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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 | } | ||