jdk.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:
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.
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 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.->reentrant-lockclj
(->reentrant-lock)(->reentrant-lock fair)Constructor.
Creates an instance of ReentrantLock with the given fairness policy.
fair - true if this lock should use a fair ordering policy - boolean
Constructor. Creates an instance of ReentrantLock with the given fairness policy. fair - true if this lock should use a fair ordering policy - `boolean`
fair?clj
(fair? this)Returns true if this lock has fairness set true.
returns: true if this lock has fairness set true - boolean
Returns true if this lock has fairness set true. returns: true if this lock has fairness set true - `boolean`
get-hold-countclj
(get-hold-count this)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 - int
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 - `int`get-queue-lengthclj
(get-queue-length this)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 - int
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 - `int`
get-wait-queue-lengthclj
(get-wait-queue-length this 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.
condition - the condition - java.util.concurrent.locks.Condition
returns: the estimated number of waiting threads - int
throws: java.lang.IllegalMonitorStateException - if this lock is not held
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 - `java.util.concurrent.locks.Condition` returns: the estimated number of waiting threads - `int` throws: java.lang.IllegalMonitorStateException - if this lock is not held
has-queued-thread?clj
(has-queued-thread? this thread)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 - java.lang.Thread
returns: true if the given thread is queued waiting for this lock - boolean
throws: java.lang.NullPointerException - if the thread 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 - `java.lang.Thread` returns: true if the given thread is queued waiting for this lock - `boolean` throws: java.lang.NullPointerException - if the thread is null
has-queued-threads?clj
(has-queued-threads? this)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.
returns: true if there may be other threads waiting to
acquire the lock - boolean
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.
returns: true if there may be other threads waiting to
acquire the lock - `boolean`has-waiters?clj
(has-waiters? this 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, 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 - java.util.concurrent.locks.Condition
returns: true if there are any waiting threads - boolean
throws: java.lang.IllegalMonitorStateException - if this lock is not held
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 - `java.util.concurrent.locks.Condition` returns: true if there are any waiting threads - `boolean` throws: java.lang.IllegalMonitorStateException - if this lock is not held
held-by-current-thread?clj
(held-by-current-thread? this)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 { 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 and
false otherwise - boolean
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 {
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 and
false otherwise - `boolean`lockclj
(lock this)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. 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.
lock-interruptiblyclj
(lock-interruptibly this)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,
then 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.
throws: java.lang.InterruptedException - if the current thread is interrupted
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, then 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. throws: java.lang.InterruptedException - if the current thread is interrupted
locked?clj
(locked? this)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 and
false otherwise - boolean
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 and
false otherwise - `boolean`new-conditionclj
(new-condition this)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.
If this lock is not held when any of the Condition waiting or signalling methods are called, then an IllegalMonitorStateException 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.
returns: the Condition object - java.util.concurrent.locks.Condition
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. If this lock is not held when any of the Condition waiting or signalling methods are called, then an IllegalMonitorStateException 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. returns: the Condition object - `java.util.concurrent.locks.Condition`
to-stringclj
(to-string this)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.
returns: a string identifying this lock, as well as its lock state - java.lang.String
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. returns: a string identifying this lock, as well as its lock state - `java.lang.String`
try-lockclj
(try-lock this)(try-lock this timeout 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.
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:
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,
then 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 - long
unit - the time unit of the timeout argument - java.util.concurrent.TimeUnit
returns: 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 - boolean
throws: java.lang.InterruptedException - if the current thread is interrupted
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:
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,
then 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 - `long`
unit - the time unit of the timeout argument - `java.util.concurrent.TimeUnit`
returns: 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 - `boolean`
throws: java.lang.InterruptedException - if the current thread is interruptedunlockclj
(unlock this)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.
throws: java.lang.IllegalMonitorStateException - if the current thread does not hold this lock
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. throws: java.lang.IllegalMonitorStateException - if the current thread does not hold this lock
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