted.Condition
newCondition()
String
toString()
Returns a string identifying this lock, as well as its lock state.boolean
tryLock()
Acquires the lock only if it is not held by another thread at the time of invocation.boolean
tryLock(long timeout, TimeUnit unit)
Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted.void
unlock()
Attempts to release this lock.
-
-
-
Constructor Detail
-
ReentrantLock
public ReentrantLock()
Creates an instance ofReentrantLock
. This is equivalent to usingReentrantLock(false)
.
-
ReentrantLock
public ReentrantLock(boolean fair)
Creates an instance ofReentrantLock
with the given fairness policy.- Parameters:
fair
-true
if this lock should use a fair ordering policy
-
-
Method Detail
-
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.
-
lockInterruptibly
public void lockInterruptibly() throws InterruptedException
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,
InterruptedException
is 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:
lockInterruptibly
in interfaceLock
- Throws:
InterruptedException
- if the current thread is interrupted
-
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
public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException
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 timedtryLock
that 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
true
is 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,
InterruptedException
is thrown and the current thread's interrupted status is cleared.If the specified waiting time elapses then the value
false
is 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:
tryLock
in interfaceLock
- Parameters:
timeout
- the time to wait for the lockunit
- the time unit of the timeout argument- Returns:
true
if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; andfalse
if the waiting time elapsed before the lock could be acquired- Throws:
InterruptedException
- if the current thread is interruptedNullPointerException
- if the time unit is null
-
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
IllegalMonitorStateException
is thrown.- Specified by:
unlock
in interfaceLock
- Throws:
IllegalMonitorStateException
- if the current thread does not hold this lock
-
newCondition
public Condition newCondition()
Returns aCondition
instance for use with thisLock
instance.The returned
Condition
instance supports the same usages as do theObject
monitor methods (wait
,notify
, andnotifyAll
) when used with the built-in monitor lock.- If this lock is not held when any of the
Condition
waiting or signalling methods are called, then anIllegalMonitorStateException
is 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
InterruptedException
will 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:
newCondition
in interfaceLock
- Returns:
- the Condition object
- If this lock is not held when any of the
-
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 { 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
-
isHeldByCurrentThread
public boolean isHeldByCurrentThread()
Queries if this lock is held by the current thread.Analogous to the
Thread.holdsLock(java.lang.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 { 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 { ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert !lock.isHeldByCurrentThread(); lock.lock(); try { // ... method body } finally { lock.unlock(); } } }
- Returns:
true
if current thread holds this lock andfalse
otherwise
-
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:
true
if any thread holds this lock andfalse
otherwise
-
isFair
public final boolean isFair()
Returnstrue
if this lock has fairness set true.- Returns:
true
if this lock has fairness set true
-
getOwner
protected Thread getOwner()
Returns the thread that currently owns this lock, ornull
if 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 momentarilynull
even 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
null
if not owned
-
hasQueuedThreads
public final boolean hasQueuedThreads()
Queries whether any threads are waiting to acquire this lock. Note that because cancellations may occur at any time, atrue
return 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:
true
if there may be other threads waiting to acquire the lock
-
hasQueuedThread
public final boolean hasQueuedThread(Thread thread)
Queries whether the given thread is waiting to acquire this lock. Note that because cancellations may occur at any time, atrue
return 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:
true
if the given thread is queued waiting for this lock- Throws:
NullPointerException
- if the thread is null
-
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 of the system state, not for synchronization control.- Returns:
- the estimated number of threads waiting for this lock
-
getQueuedThreads
protected Collection<Thread> 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
-
hasWaiters
public boolean hasWaiters(Condition condition)
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, atrue
return does not guarantee that a futuresignal
will awaken any threads. This method is designed primarily for use in monitoring of the system state.- Parameters:
condition
- the condition- Returns:
true
if 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
-
getWaitQueueLength
public int getWaitQueueLength(Condition condition)
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
-
getWaitingThreads
protected Collection<Thread> getWaitingThreads(Condition condition)
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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Dokument erstellt 11/06/2005, zuletzt geändert 04/03/2020
Quelle des gedruckten Dokuments:https://www.gaudry.be/de/java-api-rf-java/util/concurrent/locks/ReentrantLock.html
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