Improved build.xml

[vimdoclet.git] / sample / java.util.concurrent.locks.ReentrantLock.txt

*java.util.concurrent.locks.ReentrantLock* *ReentrantLock* A reentrant mutual ex

public class ReentrantLock
  extends    |java.lang.Object|
  implements |java.util.concurrent.locks.Lock|
             |java.io.Serializable|

|java.util.concurrent.locks.ReentrantLock_Description|
|java.util.concurrent.locks.ReentrantLock_Fields|
|java.util.concurrent.locks.ReentrantLock_Constructors|
|java.util.concurrent.locks.ReentrantLock_Methods|

================================================================================

*java.util.concurrent.locks.ReentrantLock_Constructors*
|java.util.concurrent.locks.ReentrantLock()|Creates an instance of ReentrantLoc
|java.util.concurrent.locks.ReentrantLock(boolean)|Creates an instance of Reent

*java.util.concurrent.locks.ReentrantLock_Methods*
|java.util.concurrent.locks.ReentrantLock.getHoldCount()|Queries the number of 
|java.util.concurrent.locks.ReentrantLock.getOwner()|Returns the thread that cu
|java.util.concurrent.locks.ReentrantLock.getQueuedThreads()|Returns a collecti
|java.util.concurrent.locks.ReentrantLock.getQueueLength()|Returns an estimate 
|java.util.concurrent.locks.ReentrantLock.getWaitingThreads(Condition)|Returns 
|java.util.concurrent.locks.ReentrantLock.getWaitQueueLength(Condition)|Returns
|java.util.concurrent.locks.ReentrantLock.hasQueuedThread(Thread)|Queries wheth
|java.util.concurrent.locks.ReentrantLock.hasQueuedThreads()|Queries whether an
|java.util.concurrent.locks.ReentrantLock.hasWaiters(Condition)|Queries whether
|java.util.concurrent.locks.ReentrantLock.isFair()|Returns true if this lock ha
|java.util.concurrent.locks.ReentrantLock.isHeldByCurrentThread()|Queries if th
|java.util.concurrent.locks.ReentrantLock.isLocked()|Queries if this lock is he
|java.util.concurrent.locks.ReentrantLock.lock()|Acquires the lock.
|java.util.concurrent.locks.ReentrantLock.lockInterruptibly()|Acquires the lock
|java.util.concurrent.locks.ReentrantLock.newCondition()|Returns aConditioninst
|java.util.concurrent.locks.ReentrantLock.toString()|Returns a string identifyi
|java.util.concurrent.locks.ReentrantLock.tryLock()|Acquires the lock only if i
|java.util.concurrent.locks.ReentrantLock.tryLock(long,TimeUnit)|Acquires the l
|java.util.concurrent.locks.ReentrantLock.unlock()|Attempts to release this loc

*java.util.concurrent.locks.ReentrantLock_Description*

A reentrant mutual exclusion (|java.util.concurrent.locks.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 (|java.util.concurrent.locks.ReentrantLock|) , and 
(|java.util.concurrent.locks.ReentrantLock|) . 

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(|java.util.concurrent.locks.ReentrantLock|) 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 (|java.util.concurrent.locks.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 2147483648 recursive locks by the same thread. 


*java.util.concurrent.locks.ReentrantLock()*

public ReentrantLock()

Creates an instance of ReentrantLock. This is equivalent to using 
ReentrantLock(false). 


*java.util.concurrent.locks.ReentrantLock(boolean)*

public ReentrantLock(boolean fair)

Creates an instance of ReentrantLock with the given fairness policy. 

    fair - true if this lock will be fair; else false 

*java.util.concurrent.locks.ReentrantLock.getHoldCount()*

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. 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. 
*java.util.concurrent.locks.ReentrantLock.getOwner()*

protected |java.lang.Thread| getOwner()

Returns the thread that currently owns this lock, or null if not owned. Note 
that 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. 


