| |
|
| java.util.concurrent.locks.Condition
| Condition | | public interface Condition (Code) | | Condition factors out the
Object monitor
methods (
Object.wait wait ,
Object.notify notify and
Object.notifyAll 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
Lock.newCondition 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
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
Object.wait wait and
Object.notify 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
and
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.
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.
since:
1.5
author:
Doug Lea |
| Method Summary | |
| void | await()
Causes the current thread to wait until it is signalled or
.
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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll method for this
Condition ; or
- Some other 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. | | 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.
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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll method for this
Condition ; or
- Some other 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. | | 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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll 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. | | boolean | awaitUntil(Date deadline)
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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll method for this
Condition ; or
- Some other 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. | | void | signal()
Wakes up one waiting thread.
If any threads are waiting on this condition then one
is selected for waking up. | | void | signalAll()
Wakes up all waiting threads.
If any threads are waiting on this condition then they are
all woken up. |
| await | | void await() throws InterruptedException(Code) | | Causes the current thread to wait until it is signalled or
.
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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll method for this
Condition ; or
- Some other 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
while waiting
and interruption of thread suspension is supported,
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.
throws:
InterruptedException - if the current thread is interrupted(and interruption of thread suspension is supported) |
| await | | boolean await(long time, TimeUnit unit) throws InterruptedException(Code) | | 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 wait
Parameters:
unit - the time unit of the time argument false if the waiting time detectably elapsedbefore return from the method, else true
throws:
InterruptedException - if the current thread is interrupted(and interruption of thread suspension is supported) |
| awaitNanos | | long awaitNanos(long nanosTimeout) throws InterruptedException(Code) | | 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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll method for this
Condition ; or
- Some other 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
while waiting
and interruption of thread suspension is supported,
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, 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.
Parameters:
nanosTimeout - the maximum time to wait, in nanoseconds an estimate of the nanosTimeout value minusthe time spent waiting upon return from this method.A positive value may be used as the argument to asubsequent call to this method to finish waiting outthe desired time. A value less than or equal to zeroindicates that no time remains.
throws:
InterruptedException - if the current thread is interrupted(and interruption of thread suspension is supported) |
| awaitUninterruptibly | | void awaitUninterruptibly()(Code) | | 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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll 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
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.
|
| awaitUntil | | boolean awaitUntil(Date deadline) throws InterruptedException(Code) | | 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
Condition.signal method for this
Condition and the current thread happens to be chosen as the
thread to be awakened; or
- Some other thread invokes the
Condition.signalAll method for this
Condition ; or
- Some other 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
while waiting
and interruption of thread suspension is supported,
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 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
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.
Parameters:
deadline - the absolute time to wait until false if the deadline has elapsed upon return, else true
throws:
InterruptedException - if the current thread is interrupted(and interruption of thread suspension is supported) |
| signal | | void signal()(Code) | | 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 .
|
| signalAll | | void signalAll()(Code) | | 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 .
|
|
|
|