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package jsr166y; |
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/** |
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* A resultless {@link ForkJoinTask} with a completion action |
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* A {@link ForkJoinTask} with a completion action |
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* performed when triggered and there are no remaining pending |
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* actions. Uses of CountedCompleter are similar to those of other |
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* completion based components (such as {@link |
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* decremented; otherwise, the completion action is performed, and if |
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* this completer itself has a completer, the process is continued |
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* with its completer. As is the case with related synchronization |
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* components such as {@link Phaser} and {@link |
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* java.util.concurrent.Semaphore} these methods affect only internal |
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* counts; they do not establish any further internal bookkeeping. In |
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* particular, the identities of pending tasks are not maintained. As |
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* illustrated below, you can create subclasses that do record some or |
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* all pended tasks or their results when needed. |
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* components such as {@link java.util.concurrent.Phaser Phaser} and |
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* {@link java.util.concurrent.Semaphore Semaphore}, these methods |
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* affect only internal counts; they do not establish any further |
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* internal bookkeeping. In particular, the identities of pending |
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* tasks are not maintained. As illustrated below, you can create |
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* subclasses that do record some or all pending tasks or their |
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* results when needed. |
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* |
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* <p>A concrete CountedCompleter class must define method {@link |
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* #compute}, that should, in almost all use cases, invoke {@code |
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* normal completion, and method {@link #onExceptionalCompletion} to |
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* perform an action upon any exception. |
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* |
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* <p>CountedCompleters most often do not bear results, in which case |
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* they are normally declared as {@code CountedCompleter<Void>}, and |
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* will always return {@code null} as a result value. In other cases, |
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* you should override method {@link #getRawResult} to provide a |
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* result from {@code join(), invoke()}, and related methods. (Method |
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* {@link #setRawResult} by default plays no role in CountedCompleters |
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* but may be overridden for example to maintain fields holding result |
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* data.) |
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* |
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* <p>A CountedCompleter that does not itself have a completer (i.e., |
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* one for which {@link #getCompleter} returns {@code null}) can be |
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* used as a regular ForkJoinTask with this added functionality. |
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* <p><b>Parallel recursive decomposition.</b> CountedCompleters may |
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* be arranged in trees similar to those often used with {@link |
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* RecursiveAction}s, although the constructions involved in setting |
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* them up typically vary. Even though they entail a bit more |
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* them up typically vary. Here, the completer of each task is its |
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* parent in the computation tree. Even though they entail a bit more |
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* bookkeeping, CountedCompleters may be better choices when applying |
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* a possibly time-consuming operation (that cannot be further |
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* subdivided) to each element of an array or collection; especially |
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* <pre> {@code |
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* class MyOperation<E> { void apply(E e) { ... } } |
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* |
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* class ForEach<E> extends CountedCompleter { |
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* class ForEach<E> extends CountedCompleter<Void> { |
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* |
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* public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) { |
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* pool.invoke(new ForEach<E>(null, array, op, 0, array.length)); |
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* } |
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* |
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* final E[] array; final MyOperation<E> op; final int lo, hi; |
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* ForEach(CountedCompleter p, E[] array, MyOperation<E> op, int lo, int hi) { |
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* ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) { |
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* super(p); |
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* this.array = array; this.op = op; this.lo = lo; this.hi = hi; |
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* } |
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* and reductions are all of type {@code E}), one way to do this in |
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* divide and conquer designs is to have each subtask record its |
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* sibling, so that it can be accessed in method {@code onCompletion}. |
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* For clarity, this class uses explicit left and right subtasks, but |
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* variants of other streamlinings seen in the above example may also |
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* apply. |
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* This technique applies to reductions in which the order of |
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* combining left and right results does not matter; ordered |
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* reductions require explicit left/right designations. Variants of |
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* other streamlinings seen in the above examples may also apply. |
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* |
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* <pre> {@code |
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* class MyMapper<E> { E apply(E v) { ... } } |
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* class MyReducer<E> { E apply(E x, E y) { ... } } |
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* class MapReducer<E> extends CountedCompleter { |
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* class MapReducer<E> extends CountedCompleter<E> { |
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* final E[] array; final MyMapper<E> mapper; |
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* final MyReducer<E> reducer; final int lo, hi; |
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* MapReducer sibling; |
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* MapReducer<E> sibling; |
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* E result; |
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* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper, |
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* MyReducer<E> reducer, int lo, int hi) { |
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* result = reducer.apply(child.result, sib.result); |
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* } |
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* } |
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* public E getRawResult() { return result; } |
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* |
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* public static <E> E mapReduce(ForkJoinPool pool, E[] array, |
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* MyMapper<E> mapper, MyReducer<E> reducer) { |
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* MapReducer<E> mr = new MapReducer<E>(null, array, mapper, |
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* reducer, 0, array.length); |
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* pool.invoke(mr); |
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* return mr.result; |
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* return pool.invoke(new MapReducer<E>(null, array, mapper, |
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* reducer, 0, array.length)); |
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* } |
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* } }</pre> |
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* |
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* triggers another async task. For example: |
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* |
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* <pre> {@code |
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* class HeaderBuilder extends CountedCompleter { ... } |
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* class BodyBuilder extends CountedCompleter { ... } |
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* class PacketSender extends CountedCompleter { |
