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package jsr166y; |
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/** |
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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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* 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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* 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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* 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()} once 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, and method {@link #onExceptionalCompletion} to |
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* perform an action upon any exception. |
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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. (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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* 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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* of method {@code compute}. Upon any exceptional completion, the |
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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. |
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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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* 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<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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* } |
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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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* return pool.invoke(new MapReducer<E>(null, array, mapper, |
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* reducer, 0, array.length)); |
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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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* Performs an action when method {@link #tryComplete} is invoked |
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* and there are no pending counts, or when the unconditional |
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* method {@link #complete} is invoked. By default, this method |
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* does nothing. |
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* does nothing. You can distinguish cases by checking the |
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* identity of the given caller argument. If not equal to {@code |
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* this}, then it is typically a subtask that may contain results |
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* (and/or links to other results) to combine. |
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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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* Regardless of pending count, invokes {@link #onCompletion}, |
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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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* #tryComplete} on this task's completer, if one exists. 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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* before invoking {@link #onCompletion} or marking this task as |
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* complete; its value is meaningful only for classes overriding |
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* {@code setRawResult}. |
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* |
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* <p>This method may be useful when forcing completion as soon as |
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* any one (versus all) of several subtask results are obtained. |
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* However, in the common (and recommended) case in which {@code |
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* setRawResult} is not overridden, this effect can be obtained |
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* more simply using {@code getRoot().quietlyComplete();}. |
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* |
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* @param rawResult the raw result |
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*/ |
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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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onCompletion(this); |
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quietlyComplete(); |
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if ((p = completer) != null) |
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p.tryComplete(); |
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/** |
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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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* If this method is overridden to update existing objects or |
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* fields, then it must in general be defined to be thread-safe. |
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*/ |
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protected void setRawResult(T t) { } |
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private static final long PENDING; |
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static { |
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try { |
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< |
U = getUnsafe(); |
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U = sun.misc.Unsafe.getUnsafe(); |
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PENDING = U.objectFieldOffset |
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(CountedCompleter.class.getDeclaredField("pending")); |
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} catch (Exception e) { |
