mirror of git://gcc.gnu.org/git/gcc.git
436 lines
19 KiB
Java
436 lines
19 KiB
Java
/*
|
|
* 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/licenses/publicdomain
|
|
*/
|
|
|
|
package java.util.concurrent.locks;
|
|
import java.util.concurrent.*;
|
|
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.
|
|
*
|
|
* <p>Conditions (also known as <em>condition queues</em> or
|
|
* <em>condition variables</em>) 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 <em>atomically</em> releases the associated lock and
|
|
* suspends the current thread, just like {@code Object.wait}.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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.
|
|
* <pre>
|
|
* class BoundedBuffer {
|
|
* <b>final Lock lock = new ReentrantLock();</b>
|
|
* final Condition notFull = <b>lock.newCondition(); </b>
|
|
* final Condition notEmpty = <b>lock.newCondition(); </b>
|
|
*
|
|
* final Object[] items = new Object[100];
|
|
* int putptr, takeptr, count;
|
|
*
|
|
* public void put(Object x) throws InterruptedException {
|
|
* <b>lock.lock();
|
|
* try {</b>
|
|
* while (count == items.length)
|
|
* <b>notFull.await();</b>
|
|
* items[putptr] = x;
|
|
* if (++putptr == items.length) putptr = 0;
|
|
* ++count;
|
|
* <b>notEmpty.signal();</b>
|
|
* <b>} finally {
|
|
* lock.unlock();
|
|
* }</b>
|
|
* }
|
|
*
|
|
* public Object take() throws InterruptedException {
|
|
* <b>lock.lock();
|
|
* try {</b>
|
|
* while (count == 0)
|
|
* <b>notEmpty.await();</b>
|
|
* Object x = items[takeptr];
|
|
* if (++takeptr == items.length) takeptr = 0;
|
|
* --count;
|
|
* <b>notFull.signal();</b>
|
|
* return x;
|
|
* <b>} finally {
|
|
* lock.unlock();
|
|
* }</b>
|
|
* }
|
|
* }
|
|
* </pre>
|
|
*
|
|
* (The {@link java.util.concurrent.ArrayBlockingQueue} class provides
|
|
* this functionality, so there is no reason to implement this
|
|
* sample usage class.)
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>Except where noted, passing a {@code null} value for any parameter
|
|
* will result in a {@link NullPointerException} being thrown.
|
|
*
|
|
* <h3>Implementation Considerations</h3>
|
|
*
|
|
* <p>When waiting upon a {@code Condition}, a "<em>spurious
|
|
* wakeup</em>" 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.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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
|
|
*/
|
|
public interface Condition {
|
|
|
|
/**
|
|
* Causes the current thread to wait until it is signalled or
|
|
* {@linkplain Thread#interrupt interrupted}.
|
|
*
|
|
* <p>The lock associated with this {@code Condition} is atomically
|
|
* released and the current thread becomes disabled for thread scheduling
|
|
* purposes and lies dormant until <em>one</em> of four things happens:
|
|
* <ul>
|
|
* <li>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
|
|
* <li>Some other thread invokes the {@link #signalAll} method for this
|
|
* {@code Condition}; or
|
|
* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
|
|
* current thread, and interruption of thread suspension is supported; or
|
|
* <li>A "<em>spurious wakeup</em>" occurs.
|
|
* </ul>
|
|
*
|
|
* <p>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 <em>guaranteed</em> to hold this lock.
|
|
*
|
|
* <p>If the current thread:
|
|
* <ul>
|
|
* <li>has its interrupted status set on entry to this method; or
|
|
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
|
* and interruption of thread suspension is supported,
|
|
* </ul>
|
|
* 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.
|
|
*
|
|
* <p><b>Implementation Considerations</b>
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>The lock associated with this condition is atomically
|
|
* released and the current thread becomes disabled for thread scheduling
|
|
* purposes and lies dormant until <em>one</em> of three things happens:
|
|
* <ul>
|
|
* <li>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
|
|
* <li>Some other thread invokes the {@link #signalAll} method for this
|
|
* {@code Condition}; or
|
|
* <li>A "<em>spurious wakeup</em>" occurs.
