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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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* 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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* java.nio.channels.CompletionHandler}) except that multiple |
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* <em>pending</em> completions may be necessary to trigger the {@link |
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* #onCompletion} action, not just one. Unless initialized otherwise, |
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* the {@link #getPendingCount pending count} starts at zero, but may |
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* be (atomically) changed using methods {@link #setPendingCount}, |
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* {@link #addToPendingCount}, and {@link |
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* A {@link ForkJoinTask} with a completion action performed when |
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* triggered and there are no remaining pending |
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* actions. CountedCompleters are in general more robust in the |
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* presence of subtask stalls and blockage than are other forms for |
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* ForkJoinTasks, but are in general less intuitive to program. Uses |
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* of CountedCompleter are similar to those of other completion based |
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* components (such as {@link java.nio.channels.CompletionHandler}) |
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* except that multiple <em>pending</em> completions may be necessary |
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* to trigger the {@link #onCompletion} action, not just one. Unless |
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* initialized otherwise, the {@link #getPendingCount pending count} |
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* starts at zero, but may be (atomically) changed using methods |
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* {@link #setPendingCount}, {@link #addToPendingCount}, and {@link |
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* #compareAndSetPendingCount}. Upon invocation of {@link |
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* #tryComplete}, if the pending action count is nonzero, it is |
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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 most basic synchronization |
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* constructs, these methods affect only internal counts; they do not |
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* establish any further internal bookkeeping. In particular, the |
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* identities of pending tasks are not maintained. As illustrated |
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* below, you can create subclasses that do record some or all pended |
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* tasks or their results when needed. |
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* with its completer. As is the case with related synchronization |
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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. Because CountedCompleters provide only basic |
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* synchronization mechanisms, it may be useful to create further |
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* abstract subclasses that maintain linkages and fields and support |
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* methods appropriate for a set of related usages. |
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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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* tryComplete()} before returning. The class may also optionally |
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* override method {@link #onCompletion} to perform an action upon |
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* normal completion. |
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* #compute}, that should in most cases (as illustrated below), invoke |
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* {@code tryComplete()} once before returning. The class may also |
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* optionally override method {@link #onCompletion} to perform an |
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* action upon normal completion, and method {@link |
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* #onExceptionalCompletion} to 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. In |
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* general, this method should return the value of a field (or a |
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* function of one or more fields) of the CountedCompleter object that |
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* holds the result upon completion. Method {@link #setRawResult} by |
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* default plays no role in CountedCompleters. It is possible, but |
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* not usually applicable, to override this method to maintain other |
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* objects or fields holding result 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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* {@link #complete}, {@link ForkJoinTask#cancel}, {@link |
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* ForkJoinTask#completeExceptionally} or upon exceptional completion |
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* of method {@code compute}. Upon any exceptional completion, the |
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* exception is relayed to a task's completer (and its completer, and |
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* so on), if one exists and it has not otherwise already completed. |
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* exception may be relayed to a task's completer (and its completer, |
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* and so on), if one exists and it has not otherwise already |
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* completed. Similarly, cancelling an internal CountedCompleter has |
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* only a local effect on that completer, so is not often useful. |
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* |
