edu.emory.mathcs.backport.java.util.concurrent.locks
public class ReentrantLock extends Object implements Lock, Serializable, CondVar.ExclusiveLock
A {@code ReentrantLock} is owned by the thread last successfully locking, but not yet unlocking it. A thread invoking {@code 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 ReentrantLock, and ReentrantLock.
The constructor for this class accepts an optional
fairness parameter. When set {@code 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 {@code lock} with a {@code 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 {@code isLocked} and {@code getLockQueueLength}, as well as some associated {@code 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.
Since: 1.5
Constructor Summary | |
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ReentrantLock()
Creates an instance of {@code ReentrantLock}.
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ReentrantLock(boolean fair)
Creates an instance of {@code ReentrantLock} with the
given fairness policy.
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Method Summary | |
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int | getHoldCount()
Queries the number of holds on this lock by the current thread.
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protected Thread | getOwner()
Returns the thread that currently owns this lock, or
{@code 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 {@code true} if this lock has fairness set true.
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boolean | isHeldByCurrentThread()
Queries if this lock is held by the current thread.
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boolean | isLocked()
Queries if this lock is held by any thread. |
void | lock()
Acquires the lock.
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void | lockInterruptibly()
Acquires the lock unless the current thread is
Thread#interrupt interrupted.
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Condition | newCondition() |
String | toString()
Returns a string identifying this lock, as well as its lock state.
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boolean | tryLock()
Acquires the lock only if it is not held by another thread at the time
of invocation.
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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
Thread#interrupt interrupted.
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void | unlock()
Attempts to release this lock.
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Parameters: fair {@code true} if this lock should use a fair ordering policy
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
Returns: the owner, or {@code null} if not owned
Returns: the collection of threads
Returns: the estimated number of threads waiting for this lock
Parameters: condition the condition
Returns: the collection of threads
Throws: IllegalMonitorStateException if this lock is not held IllegalArgumentException if the given condition is not associated with this lock NullPointerException if the condition is null
Parameters: condition the condition
Returns: the estimated number of waiting threads
Throws: IllegalMonitorStateException if this lock is not held IllegalArgumentException if the given condition is not associated with this lock NullPointerException if the condition is null
Parameters: thread the thread
Returns: {@code true} if the given thread is queued waiting for this lock
Throws: NullPointerException if the thread is null
Returns: {@code true} if there may be other threads waiting to acquire the lock
Parameters: condition the condition
Returns: {@code true} if there are any waiting threads
Throws: 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: {@code true} if this lock has fairness set true
Analogous to the Thread#holdsLock 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: {@code true} if current thread holds this lock and {@code false} otherwise
Returns: {@code true} if any thread holds this lock and {@code false} otherwise
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 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:
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: InterruptedException if the current thread is interrupted
The returned Condition instance supports the same
usages as do the Object monitor methods ( Object#wait() wait
, Object#notify notify
, and Object#notifyAll notifyAll
) when used with the built-in
monitor lock.
Returns: the Condition object
Returns: a string identifying this lock, as well as its lock state
Acquires the lock if it is not held by another thread and
returns immediately with the value {@code true}, setting the
lock hold count to one. Even when this lock has been set to use a
fair ordering policy, a call to {@code 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 {@code true}.
If the lock is held by another thread then this method will return immediately with the value {@code false}.
Returns: {@code true} if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and {@code false} otherwise
Acquires the lock if it is not held by another thread and returns immediately with the value {@code 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 {@code 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 {@code 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 {@code true} is returned and the lock hold count is set to one.
If the current thread:
If the specified waiting time elapses then the value {@code 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.
Parameters: timeout the time to wait for the lock unit the time unit of the timeout argument
Returns: {@code true} if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and {@code false} if the waiting time elapsed before the lock could be acquired
Throws: InterruptedException if the current thread is interrupted NullPointerException if the time unit is null
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: IllegalMonitorStateException if the current thread does not hold this lock