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* class HeaderBuilder extends CountedCompleter<...> { ... } |
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* class BodyBuilder extends CountedCompleter<...> { ... } |
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* class PacketSender extends CountedCompleter<...> { |
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* PacketSender(...) { super(null, 1); ... } // trigger on second completion |
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* public void compute() { } // never called |
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* public void onCompletion(CountedCompleter caller) { sendPacket(); } |
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* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); } |
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* } |
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* // sample use: |
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* PacketSender p = new PacketSender(); |
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* @since 1.8 |
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* @author Doug Lea |
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*/ |
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public abstract class CountedCompleter extends ForkJoinTask<Void> { |
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> |
public abstract class CountedCompleter<T> extends ForkJoinTask<T> { |
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private static final long serialVersionUID = 5232453752276485070L; |
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/** This task's completer, or null if none */ |
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final CountedCompleter completer; |
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final CountedCompleter<?> completer; |
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/** The number of pending tasks until completion */ |
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volatile int pending; |
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* @param completer this tasks completer, or {@code null} if none |
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* @param initialPendingCount the initial pending count |
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*/ |
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protected CountedCompleter(CountedCompleter completer, |
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protected CountedCompleter(CountedCompleter<?> completer, |
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int initialPendingCount) { |
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this.completer = completer; |
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this.pending = initialPendingCount; |
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* |
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* @param completer this tasks completer, or {@code null} if none |
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*/ |
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protected CountedCompleter(CountedCompleter completer) { |
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protected CountedCompleter(CountedCompleter<?> completer) { |
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this.completer = completer; |
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} |
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* @param caller the task invoking this method (which may |
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* be this task itself). |
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*/ |
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public void onCompletion(CountedCompleter caller) { |
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public void onCompletion(CountedCompleter<?> caller) { |
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} |
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/** |
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* @return true if this exception should be propagated to this |
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* tasks completer, if one exists. |
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*/ |
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public boolean onExceptionalCompletion(Throwable ex, CountedCompleter caller) { |
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public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) { |
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return true; |
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} |
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* |
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* @return the completer |
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*/ |
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public final CountedCompleter getCompleter() { |
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public final CountedCompleter<?> getCompleter() { |
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return completer; |
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} |
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* else marks this task as complete. |
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*/ |
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public final void tryComplete() { |
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CountedCompleter a = this, s = a; |
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CountedCompleter<?> a = this, s = a; |
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for (int c;;) { |
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if ((c = a.pending) == 0) { |
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a.onCompletion(s); |
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/** |
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* Regardless of pending count, invokes {@link #onCompletion}, |
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* marks this task as complete with a {@code null} return value, |
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* and further triggers {@link #tryComplete} on this task's |
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* completer, if one exists. This method may be useful when |
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* forcing completion as soon as any one (versus all) of several |
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* subtask results are obtained. |
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* marks this task as complete and further triggers {@link |
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* #tryComplete} on this task's completer, if one exists. This |
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* method may be useful when forcing completion as soon as any one |
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* (versus all) of several subtask results are obtained. The |
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* given rawResult is used as an argument to {@link #setRawResult} |
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* before marking this task as complete; its value is meaningful |
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* only for classes overriding {@code setRawResult}. |
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* |
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* @param mustBeNull the {@code null} completion value |
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* @param rawResult the raw result |
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*/ |
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public void complete(Void mustBeNull) { |
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CountedCompleter p; |
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public void complete(T rawResult) { |
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CountedCompleter<?> p; |
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onCompletion(this); |
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setRawResult(rawResult); |
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quietlyComplete(); |
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if ((p = completer) != null) |
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p.tryComplete(); |
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* Support for FJT exception propagation |
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*/ |
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void internalPropagateException(Throwable ex) { |
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CountedCompleter a = this, s = a; |
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CountedCompleter<?> a = this, s = a; |
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while (a.onExceptionalCompletion(ex, s) && |
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(a = (s = a).completer) != null && a.status >= 0) |
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a.recordExceptionalCompletion(ex); |
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} |
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|
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/** |
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* Always returns {@code null}. |
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> |
* Returns the result of the computation. By default |
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* returns {@code null}, which is appropriate for {@code Void} |
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* actions, but in other cases should be overridden. |
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* |
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< |
* @return {@code null} always |
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* @return the result of the computation |
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*/ |
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< |
public final Void getRawResult() { return null; } |
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> |
public T getRawResult() { return null; } |
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|
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/** |
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* Requires null completion value. |
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> |
* A method that result-bearing CountedCompleters may optionally |
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* use to help maintain result data. By default, does nothing. |
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*/ |
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< |
protected final void setRawResult(Void mustBeNull) { } |
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> |
protected void setRawResult(T t) { } |
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|
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// Unsafe mechanics |
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private static final sun.misc.Unsafe U; |