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* <p><b>Sample Usages.</b> |
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* |
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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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* continuations, other threads need not block waiting to perform |
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* them. |
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* |
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* <p> For example, here is an initial version of a class that uses |
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* <p>For example, here is an initial version of a class that uses |
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* divide-by-two recursive decomposition to divide work into single |
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* pieces (leaf tasks). Even when work is split into individual calls, |
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* tree-based techniques are usually preferable to directly forking |
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* pair of subtasks to finish triggers completion of its parent |
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* (because no result combination is performed, the default no-op |
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* implementation of method {@code onCompletion} is not overridden). A |
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* static utility method sets up the base task and invokes it: |
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* static utility method sets up the base task and invokes it |
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* (here, implicitly using the {@link ForkJoinPool#commonPool()}). |
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* |
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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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* public static <E> void forEach(E[] array, MyOperation<E> op) { |
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* new ForEach<E>(null, array, op, 0, array.length).invoke(); |
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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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* |
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* As a further improvement, notice that the left task need not even |
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* exist. Instead of creating a new one, we can iterate using the |
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* original task, and add a pending count for each fork: |
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* original task, and add a pending count for each fork. |
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* |
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* <pre> {@code |
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* class ForEach<E> ... |
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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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* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) { |
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* return new MapReducer<E>(null, array, mapper, reducer, |
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* 0, array.length).invoke(); |
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* } |
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* } }</pre> |
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* |
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* Here, method {@code onCompletion} takes a form common to many |
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* completion designs that combine results. This callback-style method |
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* is triggered once per task, in either of the two different contexts |
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* in which the pending count is, or becomes, zero: (1) by a task |
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* itself, if its pending count is zero upon invocation of {@code |
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* tryComplete}, or (2) by any of its subtasks when they complete and |
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* decrement the pending count to zero. The {@code caller} argument |
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* distinguishes cases. Most often, when the caller is {@code this}, |
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* no action is necessary. Otherwise the caller argument can be used |
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* (usually via a cast) to supply a value (and/or links to other |
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* values) to be combined. Asuuming proper use of pending counts, the |
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* actions inside {@code onCompletion} occur (once) upon completion of |
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* a task and its subtasks. No additional synchronization is required |
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* within this method to ensure thread safety of accesses to fields of |
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* this task or other completed tasks. |
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* |
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* <p><b>Searching.</b> A tree of CountedCompleters can search for a |
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* value or property in different parts of a data structure, and |
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* report a result in an {@link java.util.concurrent.AtomicReference} |
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* as soon as one is found. The others can poll the result to avoid |
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* unnecessary work. (You could additionally {@link #cancel} other |
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* tasks, but it is usually simpler and more efficient to just let |
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* them notice that the result is set and if so skip further |
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* processing.) Illustrating again with an array using full |
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* partitioning (again, in practice, leaf tasks will almost always |
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* process more than one element): |
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* |
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* <pre> {@code |
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* class Searcher<E> extends CountedCompleter<E> { |
