src/jsr166y/CountedCompleter.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/publicdomain/zero/1.0/
 */

package jsr166y;

/**
 * A resultless {@link ForkJoinTask} with a completion action
 * performed when triggered and there are no remaining pending
 * actions. Uses of CountedCompleter are similar to those of other
 * completion based components (such as {@link
 * java.nio.channels.CompletionHandler}) except that multiple
 * <em>pending</em> completions may be necessary to trigger the {@link
 * #onCompletion} action, not just one. Unless initialized otherwise,
 * the {@link #getPendingCount pending count} starts at zero, but may
 * be (atomically) changed using methods {@link #setPendingCount},
 * {@link #addToPendingCount}, and {@link
 * #compareAndSetPendingCount}. Upon invocation of {@link
 * #tryComplete}, if the pending action count is nonzero, it is
 * decremented; otherwise, the completion action is performed, and if
 * this completer itself has a completer, the process is continued
 * with its completer.  As is the case with related synchronization
 * components such as {@link Phaser} and {@link
 * java.util.concurrent.Semaphore} these methods affect only internal
 * counts; they do not establish any further internal bookkeeping. In
 * particular, the identities of pending tasks are not maintained. As
 * illustrated below, you can create subclasses that do record some or
 * all pended tasks or their results when needed.
 *
 * <p>A concrete CountedCompleter class must define method {@link
 * #compute}, that should, in almost all use cases, invoke {@code
 * tryComplete()} once before returning. The class may also optionally
 * override method {@link #onCompletion} to perform an action upon
 * normal completion, and method {@link #onExceptionalCompletion} to
 * perform an action upon any exception.
 *
 * <p>A CountedCompleter that does not itself have a completer (i.e.,
 * one for which {@link #getCompleter} returns {@code null}) can be
 * used as a regular ForkJoinTask with this added functionality.
 * However, any completer that in turn has another completer serves
 * only as an internal helper for other computations, so its own task
 * status (as reported in methods such as {@link ForkJoinTask#isDone})
 * is arbitrary; this status changes only upon explicit invocations of
 * {@link #complete}, {@link ForkJoinTask#cancel}, {@link
 * ForkJoinTask#completeExceptionally} or upon exceptional completion
 * of method {@code compute}. Upon any exceptional completion, the
 * exception may be relayed to a task's completer (and its completer,
 * and so on), if one exists and it has not otherwise already
 * completed.
 *
 * <p><b>Sample Usages.</b>
 *
 * <p><b>Parallel recursive decomposition.</b> CountedCompleters may
 * be arranged in trees similar to those often used with {@link
 * RecursiveAction}s, although the constructions involved in setting
 * them up typically vary. Even though they entail a bit more
 * bookkeeping, CountedCompleters may be better choices when applying
 * a possibly time-consuming operation (that cannot be further
 * subdivided) to each element of an array or collection; especially
 * when the operation takes a significantly different amount of time
 * to complete for some elements than others, either because of
 * intrinsic variation (for example IO) or auxiliary effects such as
 * garbage collection.  Because CountedCompleters provide their own
 * continuations, other threads need not block waiting to perform
 * them.
 *
 * <p> For example, here is an initial version of a class that uses
 * divide-by-two recursive decomposition to divide work into single
 * pieces (leaf tasks). Even when work is split into individual calls,
 * tree-based techniques are usually preferable to directly forking
 * leaf tasks, because they reduce inter-thread communication and
 * improve load balancing. In the recursive case, the second of each
 * pair of subtasks to finish triggers completion of its parent
 * (because no result combination is performed, the default no-op
 * implementation of method {@code onCompletion} is not overridden). A
 * static utility method sets up the base task and invokes it:
 *
 * <pre> {@code
 * class MyOperation<E> { void apply(E e) { ... }  }
 *
 * class ForEach<E> extends CountedCompleter {
 *
 *     public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
 *         pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
 *     }
 *
 *     final E[] array; final MyOperation<E> op; final int lo, hi;
 *     ForEach(CountedCompleter p, E[] array, MyOperation<E> op, int lo, int hi) {
 *         super(p);
 *         this.array = array; this.op = op; this.lo = lo; this.hi = hi;
 *     }
 *
 *     public void compute() { // version 1
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             setPendingCount(2); // must set pending count before fork
 *             new ForEach(this, array, op, mid, hi).fork(); // right child
 *             new ForEach(this, array, op, lo, mid).fork(); // left child
 *         }
 *         else if (hi > lo)
 *             op.apply(array[lo]);
 *         tryComplete();
 *     }
 * } }</pre>
 *
 * This design can be improved by noticing that in the recursive case,
