| java.lang.Object | |
| ↳ | java.util.concurrent.locks.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:
Developers
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.
| Public Constructors | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Creates an instance of
ReentrantLock.
| |||||||||||
Creates an instance of
ReentrantLock with the
given fairness policy.
| |||||||||||
| Public Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Queries the number of holds on this lock by the current thread.
| |||||||||||
Returns an estimate of the number of threads waiting to
acquire this lock.
| |||||||||||
Returns an estimate of the number of threads waiting on the
given condition associated with this lock.
| |||||||||||
Queries whether the given thread is waiting to acquire this
lock.
| |||||||||||
Queries whether any threads are waiting to acquire this lock.
| |||||||||||
Queries whether any threads are waiting on the given condition
associated with this lock.
| |||||||||||
Returns
true if this lock has fairness set true.
| |||||||||||
Queries if this lock is held by the current thread.
| |||||||||||
Queries if this lock is held by any thread.
| |||||||||||
Acquires the lock.
| |||||||||||
Acquires the lock unless the current thread is
interrupted.
| |||||||||||
Returns a string identifying this lock, as well as its lock state.
| |||||||||||
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 within the given
waiting time and the current thread has not been
interrupted.
| |||||||||||
Attempts to release this lock.
| |||||||||||
| Protected Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Returns the thread that currently owns this lock, or
null if not owned.
| |||||||||||
Returns a collection containing threads that may be waiting to
acquire this lock.
| |||||||||||
Returns a collection containing those threads that may be
waiting on the given condition associated with this lock.
| |||||||||||
|
[Expand]
Inherited Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
From class
java.lang.Object
| |||||||||||
|
From interface
java.util.concurrent.locks.Lock
| |||||||||||
Creates an instance of ReentrantLock.
This is equivalent to using ReentrantLock(false).
Creates an instance of ReentrantLock with the
given fairness policy.
| fair | true if this lock should use a fair ordering policy
|
|---|
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 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 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.
| condition | the condition |
|---|
| IllegalMonitorStateException | if this lock is not held |
|---|---|
| IllegalArgumentException | if the given condition is not associated with this lock |
| NullPointerException | if the condition is null |
Queries whether the given thread is waiting to acquire this
lock. Note that because cancellations may occur at any time, a
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 thread |
|---|
true if the given thread is queued waiting for this lock| NullPointerException | if the thread is null |
|---|
Queries whether any threads are waiting to acquire this lock. Note that
because cancellations may occur at any time, a 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 lock
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, a 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 condition |
|---|
true if there are any waiting threads| IllegalMonitorStateException | if this lock is not held |
|---|---|
| IllegalArgumentException | if the given condition is not associated with this lock |
| NullPointerException | if the condition is null |
Returns true if this lock has fairness set true.
true if this lock has fairness set true
Queries if this lock is held by the current thread.
Analogous to the 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 {
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 otherwise
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.
true if any thread holds this lock and
false otherwise
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.
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:
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.
| InterruptedException | if the current thread is interrupted |
|---|
Returns a 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.
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.
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 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.
true if the lock was free and was acquired by the
current thread, or the lock was already held by the current
thread; and false otherwise
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 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.
| timeout | the time to wait for the lock |
|---|---|
| unit | the time unit of the timeout argument |
true 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 acquired| InterruptedException | if the current thread is interrupted |
|---|---|
| NullPointerException | if the time unit is null |
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.
| IllegalMonitorStateException | if the current thread does not hold this lock |
|---|
Returns the thread that currently owns this lock, or
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 owned
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 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.
| condition | the condition |
|---|
| IllegalMonitorStateException | if this lock is not held |
|---|---|
| IllegalArgumentException | if the given condition is not associated with this lock |
| NullPointerException | if the condition is null |