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* final E[] array; final AtomicReference<E> result; final int lo, hi; |
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* Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) { |
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* super(p); |
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* this.array = array; this.result = result; this.lo = lo; this.hi = hi; |
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* } |
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* public E getRawResult() { return result.get(); } |
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* public void compute() { // similar to ForEach version 3 |
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* int l = lo, h = hi; |
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* while (h - l >= 2 && result.get() == null) { |
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* int mid = (l + h) >>> 1; |
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* addToPendingCount(1); |
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* new Searcher(this, array, result, mid, h).fork(); |
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* h = mid; |
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* } |
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* if (h > l && result.get() == null && matches(array[l]) && |
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* result.compareAndSet(null, array[l])) |
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* getRoot().quietlyComplete(); // root task is now joinable |
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* |
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* tryComplete(); // normally complete whether or not found |
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* } |
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* boolean matches(E e) { ... } // return true if found |
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* |
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* public static <E> E search(E[] array) { |
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* return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke(); |
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* } |
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*}}</pre> |
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* |
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* In this example, as well as others in which tasks have no other |
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* effects except to compareAndSet a common result, the trailing |
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* unconditional invocation of {@code tryComplete} could be made |
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* conditional ({@code if (result.get() == null) tryComplete();}) |
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* because no further bookkeeping is required to manage completions |
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* once the root task completes. |
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* |
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* <p><b>Triggers.</b> Some CountedCompleters are themselves never |
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* forked, but instead serve as bits of plumbing in other designs; |
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* including those in which the completion of one of more async tasks |
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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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|
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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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|
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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, |
341 |
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int initialPendingCount) { |
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this.completer = completer; |
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this.pending = initialPendingCount; |
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* |
350 |
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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) { |
352 |
> |
protected CountedCompleter(CountedCompleter<?> completer) { |
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this.completer = completer; |
354 |
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} |
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|
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public abstract void compute(); |
368 |
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|
369 |
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/** |
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* Executes the completion action when method {@link #tryComplete} |
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< |
* is invoked and there are no pending counts, or when the |
372 |
< |
* unconditional method {@link #complete} is invoked. By default, |
373 |
< |
* this method does nothing. |
370 |
> |
* Performs an action when method {@link #tryComplete} is invoked |
371 |
> |
* and there are no pending counts, or when the unconditional |
372 |
> |
* method {@link #complete} is invoked. By default, this method |
373 |
> |
* does nothing. You can distinguish cases by checking the |
374 |
> |
* identity of the given caller argument. If not equal to {@code |
375 |
> |
* this}, then it is typically a subtask that may contain results |
376 |
> |
* (and/or links to other results) to combine. |
377 |
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* |
378 |
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* @param caller the task invoking this method (which may |
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* be this task itself). |
380 |
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*/ |
381 |
< |
public void onCompletion(CountedCompleter caller) { |
381 |
> |
public void onCompletion(CountedCompleter<?> caller) { |
382 |
> |
} |
383 |
> |
|
384 |
> |
/** |
385 |
> |
* Performs an action when method {@link #completeExceptionally} |
386 |
> |
* is invoked or method {@link #compute} throws an exception, and |
387 |
> |
* this task has not otherwise already completed normally. On |
388 |
> |
* entry to this method, this task {@link |
389 |
> |
* ForkJoinTask#isCompletedAbnormally}. The return value of this |
390 |
> |
* method controls further propagation: If {@code true} and this |
391 |
> |
* task has a completer, then this completer is also completed |
392 |
> |
* exceptionally. The default implementation of this method does |
393 |
