public class ReentrantLock extends Object implements Lock, Serializable
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.
| Modifier and Type | Class and Description |
|---|---|
(package private) static class |
ReentrantLock.FairSync
Sync object for fair locks
|
(package private) static class |
ReentrantLock.NonfairSync
Sync object for non-fair locks
|
(package private) static class |
ReentrantLock.Sync
Base of synchronization control for this lock.
|
| Constructor and Description |
|---|
ReentrantLock()
Creates an instance of
ReentrantLock. |
ReentrantLock(boolean fair)
Creates an instance of
ReentrantLock with the
given fairness policy. |
| 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, or
null 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()
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()
|
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.
|
public ReentrantLock()
ReentrantLock.
This is equivalent to using ReentrantLock(false).public ReentrantLock(boolean fair)
ReentrantLock with the
given fairness policy.fair - true if this lock should use a fair ordering policypublic void 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.
public void lockInterruptibly() throws InterruptedException
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:
If the lock is acquired by the current thread then the lock hold count is set to one.
If the current thread:
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 LockInterruptedException - if the current thread is interruptedpublic boolean tryLock()
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 to tryLock() 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 use
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.
public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException
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:
If the lock is acquired then the value true is returned and
the lock hold count is set to one.
If the current thread:
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 Locktimeout - the time to wait for the lockunit - the time unit of the timeout argumenttrue if the lock was free and was acquired by the
current thread, or the lock was already held by the current
thread; and false if the waiting time elapsed before
the lock could be acquiredInterruptedException - if the current thread is interruptedNullPointerException - if the time unit is nullpublic void unlock()
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.
unlock in interface LockIllegalMonitorStateException - if the current thread does not
hold this lockpublic 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.
InterruptedException will be thrown, and the thread's
interrupted status will be cleared.
newCondition in interface Lockpublic int getHoldCount()
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();
}
}
}
public boolean isHeldByCurrentThread()
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();
}
}
}
true if current thread holds this lock and
false otherwisepublic boolean isLocked()
true if any thread holds this lock and
false otherwisepublic final boolean isFair()
true if this lock has fairness set true.true if this lock has fairness set trueprotected Thread getOwner()
null 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 momentarily null 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.null if not ownedpublic final boolean hasQueuedThreads()
true
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.true if there may be other threads waiting to
acquire the lockpublic final boolean hasQueuedThread(Thread thread)
true 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.thread - the threadtrue if the given thread is queued waiting for this lockNullPointerException - if the thread is nullpublic final int getQueueLength()
protected Collection<Thread> getQueuedThreads()
public boolean hasWaiters(Condition condition)
true return does
not guarantee that a future signal will awaken any
threads. This method is designed primarily for use in
monitoring of the system state.condition - the conditiontrue if there are any waiting threadsIllegalMonitorStateException - if this lock is not heldIllegalArgumentException - if the given condition is
not associated with this lockNullPointerException - if the condition is nullpublic int getWaitQueueLength(Condition condition)
condition - the conditionIllegalMonitorStateException - if this lock is not heldIllegalArgumentException - if the given condition is
not associated with this lockNullPointerException - if the condition is nullprotected Collection<Thread> getWaitingThreads(Condition condition)
condition - the conditionIllegalMonitorStateException - if this lock is not heldIllegalArgumentException - if the given condition is
not associated with this lockNullPointerException - if the condition is null