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/OpenJDK 21

Class RecursiveAction

java.lang.Object
java.util.concurrent.ForkJoinTask<Void>
java.util.concurrent.RecursiveAction
All Implemented Interfaces:
Serializable, Future<Void>
public abstract class RecursiveAction extends ForkJoinTask<Void>
A recursive resultless ForkJoinTask. This class establishes conventions to parameterize resultless actions as Void ForkJoinTasks. Because null is the only valid value of type Void, methods such as join always return null upon completion.

Sample Usages. Here is a simple but complete ForkJoin sort that sorts a given long[] array:

 
 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++];
   }
 }
You could then sort anArray by creating new SortTask(anArray) and invoking it in a ForkJoinPool. As a more concrete simple example, the following task increments each element of an array:
 
 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));
     }
   }
 }

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 next references. It uses a dynamic threshold based on method getSurplusQueuedTaskCount, but counterbalances potential excess partitioning by directly performing leaf actions on unstolen tasks rather than further subdividing.

 
 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;
   }
 }
Since:
1.7
See Also:

Nested Class Summary

Nested classes/interfaces declared in interface java.util.concurrent.Future

Future.State

Constructor Summary

Constructor Description
RecursiveAction()
Constructor for subclasses to call.

Method Summary

Modifier and Type Method Description
protected abstract void compute()
The main computation performed by this task.
protected final boolean exec()
Implements execution conventions for RecursiveActions.
final Void getRawResult()
Always returns null.
protected final void setRawResult(Void mustBeNull)
Requires null completion value.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods declared in interface java.util.concurrent.Future

exceptionNow, resultNow, state

Constructor Details

RecursiveAction

public RecursiveAction()
Constructor for subclasses to call.

Method Details

compute

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

getRawResult

public final Void getRawResult()
Always returns null.
Specified by:
getRawResult in class ForkJoinTask<Void>
Returns:
null always

setRawResult

protected final void setRawResult(Void mustBeNull)
Requires null completion value.
Specified by:
setRawResult in class ForkJoinTask<Void>
Parameters:
mustBeNull - the value

exec

protected final boolean exec()
Implements execution conventions for RecursiveActions.
Specified by:
exec in class ForkJoinTask<Void>
Returns:
true if this task is known to have completed normally

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/RecursiveAction.html