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

*java.util.concurrent.locks.Condition* *Condition* Condition factors out the Obj

public interface interface Condition


|java.util.concurrent.locks.Condition_Description|
|java.util.concurrent.locks.Condition_Fields|
|java.util.concurrent.locks.Condition_Constructors|
|java.util.concurrent.locks.Condition_Methods|

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

*java.util.concurrent.locks.Condition_Methods*
|java.util.concurrent.locks.Condition.await()|Causes the current thread to wait
|java.util.concurrent.locks.Condition.await(long,TimeUnit)|Causes the current t
|java.util.concurrent.locks.Condition.awaitNanos(long)|Causes the current threa
|java.util.concurrent.locks.Condition.awaitUninterruptibly()|Causes the current
|java.util.concurrent.locks.Condition.awaitUntil(Date)|Causes the current threa
|java.util.concurrent.locks.Condition.signal()|Wakes up one waiting thread.
|java.util.concurrent.locks.Condition.signalAll()|Wakes up all waiting threads.

*java.util.concurrent.locks.Condition_Description*

Condition factors out the Object monitor methods ( wait(|java.lang.Object|) , 
notify(|java.lang.Object|) and notifyAll(|java.lang.Object|) ) into distinct 
objects to give the effect of having multiple wait-sets per object, by 
combining them with the use of arbitrary (|java.util.concurrent.locks.Lock|) 
implementations. Where a Lock replaces the use of synchronized methods and 
statements, a Condition replaces the use of the Object monitor methods. 

Conditions (also known as condition queues or condition variables) provide a 
means for one thread to suspend execution (to wait) until notified by another 
thread that some state condition may now be true. Because access to this shared 
state information occurs in different threads, it must be protected, so a lock 
of some form is associated with the condition. The key property that waiting 
for a condition provides is that it atomically releases the associated lock and 
suspends the current thread, just like Object.wait. 

A Condition instance is intrinsically bound to a lock. To obtain a Condition 
instance for a particular (|java.util.concurrent.locks.Lock|) instance use its 
newCondition()(|java.util.concurrent.locks.Lock|) method. 

As an example, suppose we have a bounded buffer which supports put and take 
methods. If a take is attempted on an empty buffer, then the thread will block 
until an item becomes available; if a put is attempted on a full buffer, then 
the thread will block until a space becomes available. We would like to keep 
waiting put threads and take threads in separate wait-sets so that we can use 
the optimization of only notifying a single thread at a time when items or 
spaces become available in the buffer. This can be achieved using two 
(|java.util.concurrent.locks.Condition|) instances. 

class BoundedBuffer { final Lock lock = new ReentrantLock(); final Condition 
notFull = lock.newCondition(); final Condition notEmpty = lock.newCondition(); 

final Object[] items = new Object[100]; int putptr, takeptr, count; 

public void put(Object x) throws InterruptedException { lock.lock(); try { 
while (count == items.length) notFull.await(); items[putptr] = x; if (++putptr 
== items.length) putptr = 0; ++count; notEmpty.signal(); } finally { 
lock.unlock(); } } 

public Object take() throws InterruptedException { lock.lock(); try { while 
(count == 0) notEmpty.await(); Object x = items[takeptr]; if (++takeptr == 
items.length) takeptr = 0; --count; notFull.signal(); return x; } finally { 
lock.unlock(); } } } 

(The (|java.util.concurrent.ArrayBlockingQueue|) class provides this 
functionality, so there is no reason to implement this sample usage class.) 

A Condition implementation can provide behavior and semantics that is different 
from that of the Object monitor methods, such as guaranteed ordering for 
notifications, or not requiring a lock to be held when performing 
notifications. If an implementation provides such specialized semantics then 
the implementation must document those semantics. 

Note that Condition instances are just normal objects and can themselves be 
used as the target in a synchronized statement, and can have their own monitor 
wait(|java.lang.Object|) and notification(|java.lang.Object|) methods invoked. 
Acquiring the monitor lock of a Condition instance, or using its monitor 
methods, has no specified relationship with acquiring the 
(|java.util.concurrent.locks.Lock|) associated with that Condition or the use 
of its waiting(|java.util.concurrent.locks.Condition|) and 
signalling(|java.util.concurrent.locks.Condition|) methods. It is recommended 
that to avoid confusion you never use Condition instances in this way, except 
perhaps within their own implementation. 

Except where noted, passing a null value for any parameter will result in a 
(|java.lang.NullPointerException|) being thrown. 

Implementation Considerations 

When waiting upon a Condition, a spurious wakeup is permitted to occur, in 
general, as a concession to the underlying platform semantics. This has little 
practical impact on most application programs as a Condition should always be 
waited upon in a loop, testing the state predicate that is being waited for. An 
implementation is free to remove the possibility of spurious wakeups but it is 
recommended that applications programmers always assume that they can occur and 
so always wait in a loop. 

