src/java.base/share/classes/java/util/concurrent/RecursiveAction.java

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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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 java.util.concurrent;

/**
 * A recursive resultless {@link ForkJoinTask}.  This class
 * establishes conventions to parameterize resultless actions as
 * {@code Void} {@code ForkJoinTask}s. Because {@code null} is the
 * only valid value of type {@code Void}, methods such as {@code join}
 * always return {@code null} upon completion.
 *
 * <p><b>Sample Usages.</b> Here is a simple but complete ForkJoin
 * sort that sorts a given {@code long[]} array:
 *
 * <pre> {@code
 * static class SortTask extends RecursiveAction {
 *   final long[] array; final int lo, hi;
 *   SortTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   SortTask(long[] array) { this(array, 0, array.length); }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD)
 *       sortSequentially(lo, hi);
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new SortTask(array, lo, mid),
 *                 new SortTask(array, mid, hi));
 *       merge(lo, mid, hi);
 *     }
 *   }
 *   // implementation details follow:
 *   static final int THRESHOLD = 1000;
 *   void sortSequentially(int lo, int hi) {
 *     Arrays.sort(array, lo, hi);
 *   }
 *   void merge(int lo, int mid, int hi) {
 *     long[] buf = Arrays.copyOfRange(array, lo, mid);
 *     for (int i = 0, j = lo, k = mid; i < buf.length; j++)
 *       array[j] = (k == hi || buf[i] < array[k]) ?
 *         buf[i++] : array[k++];
 *   }
 * }}</pre>
 *
 * You could then sort {@code anArray} by creating {@code new
 * SortTask(anArray)} and invoking it in a ForkJoinPool.  As a more
 * concrete simple example, the following task increments each element
 * of an array:
 * <pre> {@code
 * class IncrementTask extends RecursiveAction {
 *   final long[] array; final int lo, hi;
 *   IncrementTask(long[] array, int lo, int hi) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *   }
 *   protected void compute() {
 *     if (hi - lo < THRESHOLD) {
 *       for (int i = lo; i < hi; ++i)
 *         array[i]++;
 *     }
 *     else {
 *       int mid = (lo + hi) >>> 1;
 *       invokeAll(new IncrementTask(array, lo, mid),
 *                 new IncrementTask(array, mid, hi));
 *     }
 *   }
 * }}</pre>
 *
 * <p>The following example illustrates some refinements and idioms
 * that may lead to better performance: RecursiveActions need not be
 * fully recursive, so long as they maintain the basic
 * divide-and-conquer approach. Here is a class that sums the squares
 * of each element of a double array, by subdividing out only the
 * right-hand-sides of repeated divisions by two, and keeping track of
 * them with a chain of {@code next} references. It uses a dynamic
 * threshold based on method {@code getSurplusQueuedTaskCount}, but
 * counterbalances potential excess partitioning by directly
 * performing leaf actions on unstolen tasks rather than further
 * subdividing.
 *
 * <pre> {@code
 * double sumOfSquares(ForkJoinPool pool, double[] array) {
 *   int n = array.length;
 *   Applyer a = new Applyer(array, 0, n, null);
 *   pool.invoke(a);
 *   return a.result;
 * }
 *
 * class Applyer extends RecursiveAction {
 *   final double[] array;
 *   final int lo, hi;
 *   double result;
 *   Applyer next; // keeps track of right-hand-side tasks
 *   Applyer(double[] array, int lo, int hi, Applyer next) {
 *     this.array = array; this.lo = lo; this.hi = hi;
 *     this.next = next;
 *   }
 *
 *   double atLeaf(int l, int h) {
 *     double sum = 0;
 *     for (int i = l; i < h; ++i) // perform leftmost base step
 *       sum += array[i] * array[i];
 *     return sum;
 *   }
 *
 *   protected void compute() {
 *     int l = lo;
 *     int h = hi;
 *     Applyer right = null;
 *     while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
 *       int mid = (l + h) >>> 1;
 *       right = new Applyer(array, mid, h, right);
 *       right.fork();
 *       h = mid;
 *     }
 *     double sum = atLeaf(l, h);
 *     while (right != null) {
 *       if (right.tryUnfork()) // directly calculate if not stolen
 *         sum += right.atLeaf(right.lo, right.hi);
 *       else {
 *         right.join();
 *         sum += right.result;
 *       }
 *       right = right.next;
 *     }
 *     result = sum;
 *   }
 * }}</pre>
 *
 * @since 1.7
 * @author Doug Lea
 */
public abstract class RecursiveAction extends ForkJoinTask<Void> {
    private static final long serialVersionUID = 5232453952276485070L;

    /**
     * Constructor for subclasses to call.
     */
    public RecursiveAction() {}

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

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

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

    /**
     * Implements execution conventions for RecursiveActions.
     */
    protected final boolean exec() {
        compute();
        return true;
    }

}