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 {@code Object.wait}.
A {@code Condition} instance is intrinsically bound to a lock.To obtain a {@code 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 {@code put} and {@code take} methods. If a{@code take} is attempted on an empty buffer, then the thread will blockuntil an item becomes available; if a {@code put} is attempted on afull buffer, then the thread will block until a space becomes available. We would like to keep waiting {@code put} threads and {@code 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 providesthis functionality, so there is no reason to implement this sample usage class.) A {@code Condition} implementation can provide behavior and semanticsthat is different from that of the {@code Object} monitor methods, such asguaranteed 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 {@code Condition} instances are just normal objects and canthemselves be used as the target in a {@code 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 {@code Condition} instance, or using itsmonitor methods, has no specified relationship with acquiring the {@link Lock} associated with that {@code Condition} or the use of its{@linkplain #await waiting} and {@linkplain #signal signalling} methods.It is recommended that to avoid confusion you never use {@code Condition}instances in this way, except perhaps within their own implementation.
Except where noted, passing a {@code null} value for any parameterwill result in a {@link NullPointerException} being thrown.
When waiting upon a {@code 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 {@code Condition} should always be waited upon in a loop, testingthe 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 @author Doug Lea
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