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java.lang.Object org.apache.openjpa.lib.util.concurrent.ReentrantLock
public class 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.
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 methods isLocked and
getLockQueueLength, as well as some associated
protected 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.
Constructor Summary | |
---|---|
ReentrantLock()
Creates an instance of ReentrantLock. |
|
ReentrantLock(boolean fair)
Creates an instance of ReentrantLock with the given fairness policy. |
Method Summary | |
---|---|
int |
getHoldCount()
Queries the number of holds on this lock by the current thread. |
protected Thread |
getOwner()
null if not owned. |
protected Collection |
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 |
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()
Returns true 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()
Returns a Condition instance for use with this
Lock instance. |
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. |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait |
Constructor Detail |
---|
public ReentrantLock()
public ReentrantLock(boolean fair)
fair
- true if this lock will be fair; else falseMethod Detail |
---|
public void lock()
lock
in interface Lock
public void lockInterruptibly() throws InterruptedException
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:
interrupts
the current
thread.
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.
lockInterruptibly
in interface Lock
InterruptedException
- if the current thread is interruptedThread.interrupt()
public boolean tryLock()
tryLock(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
in interface Lock
public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException
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 the tryLock()
method. If you want a timed tryLock 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:
interrupts
the current
thread; or
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.
tryLock
in interface Lock
timeout
- the time to wait for the lockunit
- the time unit of the timeout argument
InterruptedException
- if the current thread is interrupted
NullPointerException
- if unit is nullThread.interrupt()
public void unlock()
IllegalMonitorStateException
is thrown.
unlock
in interface Lock
IllegalMonitorStateException
- if the current thread does not
hold this lock.public Condition newCondition()
Condition
instance for use with this
Lock
instance.
The returned Condition
instance supports the same
usages as do the Object
monitor methods(wait
, notify
, and notifyAll
) when used with the built-in monitor lock.
Condition
waiting
or signalling
methods are called, then an IllegalMonitorStateException
is thrown.
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.
interrupted
while
waiting then the wait will terminate, an InterruptedException
will be thrown, and the thread's
interrupted status will be cleared.
newCondition
in interface Lock
public int getHoldCount()
class X { ReentrantLock lock = new ReentrantLock(); // ... public void m() { assert lock.getHoldCount() == 0; lock.lock(); try { // ... method body } finally { lock.unlock(); } } }
public boolean isHeldByCurrentThread()
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(); } } }
public boolean isLocked()
public final boolean isFair()
protected Thread getOwner()
public final boolean hasQueuedThreads()
public final boolean hasQueuedThread(Thread thread)
thread
- the thread
NullPointerException
- if thread is nullpublic final int getQueueLength()
protected Collection getQueuedThreads()
public boolean hasWaiters(Condition condition)
condition
- the condition
IllegalMonitorStateException
- if this lock is not held
IllegalArgumentException
- if the given condition is
not associated with this lock
NullPointerException
- if condition nullpublic int getWaitQueueLength(Condition condition)
condition
- the condition
IllegalMonitorStateException
- if this lock is not held
IllegalArgumentException
- if the given condition is
not associated with this lock
NullPointerException
- if condition nullprotected Collection getWaitingThreads(Condition condition)
condition
- the condition
IllegalMonitorStateException
- if this lock is not held
IllegalArgumentException
- if the given condition is
not associated with this lock
NullPointerException
- if condition nullpublic String toString()
Thread.getName()
of the owning thread.
toString
in class Object
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