public interface Condition
Condition factors out the Object 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 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 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 {
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 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 and
notification methods invoked.
Acquiring the monitor lock of a Condition instance, or using its
monitor methods, has no specified relationship with acquiring the
Lock associated with that Condition or the use of its
waiting and signalling 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 NullPointerException being thrown.
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 that may have unblocked the thread. An implementation should document this behavior.
| 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.
|
void await()
throws InterruptedException
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:
signal() method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
signalAll() method for this
Condition; or
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:
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.
InterruptedException - if the current thread is interrupted
(and interruption of thread suspension is supported)void awaitUninterruptibly()
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:
signal() method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
signalAll() method for this
Condition; or
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.
long awaitNanos(long nanosTimeout)
throws InterruptedException
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:
signal() method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
signalAll() method for this
Condition; or
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:
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 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 nanosecondsnanosTimeout 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.InterruptedException - if the current thread is interrupted
(and interruption of thread suspension is supported)boolean await(long time,
TimeUnit unit)
throws InterruptedException
awaitNanos(unit.toNanos(time)) > 0
time - the maximum time to waitunit - the time unit of the time argumentfalse if the waiting time detectably elapsed
before return from the method, else trueInterruptedException - if the current thread is interrupted
(and interruption of thread suspension is supported)boolean awaitUntil(Date deadline) throws InterruptedException
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:
signal() method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
signalAll() method for this
Condition; or
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:
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 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 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 untilfalse if the deadline has elapsed upon return, else
trueInterruptedException - if the current thread is interrupted
(and interruption of thread suspension is supported)void signal()
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 as IllegalMonitorStateException will be thrown.
void signalAll()
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 as IllegalMonitorStateException will be thrown.
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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