-
- All Known Implementing Classes:
- AbstractQueuedLongSynchronizer.ConditionObject, AbstractQueuedSynchronizer.ConditionObject
public interface Condition
Condition
factors out theObject
monitor methods (wait
,notify
andnotifyAll
) into distinct objects to give the effect of having multiple wait-sets per object, by combining them with the use of arbitraryLock
implementations. Where aLock
replaces the use ofsynchronized
methods and statements, aCondition
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 aCondition
instance for a particularLock
instance use itsnewCondition()
method.As an example, suppose we have a bounded buffer which supports
put
andtake
methods. If atake
is attempted on an empty buffer, then the thread will block until an item becomes available; if aput
is attempted on a full buffer, then the thread will block until a space becomes available. We would like to keep waitingput
threads andtake
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 twoCondition
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(); } } }
(TheArrayBlockingQueue
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 theObject
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 asynchronized
statement, and can have their own monitorwait
andnotification
methods invoked. Acquiring the monitor lock of aCondition
instance, or using its monitor methods, has no specified relationship with acquiring theLock
associated with thatCondition
or the use of its waiting and signalling methods. It is recommended that to avoid confusion you never useCondition
instances in this way, except perhaps within their own implementation.Except where noted, passing a
null
value for any parameter will result in aNullPointerException
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 aCondition
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 that may have unblocked the thread. An implementation should document this behavior.
- Since:
- 1.5
-
-
Method Summary
Methods Modifier and Type Method and Description void
await()
Causes the current thread to wait until it is signalled or interrupted.boolean
await(long time, TimeUnit unit)
Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses.long
awaitNanos(long nanosTimeout)
Causes the current thread to wait until it is signalled or interrupted, or the specified waiting time elapses.void
awaitUninterruptibly()
Causes the current thread to wait until it is signalled.boolean
awaitUntil(Date deadline)
Causes the current thread to wait until it is signalled or interrupted, or the specified deadline elapses.void
signal()
Wakes up one waiting thread.void
signalAll()
Wakes up all waiting threads.
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-
-
Method Detail
-
await
void await() throws InterruptedException
Causes the current thread to wait until it is signalled 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:- Some other thread invokes the
signal()
method for thisCondition
and the current thread happens to be chosen as the thread to be awakened; or - Some other thread invokes the
signalAll()
method for thisCondition
; or - Some other thread interrupts 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 while waiting and interruption of thread suspension is supported,
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 asIllegalMonitorStateException
) 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.
- Throws:
InterruptedException
- if the current thread is interrupted (and interruption of thread suspension is supported)
- Some other thread invokes the
-
awaitUninterruptibly
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
signal()
method for thisCondition
and the current thread happens to be chosen as the thread to be awakened; or - Some other thread invokes the
signalAll()
method for thisCondition
; 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 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 asIllegalMonitorStateException
) and the implementation must document that fact. - Some other thread invokes the
-
awaitNanos
long awaitNanos(long nanosTimeout) throws 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
signal()
method for thisCondition
and the current thread happens to be chosen as the thread to be awakened; or - Some other thread invokes the
signalAll()
method for thisCondition
; or - Some other thread interrupts 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 while waiting and interruption of thread suspension is supported,
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:boolean aMethod(long timeout, TimeUnit unit) { long nanos = unit.toNanos(timeout); lock.lock(); try { while (!conditionBeingWaitedFor()) { if (nanos <= 0L) return false; nanos = theCondition.awaitNanos(nanos); } // ... } finally { lock.unlock(); } }
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 asIllegalMonitorStateException
) 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.
- Parameters:
nanosTimeout
- the maximum time to wait, in nanoseconds- Returns:
- an estimate of the
nanosTimeout
value minus the time spent waiting upon return from this method. A positive value may be used as the argument to a subsequent call to this method to finish waiting out the desired time. A value less than or equal to zero indicates that no time remains. - Throws:
InterruptedException
- if the current thread is interrupted (and interruption of thread suspension is supported)
- Some other thread invokes the
-
await
boolean await(long time, TimeUnit unit) throws 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
- Parameters:
time
- the maximum time to waitunit
- the time unit of thetime
argument- Returns:
false
if the waiting time detectably elapsed before return from the method, elsetrue
- Throws:
InterruptedException
- if the current thread is interrupted (and interruption of thread suspension is supported)
-
awaitUntil
boolean awaitUntil(Date deadline) throws 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
signal()
method for thisCondition
and the current thread happens to be chosen as the thread to be awakened; or - Some other thread invokes the
signalAll()
method for thisCondition
; or - Some other thread interrupts 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 while waiting and interruption of thread suspension is supported,
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:
boolean aMethod(Date deadline) { boolean stillWaiting = true; lock.lock(); try { while (!conditionBeingWaitedFor()) { if (!stillWaiting) return false; stillWaiting = theCondition.awaitUntil(deadline); } // ... } finally { lock.unlock(); } }
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 asIllegalMonitorStateException
) 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.
- Parameters:
deadline
- the absolute time to wait until- Returns:
false
if the deadline has elapsed upon return, elsetrue
- Throws:
InterruptedException
- if the current thread is interrupted (and interruption of thread suspension is supported)
- Some other thread invokes the
-
signal
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
.Implementation Considerations
An implementation may (and typically does) require that the current thread hold the lock associated with this
Condition
when this method is called. Implementations must document this precondition and any actions taken if the lock is not held. Typically, an exception such asIllegalMonitorStateException
will be thrown.
-
signalAll
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
.Implementation Considerations
An implementation may (and typically does) require that the current thread hold the lock associated with this
Condition
when this method is called. Implementations must document this precondition and any actions taken if the lock is not held. Typically, an exception such asIllegalMonitorStateException
will be thrown.
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