 * the task has nothing to do after forking its right task, so can
 * directly invoke its left task before returning. (This is an analog
 * of tail recursion removal.)  Also, because the task returns upon
 * executing its left task (rather than falling through to invoke
 * tryComplete) the pending count is set to one:
 *
 * <pre> {@code
 * class ForEach<E> ...
 *     public void compute() { // version 2
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             setPendingCount(1); // only one pending
 *             new ForEach(this, array, op, mid, hi).fork(); // right child
 *             new ForEach(this, array, op, lo, mid).compute(); // direct invoke
 *         }
 *         else {
 *             if (hi > lo)
 *                 op.apply(array[lo]);
 *             tryComplete();
 *         }
 *     }
 * }</pre>
 *
 * As a further improvement, notice that the left task need not even
 * exist.  Instead of creating a new one, we can iterate using the
 * original task, and add a pending count for each fork:
 *
 * <pre> {@code
 * class ForEach<E> ...
 *     public void compute() { // version 3
 *         int l = lo,  h = hi;
 *         while (h - l >= 2) {
 *             int mid = (l + h) >>> 1;
 *             addToPendingCount(1);
 *             new ForEach(this, array, op, mid, h).fork(); // right child
 *             h = mid;
 *         }
 *         if (h > l)
 *             op.apply(array[l]);
 *         tryComplete();
 *     }
 * }</pre>
 *
 * Additional improvements of such classes might entail precomputing
 * pending counts so that they can be established in constructors,
 * specializing classes for leaf steps, subdividing by say, four,
 * instead of two per iteration, and using an adaptive threshold
 * instead of always subdividing down to single elements.
 *
 * <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
 * results of multiple subtasks usually need to access these results
 * in method {@link #onCompletion}. As illustrated in the following
 * class (that performs a simplified form of map-reduce where mappings
 * and reductions are all of type {@code E}), one way to do this in
 * divide and conquer designs is to have each subtask record its
 * sibling, so that it can be accessed in method {@code onCompletion}.
 * For clarity, this class uses explicit left and right subtasks, but
 * variants of other streamlinings seen in the above example may also
 * apply.
 *
 * <pre> {@code
 * class MyMapper<E> { E apply(E v) {  ...  } }
 * class MyReducer<E> { E apply(E x, E y) {  ...  } }
 * class MapReducer<E> extends CountedCompleter {
 *     final E[] array; final MyMapper<E> mapper;
 *     final MyReducer<E> reducer; final int lo, hi;
 *     MapReducer sibling;
 *     E result;
 *     MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
 *                MyReducer<E> reducer, int lo, int hi) {
 *         super(p);
 *         this.array = array; this.mapper = mapper;
 *         this.reducer = reducer; this.lo = lo; this.hi = hi;
 *     }
 *     public void compute() {
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
 *             MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
 *             left.sibling = right;
 *             right.sibling = left;
 *             setPendingCount(1); // only right is pending
 *             right.fork();
 *             left.compute();     // directly execute left
 *         }
 *         else {
 *             if (hi > lo)
 *                 result = mapper.apply(array[lo]);
 *             tryComplete();
 *         }
 *     }
 *     public void onCompletion(CountedCompleter caller) {
 *         if (caller != this) {
 *            MapReducer<E> child = (MapReducer<E>)caller;
 *            MapReducer<E> sib = child.sibling;
 *            if (sib == null || sib.result == null)
 *                result = child.result;
 *            else
 *                result = reducer.apply(child.result, sib.result);
 *         }
 *     }
 *
 *     public static <E> E mapReduce(ForkJoinPool pool, E[] array,
 *                                   MyMapper<E> mapper, MyReducer<E> reducer) {
 *         MapReducer<E> mr = new MapReducer<E>(null, array, mapper,
 *                                              reducer, 0, array.length);
 *         pool.invoke(mr);
 *         return mr.result;
 *     }
 * } }</pre>
 *
 * <p><b>Triggers.</b> Some CountedCompleters are themselves never
 * forked, but instead serve as bits of plumbing in other designs;
 * including those in which the completion of one of more async tasks
 * triggers another async task. For example:
 *
 * <pre> {@code
 * class HeaderBuilder extends CountedCompleter { ... }
 * class BodyBuilder extends CountedCompleter { ... }
 * class PacketSender extends CountedCompleter {
 *     PacketSender(...) { super(null, 1); ... } // trigger on second completion
 *     public void compute() { } // never called
 *     public void onCompletion(CountedCompleter caller) { sendPacket(); }
 * }
 * // sample use:
 * PacketSender p = new PacketSender();
 * new HeaderBuilder(p, ...).fork();
 * new BodyBuilder(p, ...).fork();
 * }</pre>
 *
 * @since 1.8
 * @author Doug Lea
 */
public abstract class CountedCompleter extends ForkJoinTask<Void> {
    private static final long serialVersionUID = 5232453752276485070L;

