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| 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 | + | |
| 36 | +package 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 | + */ | |
| 153 | +public 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 | +} | |