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| java.lang.Object java.util.concurrent.locks.ReentrantLock
ReentrantLock | public class ReentrantLock implements Lock,java.io.Serializable(Code) | | 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
ReentrantLock.isHeldByCurrentThread , and
ReentrantLock.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
ReentrantLock.tryLock() 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.
since: 1.5 author: Doug Lea |
Inner Class :final static class NonfairSync extends Sync | |
Inner Class :final static class FairSync extends Sync | |
Constructor Summary | |
public | ReentrantLock() Creates an instance of
ReentrantLock . | public | ReentrantLock(boolean fair) Creates an instance of
ReentrantLock with the
given fairness policy. |
Method Summary | |
public int | getHoldCount() 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. | protected Thread | getOwner() Returns the thread that currently owns this lock, or
null if not owned. | final public int | getQueueLength() Returns an estimate of the number of threads waiting to
acquire this lock. | protected Collection<Thread> | getQueuedThreads() Returns a collection containing threads that may be waiting to
acquire this lock. | public int | getWaitQueueLength(Condition condition) Returns an estimate of the number of threads waiting on the
given condition associated with 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.
Because the actual set of threads may change dynamically while
constructing this result, the returned collection is only a
best-effort estimate. | final public boolean | hasQueuedThread(Thread thread) Queries whether the given thread is waiting to acquire this
lock. | final public boolean | hasQueuedThreads() Queries whether any threads are waiting to acquire this lock. | public boolean | hasWaiters(Condition condition) Queries whether any threads are waiting on the given condition
associated with this lock. | final public boolean | isFair() Returns
true if this lock has fairness set true. | public boolean | isHeldByCurrentThread() Queries if this lock is held by the current thread.
Analogous to the
Thread.holdsLock method for built-in
monitor locks, this method is typically used for debugging and
testing. | public boolean | isLocked() Queries if this lock is held by any thread. | public void | lock() Acquires the lock. | public void | lockInterruptibly() Acquires the lock unless the current thread is
. | public Condition | newCondition() Returns a
Condition instance for use with this
Lock instance. | public String | toString() Returns a string identifying this lock, as well as its lock state. | public boolean | tryLock() 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. | public 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
.
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. | public void | unlock() Attempts to release this lock.
If the current thread is the holder of this lock then the hold
count is decremented. |
ReentrantLock | public ReentrantLock()(Code) | | Creates an instance of
ReentrantLock .
This is equivalent to using
ReentrantLock(false) .
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ReentrantLock | public ReentrantLock(boolean fair)(Code) | | Creates an instance of
ReentrantLock with the
given fairness policy.
Parameters: fair - true if this lock should use a fair ordering policy |
getHoldCount | public int getHoldCount()(Code) | | 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();
}
}
}
the number of holds on this lock by the current thread,or zero if this lock is not held by the current thread |
getOwner | protected Thread getOwner()(Code) | | 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.
the owner, or null if not owned |
getQueueLength | final public int getQueueLength()(Code) | | 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.
the estimated number of threads waiting for this lock |
getQueuedThreads | protected Collection<Thread> getQueuedThreads()(Code) | | 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.
the collection of threads |
getWaitQueueLength | public int getWaitQueueLength(Condition condition)(Code) | | 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.
Parameters: condition - the condition the estimated number of waiting threads throws: IllegalMonitorStateException - if this lock is not held throws: IllegalArgumentException - if the given condition isnot associated with this lock throws: NullPointerException - if the condition is null |
getWaitingThreads | protected Collection<Thread> getWaitingThreads(Condition condition)(Code) | | 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.
Parameters: condition - the condition the collection of threads throws: IllegalMonitorStateException - if this lock is not held throws: IllegalArgumentException - if the given condition isnot associated with this lock throws: NullPointerException - if the condition is null |
hasQueuedThread | final public boolean hasQueuedThread(Thread thread)(Code) | | 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.
Parameters: thread - the thread true if the given thread is queued waiting for this lock throws: NullPointerException - if the thread is null |
hasQueuedThreads | final public boolean hasQueuedThreads()(Code) | | 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 toacquire the lock |
hasWaiters | public boolean hasWaiters(Condition condition)(Code) | | 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.
Parameters: condition - the condition true if there are any waiting threads throws: IllegalMonitorStateException - if this lock is not held throws: IllegalArgumentException - if the given condition isnot associated with this lock throws: NullPointerException - if the condition is null |
isFair | final public boolean isFair()(Code) | | Returns
true if this lock has fairness set true.
true if this lock has fairness set true |
isHeldByCurrentThread | public boolean isHeldByCurrentThread()(Code) | | Queries if this lock is held by the current thread.
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();
}
}
}
true if current thread holds this lock and false otherwise |
isLocked | public boolean isLocked()(Code) | | 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 |
lock | public void lock()(Code) | | 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.
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lockInterruptibly | public void lockInterruptibly() throws InterruptedException(Code) | | Acquires the lock unless the current thread is
.
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
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
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: InterruptedException - if the current thread is interrupted |
newCondition | public Condition newCondition()(Code) | | Returns a
Condition instance for use with this
Lock instance.
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.
- If this lock is not held when any of the
Condition or
methods are called, then an
IllegalMonitorStateException is thrown.
- When the condition
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
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.
the Condition object |
toString | public String toString()(Code) | | 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
of the owning thread.
a string identifying this lock, as well as its lock state |
tryLock | public boolean tryLock()(Code) | | 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
ReentrantLock.tryLock(long,TimeUnit) 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 thecurrent thread, or the lock was already held by the currentthread; and false otherwise |
tryLock | public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException(Code) | | Acquires the lock if it is not held by another thread within the given
waiting time and the current thread has not been
.
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
ReentrantLock.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
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
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.
Parameters: timeout - the time to wait for the lock Parameters: unit - the time unit of the timeout argument true if the lock was free and was acquired by thecurrent thread, or the lock was already held by the currentthread; and false if the waiting time elapsed beforethe lock could be acquired throws: InterruptedException - if the current thread is interrupted throws: NullPointerException - if the time unit is null |
unlock | public void unlock()(Code) | | 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: IllegalMonitorStateException - if the current thread does nothold this lock |
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