    /** This task's completer, or null if none */
    final CountedCompleter completer;
    /** The number of pending tasks until completion */
    volatile int pending;

    /**
     * Creates a new CountedCompleter with the given completer
     * and initial pending count.
     *
     * @param completer this tasks completer, or {@code null} if none
     * @param initialPendingCount the initial pending count
     */
    protected CountedCompleter(CountedCompleter completer,
                               int initialPendingCount) {
        this.completer = completer;
        this.pending = initialPendingCount;
    }

    /**
     * Creates a new CountedCompleter with the given completer
     * and an initial pending count of zero.
     *
     * @param completer this tasks completer, or {@code null} if none
     */
    protected CountedCompleter(CountedCompleter completer) {
        this.completer = completer;
    }

    /**
     * Creates a new CountedCompleter with no completer
     * and an initial pending count of zero.
     */
    protected CountedCompleter() {
        this.completer = null;
    }

    /**
     * The main computation performed by this task.
     */
    public abstract void compute();

    /**
     * Performs an action when method {@link #tryComplete} is invoked
     * and there are no pending counts, or when the unconditional
     * method {@link #complete} is invoked.  By default, this method
     * does nothing.
     *
     * @param caller the task invoking this method (which may
     * be this task itself).
     */
    public void onCompletion(CountedCompleter caller) {
    }

    /**
     * Performs an action when method {@link #completeExceptionally}
     * is invoked or method {@link #compute} throws an exception, and
     * this task has not otherwise already completed normally. On
     * entry to this method, this task {@link
     * ForkJoinTask#isCompletedAbnormally}.  The return value of this
     * method controls further propagation: If {@code true} and this
     * task has a completer, then this completer is also completed
     * exceptionally.  The default implementation of this method does
     * nothing except return {@code true}.
     *
     * @param ex the exception
     * @param caller the task invoking this method (which may
     * be this task itself).
     * @return true if this exception should be propagated to this
     * tasks completer, if one exists.
     */
    public boolean onExceptionalCompletion(Throwable ex, CountedCompleter caller) {
        return true;
    }

    /**
     * Returns the completer established in this task's constructor,
     * or {@code null} if none.
     *
     * @return the completer
     */
    public final CountedCompleter getCompleter() {
        return completer;
    }

    /**
     * Returns the current pending count.
     *
     * @return the current pending count
     */
    public final int getPendingCount() {
        return pending;
    }

    /**
     * Sets the pending count to the given value.
     *
     * @param count the count
     */
    public final void setPendingCount(int count) {
        pending = count;
    }