    Returns: the owner, or null if not owned. 
*java.util.concurrent.locks.ReentrantLock.getQueuedThreads()*

protected |java.util.Collection| getQueuedThreads()

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: the collection of threads 
*java.util.concurrent.locks.ReentrantLock.getQueueLength()*

public final int getQueueLength()

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 
*java.util.concurrent.locks.ReentrantLock.getWaitingThreads(Condition)*

protected |java.util.Collection| getWaitingThreads(java.util.concurrent.locks.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. 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 

    Returns: the collection of threads 
*java.util.concurrent.locks.ReentrantLock.getWaitQueueLength(Condition)*

public int getWaitQueueLength(java.util.concurrent.locks.Condition 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 

    Returns: the estimated number of waiting threads. 
*java.util.concurrent.locks.ReentrantLock.hasQueuedThread(Thread)*

public final boolean hasQueuedThread(java.lang.Thread 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 

    Returns: true if the given thread is queued waiting for this lock. 
*java.util.concurrent.locks.ReentrantLock.hasQueuedThreads()*

public final boolean hasQueuedThreads()

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. 
*java.util.concurrent.locks.ReentrantLock.hasWaiters(Condition)*

public boolean hasWaiters(java.util.concurrent.locks.Condition 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 

    Returns: true if there are any waiting threads. 
*java.util.concurrent.locks.ReentrantLock.isFair()*

public final boolean isFair()

Returns true if this lock has fairness set true. 


    Returns: true if this lock has fairness set true. 
*java.util.concurrent.locks.ReentrantLock.isHeldByCurrentThread()*

public boolean isHeldByCurrentThread()

Queries if this lock is held by the current thread. 

Analogous to the (|java.lang.Thread|) 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. 
*java.util.concurrent.locks.ReentrantLock.isLocked()*

public boolean isLocked()

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. 
*java.util.concurrent.locks.ReentrantLock.lock()*

public void lock()

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. 


*java.util.concurrent.locks.ReentrantLock.lockInterruptibly()*

public void lockInterruptibly()
  throws |java.lang.InterruptedException|
         
Acquires the lock unless the current thread is interrupted(|java.lang.Thread|) 
. 

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(|java.lang.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 interrupted(|java.lang.Thread|) while acquiring the lock, 



then (|java.lang.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. 


*java.util.concurrent.locks.ReentrantLock.newCondition()*

public |java.util.concurrent.locks.Condition| newCondition()

Returns a (|java.util.concurrent.locks.Condition|) instance for use with this 
(|java.util.concurrent.locks.Lock|) instance. 

The returned (|java.util.concurrent.locks.Condition|) instance supports the 
same usages as do the (|java.lang.Object|) monitor methods ( 
wait(|java.lang.Object|) , notify(|java.lang.Object|) , and 
notifyAll(|java.lang.Object|) ) when used with the built-in monitor lock. 



If this lock is not held when any of the 
(|java.util.concurrent.locks.Condition|) 
waiting(|java.util.concurrent.locks.Condition|) or 
signalling(|java.util.concurrent.locks.Condition|) methods are called, then an 
(|java.lang.IllegalMonitorStateException|) is thrown. 

When the condition waiting(|java.util.concurrent.locks.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 interrupted(|java.lang.Thread|) while waiting then the wait will 
terminate, an (|java.lang.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.ReentrantLock.toString()*

public |java.lang.String| toString()

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 (|java.lang.Thread|) of the owning thread. 


    Returns: a string identifying this lock, as well as its lock state. 
*java.util.concurrent.locks.ReentrantLock.tryLock()*

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. 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) 
(|java.util.concurrent.locks.ReentrantLock|) 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. 


    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 otherwise. 
*java.util.concurrent.locks.ReentrantLock.tryLock(long,TimeUnit)*

public boolean tryLock(
  long timeout,
  java.util.concurrent.TimeUnit unit)
  throws |java.lang.InterruptedException|
         
Acquires the lock if it is not held by another thread within the given waiting 
time and the current thread has not been interrupted(|java.lang.Thread|) . 

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 
(|java.util.concurrent.locks.ReentrantLock|) 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(|java.lang.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 interrupted(|java.lang.Thread|) while acquiring the lock, 

then (|java.lang.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 

    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. 
*java.util.concurrent.locks.ReentrantLock.unlock()*

public void unlock()

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 
(|java.lang.IllegalMonitorStateException|) is thrown.