|
|
* </ul>
|
|
*
|
|
* <p>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 <em>guaranteed</em> to hold this lock.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p><b>Implementation Considerations</b>
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>The lock associated with this condition is atomically
|
|
* released and the current thread becomes disabled for thread scheduling
|
|
* purposes and lies dormant until <em>one</em> of five things happens:
|
|
* <ul>
|
|
* <li>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
|
|
* <li>Some other thread invokes the {@link #signalAll} method for this
|
|
* {@code Condition}; or
|
|
* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
|
|
* current thread, and interruption of thread suspension is supported; or
|
|
* <li>The specified waiting time elapses; or
|
|
* <li>A "<em>spurious wakeup</em>" occurs.
|
|
* </ul>
|
|
*
|
|
* <p>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 <em>guaranteed</em> to hold this lock.
|
|
*
|
|
* <p>If the current thread:
|
|
* <ul>
|
|
* <li>has its interrupted status set on entry to this method; or
|
|
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
|
* and interruption of thread suspension is supported,
|
|
* </ul>
|
|
* 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.
|
|
*
|
|
* <p>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:
|
|
*
|
|
* <pre>
|
|
* synchronized boolean aMethod(long timeout, TimeUnit unit) {
|
|
* long nanosTimeout = unit.toNanos(timeout);
|
|
* while (!conditionBeingWaitedFor) {
|
|
* if (nanosTimeout > 0)
|
|
* nanosTimeout = theCondition.awaitNanos(nanosTimeout);
|
|
* else
|
|
* return false;
|
|
* }
|
|
* // ...
|
|
* }
|
|
* </pre>
|
|
*
|
|
* <p> 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.
|
|
*
|
|
* <p><b>Implementation Considerations</b>
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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:<br>
|
|
* <pre>
|
|
* awaitNanos(unit.toNanos(time)) > 0
|
|
* </pre>
|
|
* @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.
|
|
*
|
|
* <p>The lock associated with this condition is atomically
|
|
* released and the current thread becomes disabled for thread scheduling
|
|
* purposes and lies dormant until <em>one</em> of five things happens:
|
|
* <ul>
|
|
* <li>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
|
|
* <li>Some other thread invokes the {@link #signalAll} method for this
|
|
* {@code Condition}; or
|
|
* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
|
|
* current thread, and interruption of thread suspension is supported; or
|
|
* <li>The specified deadline elapses; or
|
|
* <li>A "<em>spurious wakeup</em>" occurs.
|
|
* </ul>
|
|
*
|
|
* <p>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 <em>guaranteed</em> to hold this lock.
|
|
*
|
|
*
|
|
* <p>If the current thread:
|
|
* <ul>
|
|
* <li>has its interrupted status set on entry to this method; or
|
|
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
|
* and interruption of thread suspension is supported,
|
|
* </ul>
|
|
* 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.
|
|
*
|
|
*
|
|
* <p>The return value indicates whether the deadline has elapsed,
|
|
* which can be used as follows:
|
|
* <pre>
|
|
* synchronized boolean aMethod(Date deadline) {
|
|
* boolean stillWaiting = true;
|
|
* while (!conditionBeingWaitedFor) {
|
|
* if (stillWaiting)
|
|
* stillWaiting = theCondition.awaitUntil(deadline);
|
|
* else
|
|
* return false;
|
|
* }
|
|
* // ...
|
|
* }
|
|
* </pre>
|
|
*
|
|
* <p><b>Implementation Considerations</b>
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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.
|
|
*
|
|
* <p>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}.
|
|
*/
|
|
void signal();
|
|
|
|
/**
|
|
* Wakes up all waiting threads.
|
|
*
|
|
* <p>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}.
|
|
*/
|
|
void signalAll();
|
|
}
|