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* nothing except return {@code true}. |
394 |
> |
* |
395 |
> |
* @param ex the exception |
396 |
> |
* @param caller the task invoking this method (which may |
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* be this task itself). |
398 |
> |
* @return true if this exception should be propagated to this |
399 |
> |
* tasks completer, if one exists. |
400 |
> |
*/ |
401 |
> |
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) { |
402 |
> |
return true; |
403 |
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} |
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|
405 |
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/** |
408 |
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* |
409 |
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* @return the completer |
410 |
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*/ |
411 |
< |
public final CountedCompleter getCompleter() { |
411 |
> |
public final CountedCompleter<?> getCompleter() { |
412 |
|
return completer; |
413 |
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} |
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|
453 |
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} |
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|
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/** |
456 |
+ |
* Returns the root of the current computation; i.e., this |
457 |
+ |
* task if it has no completer, else its completer's root. |
458 |
+ |
* |
459 |
+ |
* @return the root of the current computation |
460 |
+ |
*/ |
461 |
+ |
public final CountedCompleter<?> getRoot() { |
462 |
+ |
CountedCompleter<?> a = this, p; |
463 |
+ |
while ((p = a.completer) != null) |
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a = p; |
465 |
+ |
return a; |
466 |
+ |
} |
467 |
+ |
|
468 |
+ |
/** |
469 |
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* If the pending count is nonzero, decrements the count; |
470 |
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* otherwise invokes {@link #onCompletion} and then similarly |
471 |
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* tries to complete this task's completer, if one exists, |
472 |
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* else marks this task as complete. |
473 |
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*/ |
474 |
|
public final void tryComplete() { |
475 |
< |
for (CountedCompleter a = this, s = a;;) { |
476 |
< |
int c; |
475 |
> |
CountedCompleter<?> a = this, s = a; |
476 |
> |
for (int c;;) { |
477 |
|
if ((c = a.pending) == 0) { |
478 |
|
a.onCompletion(s); |
479 |
|
if ((a = (s = a).completer) == null) { |
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|
489 |
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/** |
490 |
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* Regardless of pending count, invokes {@link #onCompletion}, |
491 |
< |
* marks this task as complete with a {@code null} return value, |
492 |
< |
* and further triggers {@link #tryComplete} on this task's |
493 |
< |
* completer, if one exists. This method may be useful when |
494 |
< |
* forcing completion as soon as any one (versus all) of several |
495 |
< |
* subtask results are obtained. |
496 |
< |
* |
497 |
< |
* @param mustBeNull the {@code null} completion value |
498 |
< |
*/ |
499 |
< |
public void complete(Void mustBeNull) { |
500 |
< |
CountedCompleter p; |
491 |
> |
* marks this task as complete and further triggers {@link |
492 |
> |
* #tryComplete} on this task's completer, if one exists. The |
493 |
> |
* given rawResult is used as an argument to {@link #setRawResult} |
494 |
> |
* before invoking {@link #onCompletion} or marking this task as |
495 |
> |
* complete; its value is meaningful only for classes overriding |
496 |
> |
* {@code setRawResult}. |
497 |
> |
* |
498 |
> |
* <p>This method may be useful when forcing completion as soon as |
499 |
> |
* any one (versus all) of several subtask results are obtained. |
500 |
> |
* However, in the common (and recommended) case in which {@code |
501 |
> |
* setRawResult} is not overridden, this effect can be obtained |
502 |
> |
* more simply using {@code getRoot().quietlyComplete();}. |
503 |
> |
* |
504 |
> |
* @param rawResult the raw result |
505 |
> |
*/ |
506 |
> |
public void complete(T rawResult) { |
507 |
> |
CountedCompleter<?> p; |
508 |
> |
setRawResult(rawResult); |
509 |
|
onCompletion(this); |
510 |
|
quietlyComplete(); |
511 |
|
if ((p = completer) != null) |
513 |
|
} |
514 |
|
|
515 |
|
/** |
516 |
+ |
* Support for FJT exception propagation |
517 |
+ |
*/ |
518 |
+ |
void internalPropagateException(Throwable ex) { |
519 |
+ |
CountedCompleter<?> a = this, s = a; |
520 |
+ |
while (a.onExceptionalCompletion(ex, s) && |
521 |
+ |
(a = (s = a).completer) != null && a.status >= 0) |
522 |
+ |
a.recordExceptionalCompletion(ex); |
523 |
+ |
} |
524 |
+ |
|
525 |
+ |
/** |
526 |
|
* Implements execution conventions for CountedCompleters |
527 |
|
*/ |
528 |
|
protected final boolean exec() { |
531 |
|
} |
532 |
|
|
533 |
|
/** |
534 |
< |
* Always returns {@code null}. |
534 |
> |
* Returns the result of the computation. By default |
535 |
> |
* returns {@code null}, which is appropriate for {@code Void} |
536 |
> |
* actions, but in other cases should be overridden. |
537 |
|
* |
538 |
< |
* @return {@code null} always |
394 |
< |
*/ |
395 |
< |
public final Void getRawResult() { return null; } |
396 |
< |
|
397 |
< |
/** |
398 |
< |
* Requires null completion value. |
538 |
> |
* @return the result of the computation |
539 |
|
*/ |
540 |
< |
protected final void setRawResult(Void mustBeNull) { } |
540 |
> |
public T getRawResult() { return null; } |
541 |
|
|
542 |
|
/** |
543 |
< |
* Support for FJT exception propagation |
543 |
> |
* A method that result-bearing CountedCompleters may optionally |
544 |
> |
* use to help maintain result data. By default, does nothing. |
545 |
> |
* If this method is overridden to update existing objects or |
546 |
> |
* fields, then it must in general be defined to be thread-safe. |
547 |
|
*/ |
548 |
< |
final ForkJoinTask<?> internalGetCompleter() { return completer; } |
548 |
> |
protected void setRawResult(T t) { } |
549 |
|
|
550 |
|
// Unsafe mechanics |
551 |
|
private static final sun.misc.Unsafe U; |
552 |
|
private static final long PENDING; |
553 |
|
static { |
554 |
|
try { |
555 |
< |
U = getUnsafe(); |
555 |
> |
U = sun.misc.Unsafe.getUnsafe(); |
556 |
|
PENDING = U.objectFieldOffset |
557 |
|
(CountedCompleter.class.getDeclaredField("pending")); |
558 |
|
} catch (Exception e) { |
560 |
|
} |
561 |
|
} |
562 |
|
|
420 |
– |
|
563 |
|
/** |
564 |
|
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
565 |
|
* Replace with a simple call to Unsafe.getUnsafe when integrating |
587 |
|
} |
588 |
|
} |
589 |
|
} |
448 |
– |
|
590 |
|
} |