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. * * <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 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}. * * <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> {@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(); * } * }}</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: * <pre> {@code 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> {@code * boolean aMethod(Date deadline) { * boolean stillWaiting = true; * lock.lock(); * try { * while (!conditionBeingWaitedFor()) { * if (!stillWaiting) * return false; * stillWaiting = theCondition.awaitUntil(deadline); * } * // ... * } finally { * lock.unlock(); * } * }}</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}. * * <p><b>Implementation Considerations</b> * * <p>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. * * <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}. * * <p><b>Implementation Considerations</b> * * <p>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();}
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