activemq-cpp-3.6.0
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Condition factors out the Mutex monitor methods (wait, notify and notifyAll) into distinct objects to give the effect of having multiple wait-sets per object, by combining them with the use of arbitrary Lock implementations. More...
#include <src/main/decaf/util/concurrent/locks/Condition.h>
Public Member Functions | |
virtual | ~Condition () |
virtual void | await ()=0 |
Causes the current thread to wait until it is signaled or interrupted. | |
virtual void | awaitUninterruptibly ()=0 |
Causes the current thread to wait until it is signalled. | |
virtual long long | awaitNanos (long long nanosTimeout)=0 |
Causes the current thread to wait until it is signaled or interrupted, or the specified waiting time elapses. | |
virtual bool | await (long long time, const TimeUnit &unit)=0 |
Causes the current thread to wait until it is signaled or interrupted, or the specified waiting time elapses. | |
virtual bool | awaitUntil (const Date &deadline)=0 |
virtual void | signal ()=0 |
Wakes up one waiting thread. | |
virtual void | signalAll ()=0 |
Wakes up all waiting threads. |
Condition factors out the Mutex monitor methods (wait, notify and notifyAll) into distinct objects to give the effect of having multiple wait-sets per object, by combining them with the use of arbitrary Lock implementations.
Where a Lock replaces the use of synchronized 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.
A Condition instance is intrinsically bound to a lock. To obtain a Condition instance for a particular Lock instance use its newCondition() 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 Condition instances.
class BoundedBuffer { Lock* lock = new ReentrantLock(); Condition* notFull = lock->newCondition(); Condition* notEmpty = lock->newCondition();
Object* items = new Object[100]; int putptr, takeptr, count;
public void put( Object* x ) { lock->lock(); try { while( count == 100 ) notFull->await(); items[putptr] = x; if (++putptr == 100) putptr = 0; ++count; notEmpty->signal(); } catch(...) { lock->unlock(); } }
public Object take() { lock->lock(); try { while(count == 0) notEmpty->await(); Object x = items[takeptr]; if (++takeptr == 100) takeptr = 0; –count; notFull->signal(); return x; } catch(...) { lock->unlock(); } } }
(The ArrayBlockingQueue class provides this functionality, so there is no reason to implement this sample usage class.)
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.
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virtual |
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pure virtual |
Causes the current thread to wait until it is signaled or interrupted.
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:
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:
then 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 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.
RuntimeException | if an unexpected error occurs while trying to wait on the Condition. |
InterruptedException | if the current thread is interrupted (and interruption of thread suspension is supported) |
IllegalMonitorStateException | if the caller is not the lock owner. |
Referenced by decaf::util::concurrent::CopyOnWriteArrayList< E >::wait().
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pure virtual |
Causes the current thread to wait until it is signaled 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 |
RuntimeException | if an unexpected error occurs while trying to wait on the Condition. |
InterruptedException | if the current thread is interrupted (and interruption of thread suspension is supported) |
IllegalMonitorStateException | if the caller is not the lock owner. |
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pure virtual |
Causes the current thread to wait until it is signaled 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:
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:
then 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, const 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 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 |
RuntimeException | if an unexpected error occurs while trying to wait on the Condition. |
InterruptedException | if the current thread is interrupted (and interruption of thread suspension is supported) |
IllegalMonitorStateException | if the caller is not the lock owner. |
Referenced by decaf::util::concurrent::CopyOnWriteArrayList< E >::wait().
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pure virtual |
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:
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 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 IllegalMonitorStateException) and the implementation must document that fact.
RuntimeException | if an unexpected error occurs while trying to wait on the Condition. |
IllegalMonitorStateException | if the caller is not the lock owner. |
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pure virtual |
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pure virtual |
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.
RuntimeException | if an unexpected error occurs while trying to wait on the Condition. |
Referenced by decaf::util::concurrent::CopyOnWriteArrayList< E >::notify().
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pure virtual |
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.
RuntimeException | if an unexpected error occurs while trying to wait on the Condition. |
Referenced by decaf::util::concurrent::CopyOnWriteArrayList< E >::notifyAll().