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/*

 * 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 &quot;<em>spurious

 * wakeup</em>&quot; 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 &quot;<em>spurious wakeup</em>&quot; 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 &quot;<em>spurious wakeup</em>&quot; 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 &quot;<em>spurious wakeup</em>&quot; 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 &quot;<em>spurious wakeup</em>&quot; 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();

}