The three forms of condition waiting (interruptible, non-interruptible, and 
timed) may differ in their ease of implementation on some platforms and in 
their performance characteristics. In particular, it may be difficult to 
provide these features and maintain specific semantics such as ordering 
guarantees. Further, the ability to interrupt the actual suspension of the 
thread may not always be feasible to implement on all platforms. Consequently, 
an implementation is not required to define exactly the same guarantees or 
semantics for all three forms of waiting, nor is it required to support 
interruption of the actual suspension of the thread. An implementation is 
required to clearly document the semantics and guarantees provided by each of 
the waiting methods, and when an implementation does support interruption of 
thread suspension then it must obey the interruption semantics as defined in 
this interface. As interruption generally implies cancellation, and checks for 
interruption are often infrequent, an implementation can favor responding to an 
interrupt over normal method return. This is true even if it can be shown that 
the interrupt occurred after another action may have unblocked the thread. An 
implementation should document this behavior. 


*java.util.concurrent.locks.Condition.await()*

public void await()
  throws |java.lang.InterruptedException|
         
Causes the current thread to wait until it is signalled or 
interrupted(|java.lang.Thread|) . 

The lock associated with this Condition is atomically released and the current 
thread becomes disabled for thread scheduling purposes and lies dormant until 
one of four things happens: 

Some other thread invokes the (|java.util.concurrent.locks.Condition|) method 
for this Condition and the current thread happens to be chosen as the thread to 
be awakened; or Some other thread invokes the 
(|java.util.concurrent.locks.Condition|) method for this Condition; or Some 
other thread interrupts(|java.lang.Thread|) the current thread, and 
interruption of thread suspension is supported; or A spurious wakeup occurs 

In all cases, before this method can return the current thread must re-acquire 
the lock associated with this condition. When the thread returns it is 
guaranteed to hold this lock. 

If the current thread: 

has its interrupted status set on entry to this method; or is 
interrupted(|java.lang.Thread|) while waiting and interruption of thread 
suspension is supported, 

then (|java.lang.InterruptedException|) is thrown and the current thread's 
interrupted status is cleared. It is not specified, in the first case, whether 
or not the test for interruption occurs before the lock is released. 

Implementation Considerations The current thread is assumed to hold the lock 
associated with this Condition when this method is called. It is up to the 
implementation to determine if this is the case and if not, how to respond. 
Typically, an exception will be thrown (such as 
(|java.lang.IllegalMonitorStateException|) ) and the implementation must 
document that fact. 

An implementation can favor responding to an interrupt over normal method 
return in response to a signal. In that case the implementation must ensure 
that the signal is redirected to another waiting thread, if there is one. 


*java.util.concurrent.locks.Condition.await(long,TimeUnit)*

public boolean await(
  long time,
  java.util.concurrent.TimeUnit unit)
  throws |java.lang.InterruptedException|
         
Causes the current thread to wait until it is signalled or interrupted, or the 
specified waiting time elapses. This method is behaviorally equivalent to: 

awaitNanos(unit.toNanos(time)) > 0 

    time - the maximum time to wait 
    unit - the time unit of the time argument. 

    Returns: false if the waiting time detectably elapsed before return from the method, 
             else true. 
*java.util.concurrent.locks.Condition.awaitNanos(long)*

public long awaitNanos(long nanosTimeout)
  throws |java.lang.InterruptedException|
         
Causes the current thread to wait until it is signalled or interrupted, or the 
specified waiting time elapses. 

The lock associated with this condition is atomically released and the current 
thread becomes disabled for thread scheduling purposes and lies dormant until 
one of five things happens: 

Some other thread invokes the (|java.util.concurrent.locks.Condition|) method 
for this Condition and the current thread happens to be chosen as the thread to 
be awakened; or Some other thread invokes the 
(|java.util.concurrent.locks.Condition|) method for this Condition; or Some 
other thread interrupts(|java.lang.Thread|) the current thread, and 
interruption of thread suspension is supported; or The specified waiting time 
elapses; or A spurious wakeup occurs. 

In all cases, before this method can return the current thread must re-acquire 
the lock associated with this condition. When the thread returns it is 
guaranteed to hold this lock. 

If the current thread: 

has its interrupted status set on entry to this method; or is 
interrupted(|java.lang.Thread|) while waiting and interruption of thread 
suspension is supported, 

then (|java.lang.InterruptedException|) is thrown and the current thread's 
interrupted status is cleared. It is not specified, in the first case, whether 
or not the test for interruption occurs before the lock is released. 

The method returns an estimate of the number of nanoseconds remaining to wait 
given the supplied nanosTimeout value upon return, or a value less than or 
equal to zero if it timed out. This value can be used to determine whether and 
how long to re-wait in cases where the wait returns but an awaited condition 
still does not hold. Typical uses of this method take the following form: 



synchronized boolean aMethod(long timeout, TimeUnit unit) { long nanosTimeout = 
unit.toNanos(timeout); while (!conditionBeingWaitedFor) { if (nanosTimeout > 0) 
nanosTimeout = theCondition.awaitNanos(nanosTimeout); else return false; } // 
... } 

Design note: This method requires a nanosecond argument so as to avoid 
truncation errors in reporting remaining times. Such precision loss would make 
it difficult for programmers to ensure that total waiting times are not 
systematically shorter than specified when re-waits occur. 

