java.util.concurrent.locks.Condition Maven / Gradle / Ivy
/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
import java.util.Date;
/**
* {@code Condition} factors out the {@code 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 {@code Lock} replaces the use of {@code synchronized} methods
* and statements, a {@code 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 {@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 block
* until an item becomes available; if a {@code put} is attempted on a
* full 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 provides
* this functionality, so there is no reason to implement this
* sample usage class.)
*
* A {@code Condition} implementation can provide behavior and semantics
* that is
* different from that of the {@code 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 {@code Condition} instances are just normal objects and can
* themselves 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 its
* monitor 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 parameter
* will result in a {@link NullPointerException} being thrown.
*
*
Implementation Considerations
*
* 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, 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
* @author Doug Lea
*/
public interface Condition {
/**
* Causes the current thread to wait until it is signalled or
* {@linkplain Thread#interrupt interrupted}.
*
*
The lock associated with this {@code 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 {@link #signal} method for this
* {@code Condition} and the current thread happens to be chosen as the
* thread to be awakened; or
*
- Some other thread invokes the {@link #signalAll} method for this
* {@code Condition}; or
*
- Some other thread {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt interrupted} while waiting
* and interruption of thread suspension is supported,
*
* then {@link 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
* {@code 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 {@link 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)
*/
void await() throws InterruptedException;
/**
* 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 {@link #signal} method for this
* {@code Condition} and the current thread happens to be chosen as the
* thread to be awakened; or
*
- Some other thread invokes the {@link #signalAll} method for this
* {@code 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 {@linkplain Thread#interrupt 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
* {@code 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 {@link IllegalMonitorStateException}) and the
* implementation must document that fact.
*/
void awaitUninterruptibly();
/**
* 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 {@link #signal} method for this
* {@code Condition} and the current thread happens to be chosen as the
* thread to be awakened; or
*
- Some other thread invokes the {@link #signalAll} method for this
* {@code Condition}; or
*
- Some other thread {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt interrupted} while waiting
* and interruption of thread suspension is supported,
*
* then {@link 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 {@code 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:
*
*
{@code
* 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
* {@code 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 {@link 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.
*
* @param nanosTimeout the maximum time to wait, in nanoseconds
* @return an estimate of the {@code 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)
*/
long awaitNanos(long nanosTimeout) 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:
*
{@code awaitNanos(unit.toNanos(time)) > 0}
*
* @param time the maximum time to wait
* @param unit the time unit of the {@code time} argument
* @return {@code false} if the waiting time detectably elapsed
* before return from the method, else {@code true}
* @throws InterruptedException if the current thread is interrupted
* (and interruption of thread suspension is supported)
*/
boolean await(long time, TimeUnit unit) 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 {@link #signal} method for this
* {@code Condition} and the current thread happens to be chosen as the
* thread to be awakened; or
*
- Some other thread invokes the {@link #signalAll} method for this
* {@code Condition}; or
*
- Some other thread {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt interrupted} while waiting
* and interruption of thread suspension is supported,
*
* then {@link 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:
*
{@code
* 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
* {@code 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 {@link 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.
*
* @param deadline the absolute time to wait until
* @return {@code false} if the deadline has elapsed upon return, else
* {@code true}
* @throws InterruptedException if the current thread is interrupted
* (and interruption of thread suspension is supported)
*/
boolean awaitUntil(Date deadline) throws InterruptedException;
/**
* 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 {@code await}.
*
*
Implementation Considerations
*
*
An implementation may (and typically does) require that the
* current thread hold the lock associated with this {@code
* 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 {@link
* IllegalMonitorStateException} will be thrown.
*/
void signal();
/**
* 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 {@code await}.
*
*
Implementation Considerations
*
*
An implementation may (and typically does) require that the
* current thread hold the lock associated with this {@code
* 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 {@link
* IllegalMonitorStateException} will be thrown.
*/
void signalAll();
}