    /**
     * Adds (atomically) the given value to the pending count.
     *
     * @param delta the value to add
     */
    public final void addToPendingCount(int delta) {
        int c; // note: can replace with intrinsic in jdk8
        do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
    }

    /**
     * Sets (atomically) the pending count to the given count only if
     * it currently holds the given expected value.
     *
     * @param expected the expected value
     * @param count the new value
     * @return true is successful
     */
    public final boolean compareAndSetPendingCount(int expected, int count) {
        return U.compareAndSwapInt(this, PENDING, expected, count);
    }

    /**
     * If the pending count is nonzero, decrements the count;
     * otherwise invokes {@link #onCompletion} and then similarly
     * tries to complete this task's completer, if one exists,
     * else marks this task as complete.
     */
    public final void tryComplete() {
        CountedCompleter a = this, s = a;
        for (int c;;) {
            if ((c = a.pending) == 0) {
                a.onCompletion(s);
                if ((a = (s = a).completer) == null) {
                    s.quietlyComplete();
                    return;
                }
            }
            else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
                return;
        }
    }

    /**
     * Regardless of pending count, invokes {@link #onCompletion},
     * marks this task as complete with a {@code null} return value,
     * and further triggers {@link #tryComplete} on this task's
     * completer, if one exists. This method may be useful when
     * forcing completion as soon as any one (versus all) of several
     * subtask results are obtained.
     *
     * @param mustBeNull the {@code null} completion value
     */
    public void complete(Void mustBeNull) {
        CountedCompleter p;
        onCompletion(this);
        quietlyComplete();
        if ((p = completer) != null)
            p.tryComplete();
    }

    /**
     * Support for FJT exception propagation
     */
    void internalPropagateException(Throwable ex) {
        CountedCompleter a = this, s = a;
        while (a.onExceptionalCompletion(ex, s) &&
               (a = (s = a).completer) != null && a.status >= 0)
            a.recordExceptionalCompletion(ex);
    }

    /**
     * Implements execution conventions for CountedCompleters
     */
    protected final boolean exec() {
        compute();
        return false;
    }

    /**
     * Always returns {@code null}.
     *
     * @return {@code null} always
     */
    public final Void getRawResult() { return null; }

    /**
     * Requires null completion value.
     */
    protected final void setRawResult(Void mustBeNull) { }

    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final long PENDING;
    static {
        try {
            U = getUnsafe();
            PENDING = U.objectFieldOffset
                (CountedCompleter.class.getDeclaredField("pending"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }


    /**
     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating
     * into a jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException se) {
            try {
                return java.security.AccessController.doPrivileged
                    (new java.security
                     .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                        public sun.misc.Unsafe run() throws Exception {
                            java.lang.reflect.Field f = sun.misc
                                .Unsafe.class.getDeclaredField("theUnsafe");
                            f.setAccessible(true);
                            return (sun.misc.Unsafe) f.get(null);
                        }});
            } catch (java.security.PrivilegedActionException e) {
                throw new RuntimeException("Could not initialize intrinsics",
                                           e.getCause());
            }
        }
    }

