| Condition factors out the Object monitor
methods ({@link Object#wait() wait}, {@link Object#notify notify}
and {@link 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 {@link 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 {@link Lock}
instance use its {@link 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
{@link 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 {@link 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 {@link Object#wait wait} and
{@link 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
{@link Lock} associated with that Condition or the use of its
{@link #await waiting} and {@link #signal 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 {@link 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. | 