Implementation Considerations The current thread is assumed to hold the lock 
associated with this Condition when this method is called. It is up to the 
implementation to determine if this is the case and if not, how to respond. 
Typically, an exception will be thrown (such as 
(|java.lang.IllegalMonitorStateException|) ) and the implementation must 
document that fact. 

An implementation can favor responding to an interrupt over normal method 
return in response to a signal, or over indicating the elapse of the specified 
waiting time. In either case the implementation must ensure that the signal is 
redirected to another waiting thread, if there is one. 

    nanosTimeout - the maximum time to wait, in nanoseconds 

    Returns: A value less than or equal to zero if the wait has timed out; otherwise an 
             estimate, that is strictly less than the nanosTimeout argument, of 
             the time still remaining when this method returned. 
*java.util.concurrent.locks.Condition.awaitUninterruptibly()*

public void awaitUninterruptibly()

Causes the current thread to wait until it is signalled. 

The lock associated with this condition is atomically released and the current 
thread becomes disabled for thread scheduling purposes and lies dormant until 
one of three things happens: 

Some other thread invokes the (|java.util.concurrent.locks.Condition|) method 
for this Condition and the current thread happens to be chosen as the thread to 
be awakened; or Some other thread invokes the 
(|java.util.concurrent.locks.Condition|) method for this Condition; or A 
spurious wakeup occurs 

In all cases, before this method can return the current thread must re-acquire 
the lock associated with this condition. When the thread returns it is 
guaranteed to hold this lock. 

If the current thread's interrupted status is set when it enters this method, 
or it is interrupted(|java.lang.Thread|) while waiting, it will continue to 
wait until signalled. When it finally returns from this method its interrupted 
status will still be set. 

Implementation Considerations The current thread is assumed to hold the lock 
associated with this Condition when this method is called. It is up to the 
implementation to determine if this is the case and if not, how to respond. 
Typically, an exception will be thrown (such as 
(|java.lang.IllegalMonitorStateException|) ) and the implementation must 
document that fact. 


*java.util.concurrent.locks.Condition.awaitUntil(Date)*

public boolean awaitUntil(java.util.Date deadline)
  throws |java.lang.InterruptedException|
         
Causes the current thread to wait until it is signalled or interrupted, or the 
specified deadline elapses. 

The lock associated with this condition is atomically released and the current 
thread becomes disabled for thread scheduling purposes and lies dormant until 
one of five things happens: 

Some other thread invokes the (|java.util.concurrent.locks.Condition|) method 
for this Condition and the current thread happens to be chosen as the thread to 
be awakened; or Some other thread invokes the 
(|java.util.concurrent.locks.Condition|) method for this Condition; or Some 
other thread interrupts(|java.lang.Thread|) the current thread, and 
interruption of thread suspension is supported; or The specified deadline 
elapses; or A spurious wakeup occurs. 

In all cases, before this method can return the current thread must re-acquire 
the lock associated with this condition. When the thread returns it is 
guaranteed to hold this lock. 

If the current thread: 

has its interrupted status set on entry to this method; or is 
interrupted(|java.lang.Thread|) while waiting and interruption of thread 
suspension is supported, 

then (|java.lang.InterruptedException|) is thrown and the current thread's 
interrupted status is cleared. It is not specified, in the first case, whether 
or not the test for interruption occurs before the lock is released. 

The return value indicates whether the deadline has elapsed, which can be used 
as follows: 

synchronized boolean aMethod(Date deadline) { boolean stillWaiting = true; 
while (!conditionBeingWaitedFor) { if (stillwaiting) stillWaiting = 
theCondition.awaitUntil(deadline); else return false; } // ... } 

Implementation Considerations The current thread is assumed to hold the lock 
associated with this Condition when this method is called. It is up to the 
implementation to determine if this is the case and if not, how to respond. 
Typically, an exception will be thrown (such as 
(|java.lang.IllegalMonitorStateException|) ) and the implementation must 
document that fact. 

An implementation can favor responding to an interrupt over normal method 
return in response to a signal, or over indicating the passing of the specified 
deadline. In either case the implementation must ensure that the signal is 
redirected to another waiting thread, if there is one. 

    deadline - the absolute time to wait until 

    Returns: false if the deadline has elapsed upon return, else true. 
*java.util.concurrent.locks.Condition.signal()*

public void signal()

Wakes up one waiting thread. 

If any threads are waiting on this condition then one is selected for waking 
up. That thread must then re-acquire the lock before returning from await. 


*java.util.concurrent.locks.Condition.signalAll()*

public void signalAll()

Wakes up all waiting threads. 

If any threads are waiting on this condition then they are all woken up. Each 
thread must re-acquire the lock before it can return from await.