}
Revision:	1.2
Committed:	Sat Apr 21 11:45:20 2012 UTC (12 years ago) by dl
Branch:	MAIN
Changes since 1.1:	+52 -21 lines
Log Message:	add CountedCompleter.onExceptionalCompletion
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package jsr166y;
8
9	/**
10	* A resultless {@link ForkJoinTask} with a completion action
11	* performed when triggered and there are no remaining pending
12	* actions. Uses of CountedCompleter are similar to those of other
13	* completion based components (such as {@link
14	* java.nio.channels.CompletionHandler}) except that multiple
15	* <em>pending</em> completions may be necessary to trigger the {@link
16	* #onCompletion} action, not just one. Unless initialized otherwise,
17	* the {@link #getPendingCount pending count} starts at zero, but may
18	* be (atomically) changed using methods {@link #setPendingCount},
19	* {@link #addToPendingCount}, and {@link
20	* #compareAndSetPendingCount}. Upon invocation of {@link
21	* #tryComplete}, if the pending action count is nonzero, it is
22	* decremented; otherwise, the completion action is performed, and if
23	* this completer itself has a completer, the process is continued
24	* with its completer. As is the case with related synchronization
25	* components such as {@link Phaser} and {@link
26	* java.util.concurrent.Semaphore} these methods affect only internal
27	* counts; they do not establish any further internal bookkeeping. In
28	* particular, the identities of pending tasks are not maintained. As
29	* illustrated below, you can create subclasses that do record some or
30	* all pended tasks or their results when needed.
31	*
32	* <p>A concrete CountedCompleter class must define method {@link
33	* #compute}, that should, in almost all use cases, invoke {@code
34	* tryComplete()} once before returning. The class may also optionally
35	* override method {@link #onCompletion} to perform an action upon
36	* normal completion, and method {@link #onExceptionalCompletion} to
37	* perform an action upon any exception.
38	*
39	* <p>A CountedCompleter that does not itself have a completer (i.e.,
40	* one for which {@link #getCompleter} returns {@code null}) can be
41	* used as a regular ForkJoinTask with this added functionality.
42	* However, any completer that in turn has another completer serves
43	* only as an internal helper for other computations, so its own task
44	* status (as reported in methods such as {@link ForkJoinTask#isDone})
45	* is arbitrary; this status changes only upon explicit invocations of
46	* {@link #complete}, {@link ForkJoinTask#cancel}, {@link
47	* ForkJoinTask#completeExceptionally} or upon exceptional completion
48	* of method {@code compute}. Upon any exceptional completion, the
49	* exception may be relayed to a task's completer (and its completer,
50	* and so on), if one exists and it has not otherwise already
51	* completed.
52	*
53	* <p><b>Sample Usages.</b>
54	*
55	* <p><b>Parallel recursive decomposition.</b> CountedCompleters may
56	* be arranged in trees similar to those often used with {@link
57	* RecursiveAction}s, although the constructions involved in setting
58	* them up typically vary. Even though they entail a bit more
59	* bookkeeping, CountedCompleters may be better choices when applying
60	* a possibly time-consuming operation (that cannot be further
61	* subdivided) to each element of an array or collection; especially
62	* when the operation takes a significantly different amount of time
63	* to complete for some elements than others, either because of
64	* intrinsic variation (for example IO) or auxiliary effects such as
65	* garbage collection. Because CountedCompleters provide their own
66	* continuations, other threads need not block waiting to perform
67	* them.
68	*
69	* <p> For example, here is an initial version of a class that uses
70	* divide-by-two recursive decomposition to divide work into single
71	* pieces (leaf tasks). Even when work is split into individual calls,
72	* tree-based techniques are usually preferable to directly forking
73	* leaf tasks, because they reduce inter-thread communication and
74	* improve load balancing. In the recursive case, the second of each
75	* pair of subtasks to finish triggers completion of its parent
76	* (because no result combination is performed, the default no-op
77	* implementation of method {@code onCompletion} is not overridden). A
