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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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import java.util.*; |
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import java.util.concurrent.*; |
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|
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/** |
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* A computation that is broken into a series of task executions, each |
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* separated by a Phaser arrival. Concrete subclasses must |
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* define method {@code compute}, that performs the action occurring |
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* at each step of the barrier. Upon invocation of this task, the |
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* {@code compute} method is repeatedly invoked until the barrier |
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* {@code isTerminated} or until its execution throws an exception. |
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* |
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* <p><b>Sample Usage.</b> Here is a sketch of a set of CyclicActions |
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* that each perform 500 iterations of an imagined image smoothing |
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* operation. Note that the aggregate ImageSmoother task itself is not |
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* a CyclicTask. |
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* |
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* <pre> |
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* class ImageSmoother extends RecursiveAction { |
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* protected void compute() { |
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* Phaser b = new Phaser() { |
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* protected boolean onAdvance(int cycle, int registeredParties) { |
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* return registeredParties <= 0 || cycle >= 500; |
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* } |
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* } |
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* int n = pool.getParallelismLevel(); |
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* CyclicAction[] actions = new CyclicAction[n]; |
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* for (int i = 0; i < n; ++i) { |
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* action[i] = new CyclicAction(b) { |
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* protected void compute() { |
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* smoothImagePart(i); |
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* } |
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* } |
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* } |
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* invokeAll(actions); |
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* } |
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* } |
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* </pre> |
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*/ |
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public abstract class CyclicAction extends ForkJoinTask<Void> { |
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final Phaser barrier; |
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boolean deregistered; |
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int lastArrived; |
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|
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/** |
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* Constructs a new CyclicAction using the supplied barrier, |
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* registering for this barrier upon construction. |
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* @param barrier the barrier |
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*/ |
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public CyclicAction(Phaser barrier) { |
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this.barrier = barrier; |
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lastArrived = barrier.register() - 1; |
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} |
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|
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/** |
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* The computation performed by this task on each cycle of the |
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* barrier. While you must define this method, you should not in |
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* general call it directly. |
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*/ |
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protected abstract void step(); |
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|
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/** |
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* Returns the barrier |
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*/ |
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public final Phaser getBarrier() { |
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return barrier; |
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} |
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|
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/** |
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* Returns the current cycle of the barrier |
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*/ |
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public final int getCycle() { |
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return barrier.getPhase(); |
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} |
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|
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public final Void getRawResult() { return null; } |
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protected final void setRawResult(Void mustBeNull) { } |
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|
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private void deregister() { |
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if (!deregistered) { |
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deregistered = true; |
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barrier.arriveAndDeregister(); |
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} |
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} |
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|
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protected final boolean exec() { |
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Phaser b = barrier; |
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if (!isDone()) { |
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b.awaitAdvance(lastArrived); |
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if (b.getPhase() >= 0) { |
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try { |
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step(); |
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} catch (Throwable rex) { |
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deregister(); |
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completeExceptionally(rex); |
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return false; |
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} |
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if ((lastArrived = b.arrive()) >= 0) { |
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this.fork(); |
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return false; |
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} |
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} |
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} |
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deregister(); |
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return true; |
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} |
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|
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} |
