RecursiveAction.java revision 51b1b6997fd3f980076b8081f7f1165ccc2a4008 
1/*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.  Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25/*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36package java.util.concurrent;
37
38/**
39 * A recursive resultless {@link ForkJoinTask}.  This class
40 * establishes conventions to parameterize resultless actions as
41 * {@code Void} {@code ForkJoinTask}s. Because {@code null} is the
42 * only valid value of type {@code Void}, methods such as join always
43 * return {@code null} upon completion.
44 *
45 * <p><b>Sample Usages.</b> Here is a sketch of a ForkJoin sort that
46 * sorts a given {@code long[]} array:
47 *
48 *  <pre> {@code
49 * class SortTask extends RecursiveAction {
50 *   final long[] array; final int lo; final int hi;
51 *   SortTask(long[] array, int lo, int hi) {
52 *     this.array = array; this.lo = lo; this.hi = hi;
53 *   }
54 *   protected void compute() {
55 *     if (hi - lo < THRESHOLD)
56 *       sequentiallySort(array, lo, hi);
57 *     else {
58 *       int mid = (lo + hi) >>> 1;
59 *       invokeAll(new SortTask(array, lo, mid),
60 *                 new SortTask(array, mid, hi));
61 *       merge(array, lo, hi);
62 *     }
63 *   }
64 * }}</pre>
65 *
66 * You could then sort {@code anArray} by creating {@code new
67 * SortTask(anArray, 0, anArray.length-1) } and invoking it in a
68 * ForkJoinPool.  As a more concrete simple example, the following
69 * task increments each element of an array:
70 *  <pre> {@code
71 * class IncrementTask extends RecursiveAction {
72 *   final long[] array; final int lo; final int hi;
73 *   IncrementTask(long[] array, int lo, int hi) {
74 *     this.array = array; this.lo = lo; this.hi = hi;
75 *   }
76 *   protected void compute() {
77 *     if (hi - lo < THRESHOLD) {
78 *       for (int i = lo; i < hi; ++i)
79 *         array[i]++;
80 *     }
81 *     else {
82 *       int mid = (lo + hi) >>> 1;
83 *       invokeAll(new IncrementTask(array, lo, mid),
84 *                 new IncrementTask(array, mid, hi));
85 *     }
86 *   }
87 * }}</pre>
88 *
89 * <p>The following example illustrates some refinements and idioms
90 * that may lead to better performance: RecursiveActions need not be
91 * fully recursive, so long as they maintain the basic
92 * divide-and-conquer approach. Here is a class that sums the squares
93 * of each element of a double array, by subdividing out only the
94 * right-hand-sides of repeated divisions by two, and keeping track of
95 * them with a chain of {@code next} references. It uses a dynamic
96 * threshold based on method {@code getSurplusQueuedTaskCount}, but
97 * counterbalances potential excess partitioning by directly
98 * performing leaf actions on unstolen tasks rather than further
99 * subdividing.
100 *
101 *  <pre> {@code
102 * double sumOfSquares(ForkJoinPool pool, double[] array) {
103 *   int n = array.length;
104 *   Applyer a = new Applyer(array, 0, n, null);
105 *   pool.invoke(a);
106 *   return a.result;
107 * }
108 *
109 * class Applyer extends RecursiveAction {
110 *   final double[] array;
111 *   final int lo, hi;
112 *   double result;
113 *   Applyer next; // keeps track of right-hand-side tasks
114 *   Applyer(double[] array, int lo, int hi, Applyer next) {
115 *     this.array = array; this.lo = lo; this.hi = hi;
116 *     this.next = next;
117 *   }
118 *
119 *   double atLeaf(int l, int h) {
120 *     double sum = 0;
121 *     for (int i = l; i < h; ++i) // perform leftmost base step
122 *       sum += array[i] * array[i];
123 *     return sum;
124 *   }
125 *
126 *   protected void compute() {
127 *     int l = lo;
128 *     int h = hi;
129 *     Applyer right = null;
130 *     while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
131 *        int mid = (l + h) >>> 1;
132 *        right = new Applyer(array, mid, h, right);
133 *        right.fork();
134 *        h = mid;
135 *     }
136 *     double sum = atLeaf(l, h);
137 *     while (right != null) {
138 *        if (right.tryUnfork()) // directly calculate if not stolen
139 *          sum += right.atLeaf(right.lo, right.hi);
140 *       else {
141 *          right.join();
142 *          sum += right.result;
143 *        }
144 *        right = right.next;
145 *      }
146 *     result = sum;
147 *   }
148 * }}</pre>
149 *
150 * @since 1.7
151 * @author Doug Lea
152 */
153public abstract class RecursiveAction extends ForkJoinTask<Void> {
154    private static final long serialVersionUID = 5232453952276485070L;
155
156    /**
157     * The main computation performed by this task.
158     */
159    protected abstract void compute();
160
161    /**
162     * Always returns {@code null}.
163     *
164     * @return {@code null} always
165     */
166    public final Void getRawResult() { return null; }
167
168    /**
169     * Requires null completion value.
170     */
171    protected final void setRawResult(Void mustBeNull) { }
172
173    /**
174     * Implements execution conventions for RecursiveActions.
175     */
176    protected final boolean exec() {
177        compute();
178        return true;
179    }
180
181}
182