RecursiveAction

概述

一个递归的无结果的ForkJoinTask。此类建立了将无结果操作参数化为Void ForkJoinTasks的约定。因为null是Void类型的唯一有效值，因此完成后诸如join等方法总是返回null。 样例用法。这是一个简单但完整的ForkJoin排序，可对给定的long []数组进行排序：

 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++];
   }
 }

您可以通过创建new SortTask（anArray）并在ForkJoinPool中调用它来对anArray进行排序。作为一个更具体的简单示例，以下任务增加数组的每个元素：

 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));
     }
   }
 }

以下示例说明了一些可能导致更好性能的改进和惯用法：RecursiveActions不需要完全递归，只要它们保持基本的分治方法即可。这是一个类，通过仅对重复除以二的右侧进行分裂，并通过next引用链跟踪它们，对double数组的每个元素的平方和。它使用基于方法getSurplusQueuedTaskCount的动态阈值，但通过直接对未被盗用的任务执行叶动作而不是进一步细分来平衡潜在的过度分割。

 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;
   }
 }