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java.lang.Objectjsr166y.ForkJoinTask<java.lang.Void>
jsr166y.RecursiveAction
public abstract class RecursiveAction
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 sketch of a ForkJoin sort that
sorts a given long[] array:
class SortTask extends RecursiveAction {
final long[] array; final int lo; final int hi;
SortTask(long[] array, int lo, int hi) {
this.array = array; this.lo = lo; this.hi = hi;
}
protected void compute() {
if (hi - lo < THRESHOLD)
sequentiallySort(array, lo, hi);
else {
int mid = (lo + hi) >>> 1;
invokeAll(new SortTask(array, lo, mid),
new SortTask(array, mid, hi));
merge(array, lo, hi);
}
}
}
You could then sort anArray by creating new
SortTask(anArray, 0, anArray.length-1) 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; final int 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;
}
}
| Constructor Summary | |
|---|---|
RecursiveAction()
|
|
| Method Summary | |
|---|---|
protected abstract void |
compute()
The main computation performed by this task. |
protected boolean |
exec()
Implements execution conventions for RecursiveActions. |
java.lang.Void |
getRawResult()
Always returns null. |
protected void |
setRawResult(java.lang.Void mustBeNull)
Requires null completion value. |
| Methods inherited from class jsr166y.ForkJoinTask |
|---|
adapt, adapt, adapt, cancel, complete, completeExceptionally, fork, get, get, getException, getPool, getQueuedTaskCount, getSurplusQueuedTaskCount, helpQuiesce, inForkJoinPool, invoke, invokeAll, invokeAll, invokeAll, isCancelled, isCompletedAbnormally, isCompletedNormally, isDone, join, peekNextLocalTask, pollNextLocalTask, pollTask, quietlyInvoke, quietlyJoin, reinitialize, tryUnfork |
| Methods inherited from class java.lang.Object |
|---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
| Constructor Detail |
|---|
public RecursiveAction()
| Method Detail |
|---|
protected abstract void compute()
public final java.lang.Void getRawResult()
null.
getRawResult in class ForkJoinTask<java.lang.Void>null alwaysprotected final void setRawResult(java.lang.Void mustBeNull)
setRawResult in class ForkJoinTask<java.lang.Void>mustBeNull - the valueprotected final boolean exec()
exec in class ForkJoinTask<java.lang.Void>true if completed normally
|
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