78	* static utility method sets up the base task and invokes it:
79	*
80	* <pre> {@code
81	* class MyOperation<E> { void apply(E e) { ... } }
82	*
83	* class ForEach<E> extends CountedCompleter {
84	*
85	* public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
86	* pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
87	* }
88	*
89	* final E[] array; final MyOperation<E> op; final int lo, hi;
90	* ForEach(CountedCompleter p, E[] array, MyOperation<E> op, int lo, int hi) {
91	* super(p);
92	* this.array = array; this.op = op; this.lo = lo; this.hi = hi;
93	* }
94	*
95	* public void compute() { // version 1
96	* if (hi - lo >= 2) {
97	* int mid = (lo + hi) >>> 1;
98	* setPendingCount(2); // must set pending count before fork
99	* new ForEach(this, array, op, mid, hi).fork(); // right child
100	* new ForEach(this, array, op, lo, mid).fork(); // left child
101	* }
102	* else if (hi > lo)
103	* op.apply(array[lo]);
104	* tryComplete();
105	* }
106	* } }</pre>
107	*
108	* This design can be improved by noticing that in the recursive case,
109	* the task has nothing to do after forking its right task, so can
110	* directly invoke its left task before returning. (This is an analog
111	* of tail recursion removal.) Also, because the task returns upon
112	* executing its left task (rather than falling through to invoke
113	* tryComplete) the pending count is set to one:
114	*
115	* <pre> {@code
116	* class ForEach<E> ...
117	* public void compute() { // version 2
118	* if (hi - lo >= 2) {
119	* int mid = (lo + hi) >>> 1;
120	* setPendingCount(1); // only one pending
121	* new ForEach(this, array, op, mid, hi).fork(); // right child
122	* new ForEach(this, array, op, lo, mid).compute(); // direct invoke
123	* }
124	* else {
125	* if (hi > lo)
126	* op.apply(array[lo]);
127	* tryComplete();
128	* }
129	* }
130	* }</pre>
131	*
132	* As a further improvement, notice that the left task need not even
133	* exist. Instead of creating a new one, we can iterate using the
134	* original task, and add a pending count for each fork:
135	*
136	* <pre> {@code
137	* class ForEach<E> ...
138	* public void compute() { // version 3
139	* int l = lo, h = hi;
140	* while (h - l >= 2) {
141	* int mid = (l + h) >>> 1;
142	* addToPendingCount(1);
143	* new ForEach(this, array, op, mid, h).fork(); // right child
144	* h = mid;
145	* }
146	* if (h > l)
147	* op.apply(array[l]);
148	* tryComplete();
149	* }
150	* }</pre>
151	*
152	* Additional improvements of such classes might entail precomputing
153	* pending counts so that they can be established in constructors,
154	* specializing classes for leaf steps, subdividing by say, four,
155	* instead of two per iteration, and using an adaptive threshold
156	* instead of always subdividing down to single elements.
157	*
158	* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
159	* results of multiple subtasks usually need to access these results
160	* in method {@link #onCompletion}. As illustrated in the following
161	* class (that performs a simplified form of map-reduce where mappings
162	* and reductions are all of type {@code E}), one way to do this in
163	* divide and conquer designs is to have each subtask record its
164	* sibling, so that it can be accessed in method {@code onCompletion}.
165	* For clarity, this class uses explicit left and right subtasks, but
166	* variants of other streamlinings seen in the above example may also
167	* apply.
168	*
169	* <pre> {@code
170	* class MyMapper<E> { E apply(E v) { ... } }
171	* class MyReducer<E> { E apply(E x, E y) { ... } }
172	* class MapReducer<E> extends CountedCompleter {
173	* final E[] array; final MyMapper<E> mapper;
174	* final MyReducer<E> reducer; final int lo, hi;
175	* MapReducer sibling;
176	* E result;
177	* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
178	* MyReducer<E> reducer, int lo, int hi) {
179	* super(p);
180	* this.array = array; this.mapper = mapper;
181	* this.reducer = reducer; this.lo = lo; this.hi = hi;
182	* }
183	* public void compute() {
184	* if (hi - lo >= 2) {
185	* int mid = (lo + hi) >>> 1;
186	* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
187	* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
188	* left.sibling = right;
189	* right.sibling = left;
190	* setPendingCount(1); // only right is pending
