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Diffstat (limited to 'luni/src/main/java/java/util/concurrent/RecursiveAction.java')
-rw-r--r-- | luni/src/main/java/java/util/concurrent/RecursiveAction.java | 165 |
1 files changed, 165 insertions, 0 deletions
diff --git a/luni/src/main/java/java/util/concurrent/RecursiveAction.java b/luni/src/main/java/java/util/concurrent/RecursiveAction.java new file mode 100644 index 0000000..48066c9 --- /dev/null +++ b/luni/src/main/java/java/util/concurrent/RecursiveAction.java @@ -0,0 +1,165 @@ +/* + * 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: + * final static 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 + * @hide + * @author Doug Lea + */ +public abstract class RecursiveAction extends ForkJoinTask<Void> { + private static final long serialVersionUID = 5232453952276485070L; + + /** + * 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; + } + +} |