condition.java

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/*
 * @(#)Condition.java	1.5 04/07/12
 *
 * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util.concurrent.locks;
import java.util.concurrent.*;
import java.util.Date;

/**
 * <tt>Condition</tt> factors out the <tt>Object</tt> 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 <tt>Lock</tt> replaces the use of <tt>synchronized</tt> methods
 * and statements, a <tt>Condition</tt> 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 &quot;wait&quot;) 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 <tt>Object.wait</tt>.
 *
 * <p>A <tt>Condition</tt> instance is intrinsically bound to a lock.
 * To obtain a <tt>Condition</tt> 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
 * <tt>put</tt> and <tt>take</tt> methods.  If a
 * <tt>take</tt> is attempted on an empty buffer, then the thread will block
 * until an item becomes available; if a <tt>put</tt> is attempted on a
 * full buffer, then the thread will block until a space becomes available.
 * We would like to keep waiting <tt>put</tt> threads and <tt>take</tt>
 * 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 <tt>Condition</tt> implementation can provide behavior and semantics 
 * that is 
 * different from that of the <tt>Object</tt> 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 <tt>Condition</tt> instances are just normal objects and can 
 * themselves be used as the target in a <tt>synchronized</tt> statement,
 * and can have their own monitor {@link Object#wait wait} and
 * {@link Object#notify notification} methods invoked.
 * Acquiring the monitor lock of a <tt>Condition</tt> instance, or using its
 * monitor methods, has no specified relationship with acquiring the
 * {@link Lock} associated with that <tt>Condition</tt> or the use of its
 * {@link #await waiting} and {@link #signal signalling} methods.
 * It is recommended that to avoid confusion you never use <tt>Condition</tt>
 * instances in this way, except perhaps within their own implementation.
 *
 * <p>Except where noted, passing a <tt>null</tt> value for any parameter 
 * will result in a {@link NullPointerException} being thrown.
 *
 * <h3>Implementation Considerations</h3>
 *
 * <p>When waiting upon a <tt>Condition</tt>, 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
 * <tt>Condition</tt> 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 
     * {@link Thread#interrupt interrupted}.
     *
     * <p>The lock associated with this <tt>Condition</tt> 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 
     * <tt>Condition</tt> 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 
     * <tt>Condition</tt>; or
     * <li>Some other thread {@link 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 {@link 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
     * <tt>Condition</tt> 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 
     * <tt>Condition</tt> 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 
     * <tt>Condition</tt>; 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