191	* right.fork();
192	* left.compute(); // directly execute left
193	* }
194	* else {
195	* if (hi > lo)
196	* result = mapper.apply(array[lo]);
197	* tryComplete();
198	* }
199	* }
200	* public void onCompletion(CountedCompleter caller) {
201	* if (caller != this) {
202	* MapReducer<E> child = (MapReducer<E>)caller;
203	* MapReducer<E> sib = child.sibling;
204	* if (sib == null \|\| sib.result == null)
205	* result = child.result;
206	* else
207	* result = reducer.apply(child.result, sib.result);
208	* }
209	* }
210	*
211	* public static <E> E mapReduce(ForkJoinPool pool, E[] array,
212	* MyMapper<E> mapper, MyReducer<E> reducer) {
213	* MapReducer<E> mr = new MapReducer<E>(null, array, mapper,
214	* reducer, 0, array.length);
215	* pool.invoke(mr);
216	* return mr.result;
217	* }
218	* } }</pre>
219	*
220	* <p><b>Triggers.</b> Some CountedCompleters are themselves never
221	* forked, but instead serve as bits of plumbing in other designs;
222	* including those in which the completion of one of more async tasks
223	* triggers another async task. For example:
224	*
225	* <pre> {@code
226	* class HeaderBuilder extends CountedCompleter { ... }
227	* class BodyBuilder extends CountedCompleter { ... }
228	* class PacketSender extends CountedCompleter {
229	* PacketSender(...) { super(null, 1); ... } // trigger on second completion
230	* public void compute() { } // never called
231	* public void onCompletion(CountedCompleter caller) { sendPacket(); }
232	* }
233	* // sample use:
234	* PacketSender p = new PacketSender();
235	* new HeaderBuilder(p, ...).fork();
236	* new BodyBuilder(p, ...).fork();
237	* }</pre>
238	*
239	* @since 1.8
240	* @author Doug Lea
241	*/
242	public abstract class CountedCompleter extends ForkJoinTask<Void> {
243	private static final long serialVersionUID = 5232453752276485070L;
244
245	/** This task's completer, or null if none */
246	final CountedCompleter completer;
247	/** The number of pending tasks until completion */
248	volatile int pending;
249
250	/**
251	* Creates a new CountedCompleter with the given completer
252	* and initial pending count.
253	*
254	* @param completer this tasks completer, or {@code null} if none
255	* @param initialPendingCount the initial pending count
256	*/
257	protected CountedCompleter(CountedCompleter completer,
258	int initialPendingCount) {
259	this.completer = completer;
260	this.pending = initialPendingCount;
261	}
262
263	/**
264	* Creates a new CountedCompleter with the given completer
265	* and an initial pending count of zero.
266	*
267	* @param completer this tasks completer, or {@code null} if none
268	*/
269	protected CountedCompleter(CountedCompleter completer) {
270	this.completer = completer;
271	}
272
273	/**
274	* Creates a new CountedCompleter with no completer
275	* and an initial pending count of zero.
276	*/
277	protected CountedCompleter() {
278	this.completer = null;
279	}
280
281	/**
282	* The main computation performed by this task.
283	*/
284	public abstract void compute();
285
286	/**
287	* Performs an action when method {@link #tryComplete} is invoked
288	* and there are no pending counts, or when the unconditional
289	* method {@link #complete} is invoked. By default, this method
290	* does nothing.
291	*
292	* @param caller the task invoking this method (which may
293	* be this task itself).
294	*/
295	public void onCompletion(CountedCompleter caller) {
296	}
297
298	/**
299	* Performs an action when method {@link #completeExceptionally}
300	* is invoked or method {@link #compute} throws an exception, and
301	* this task has not otherwise already completed normally. On
302	* entry to this method, this task {@link
303	* ForkJoinTask#isCompletedAbnormally}. The return value of this
304	* method controls further propagation: If {@code true} and this
305	* task has a completer, then this completer is also completed
306	* exceptionally. The default implementation of this method does
307	* nothing except return {@code true}.
308	*
309	* @param ex the exception
310	* @param caller the task invoking this method (which may
311	* be this task itself).
312	* @return true if this exception should be propagated to this
313	* tasks completer, if one exists.
314	*/
315	public boolean onExceptionalCompletion(Throwable ex, CountedCompleter caller) {
316	return true;
317	}
318
319	/**
320	* Returns the completer established in this task's constructor,
321	* or {@code null} if none.
322	*
323	* @return the completer
324	*/
325	public final CountedCompleter getCompleter() {
326	return completer;
327	}
328
329	/**
330	* Returns the current pending count.
331	*
332	* @return the current pending count
333	*/
334	public final int getPendingCount() {
335	return pending;
336	}
337
338	/**
339	* Sets the pending count to the given value.
340	*
341	* @param count the count
342	*/
343	public final void setPendingCount(int count) {
344	pending = count;
345	}
346
347	/**
348	* Adds (atomically) the given value to the pending count.
349	*
350	* @param delta the value to add
351	*/
352	public final void addToPendingCount(int delta) {
353	int c; // note: can replace with intrinsic in jdk8
354	do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
355	}
356
357	/**
358	* Sets (atomically) the pending count to the given count only if
359	* it currently holds the given expected value.
360	*
361	* @param expected the expected value
362	* @param count the new value
363	* @return true is successful
364	*/
365	public final boolean compareAndSetPendingCount(int expected, int count) {
366	return U.compareAndSwapInt(this, PENDING, expected, count);
367	}
368
369	/**
370	* If the pending count is nonzero, decrements the count;
371	* otherwise invokes {@link #onCompletion} and then similarly
372	* tries to complete this task's completer, if one exists,
373	* else marks this task as complete.
374	*/
375	public final void tryComplete() {
376	CountedCompleter a = this, s = a;
377	for (int c;;) {
378	if ((c = a.pending) == 0) {
379	a.onCompletion(s);
380	if ((a = (s = a).completer) == null) {
381	s.quietlyComplete();
382	return;
383	}
384	}
385	else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
386	return;
387	}
388	}
389
390	/**
391	* Regardless of pending count, invokes {@link #onCompletion},
392	* marks this task as complete with a {@code null} return value,
393	* and further triggers {@link #tryComplete} on this task's
394	* completer, if one exists. This method may be useful when
395	* forcing completion as soon as any one (versus all) of several
396	* subtask results are obtained.
397	*
398	* @param mustBeNull the {@code null} completion value
399	*/
400	public void complete(Void mustBeNull) {
401	CountedCompleter p;
402	onCompletion(this);
403	quietlyComplete();
404	if ((p = completer) != null)
405	p.tryComplete();
406	}
407
408	/**
409	* Support for FJT exception propagation
410	*/
411	void internalPropagateException(Throwable ex) {
412	CountedCompleter a = this, s = a;
413	while (a.onExceptionalCompletion(ex, s) &&
414	(a = (s = a).completer) != null && a.status >= 0)
415	a.recordExceptionalCompletion(ex);
416	}
417
418	/**
419	* Implements execution conventions for CountedCompleters
420	*/
421	protected final boolean exec() {
422	compute();
423	return false;
424	}
425
426	/**
427	* Always returns {@code null}.
428	*
429	* @return {@code null} always
430	*/
431	public final Void getRawResult() { return null; }
432
433	/**
434	* Requires null completion value.
435	*/
436	protected final void setRawResult(Void mustBeNull) { }
437
438	// Unsafe mechanics
439	private static final sun.misc.Unsafe U;
440	private static final long PENDING;
441	static {
442	try {
443	U = getUnsafe();
444	PENDING = U.objectFieldOffset
445	(CountedCompleter.class.getDeclaredField("pending"));
446	} catch (Exception e) {
447	throw new Error(e);
448	}
449	}
450
451
452	/**
453	* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
454	* Replace with a simple call to Unsafe.getUnsafe when integrating
455	* into a jdk.
456	*
457	* @return a sun.misc.Unsafe
458	*/
459	private static sun.misc.Unsafe getUnsafe() {
460	try {
461	return sun.misc.Unsafe.getUnsafe();
462	} catch (SecurityException se) {
463	try {
464	return java.security.AccessController.doPrivileged
465	(new java.security
466	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
467	public sun.misc.Unsafe run() throws Exception {
468	java.lang.reflect.Field f = sun.misc
469	.Unsafe.class.getDeclaredField("theUnsafe");
470	f.setAccessible(true);
471	return (sun.misc.Unsafe) f.get(null);
472	}});
473	} catch (java.security.PrivilegedActionException e) {
474	throw new RuntimeException("Could not initialize intrinsics",
475	e.getCause());
476	}
477	}
478	}
479
480	}