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package jsr166e; |
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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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* #compareAndSetPendingCount}. Upon invocation of {@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 actions. |
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* CountedCompleters are in general more robust in the |
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* presence of subtask stalls and blockage than are other forms of |
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* ForkJoinTasks, but are less intuitive to program. Uses of |
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* 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 completion action {@link #onCompletion(CountedCompleter)}, |
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* not just one. |
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* Unless initialized otherwise, the {@linkplain #getPendingCount pending |
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* count} starts at zero, but may be (atomically) changed using |
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* methods {@link #setPendingCount}, {@link #addToPendingCount}, and |
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* {@link #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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* 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. As illustrated below, utility methods |
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* supporting customization of completion traversals are also |
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* provided. However, 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, fields, and additional |
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* support 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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* #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(CountedCompleter)} |
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* to perform an action upon normal completion, and method |
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* {@link #onExceptionalCompletion(Throwable, CountedCompleter)} 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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* 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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* rarely 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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* only as an internal helper for other computations, so its own task |
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* status (as reported in methods such as {@link ForkJoinTask#isDone}) |
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* is arbitrary; this status changes only upon explicit invocations of |
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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 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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* {@link #complete}, {@link ForkJoinTask#cancel}, |
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* {@link ForkJoinTask#completeExceptionally(Throwable)} or upon |
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* exceptional completion of method {@code compute}. Upon any |
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* exceptional completion, the exception may be relayed to a task's |
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* completer (and its completer, and so on), if one exists and it has |
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* not otherwise already completed. Similarly, cancelling an internal |
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* CountedCompleter has only a local effect on that completer, so is |
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* not often useful. |
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* |
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* <p><b>Sample Usages.</b> |
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* |
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* subdivided) to each element of an array or collection; especially |
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* when the operation takes a significantly different amount of time |
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* to complete for some elements than others, either because of |
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* intrinsic variation (for example IO) or auxiliary effects such as |
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* intrinsic variation (for example I/O) or auxiliary effects such as |
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* garbage collection. Because CountedCompleters provide their own |
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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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* improve load balancing. In the recursive case, the second of each |
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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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* implementation of method {@code onCompletion} is not overridden). |
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* A 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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* 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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* public void compute() { // version 1 |
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* if (hi - lo >= 2) { |
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* int mid = (lo + hi) >>> 1; |
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* setPendingCount(2); // must set pending count before fork |
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* new ForEach(this, array, op, mid, hi).fork(); // right child |
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* new ForEach(this, array, op, lo, mid).fork(); // left child |
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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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* 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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* public void compute() { // version 1 |
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* if (hi - lo >= 2) { |
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* int mid = (lo + hi) >>> 1; |
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* setPendingCount(2); // must set pending count before fork |
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* new ForEach(this, array, op, mid, hi).fork(); // right child |
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* new ForEach(this, array, op, lo, mid).fork(); // left child |
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* } |
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* else if (hi > lo) |
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* op.apply(array[lo]); |
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* tryComplete(); |
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* } |
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* } }</pre> |
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* else if (hi > lo) |
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* op.apply(array[lo]); |
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* tryComplete(); |
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* } |
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* }}</pre> |
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* |
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* This design can be improved by noticing that in the recursive case, |
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* the task has nothing to do after forking its right task, so can |
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* directly invoke its left task before returning. (This is an analog |
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* of tail recursion removal.) Also, because the task returns upon |
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* executing its left task (rather than falling through to invoke |
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* tryComplete) the pending count is set to one: |
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* {@code tryComplete}) the pending count is set to one: |
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* |
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* <pre> {@code |
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* class ForEach<E> ... |
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* public void compute() { // version 2 |
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* if (hi - lo >= 2) { |
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* int mid = (lo + hi) >>> 1; |
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* setPendingCount(1); // only one pending |
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* new ForEach(this, array, op, mid, hi).fork(); // right child |
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* new ForEach(this, array, op, lo, mid).compute(); // direct invoke |
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* } |
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* else { |
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* if (hi > lo) |
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* op.apply(array[lo]); |
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* tryComplete(); |
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* } |
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* public void compute() { // version 2 |
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* if (hi - lo >= 2) { |
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* int mid = (lo + hi) >>> 1; |
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* setPendingCount(1); // only one pending |
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* new ForEach(this, array, op, mid, hi).fork(); // right child |
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* new ForEach(this, array, op, lo, mid).compute(); // direct invoke |
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* } |
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* else { |
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* if (hi > lo) |
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* op.apply(array[lo]); |
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* tryComplete(); |
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* } |
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* } |
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* }</pre> |
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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. Additionally, |
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* this version uses {@code helpComplete} to streamline assistance in |
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* the execution of forked tasks. |
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* As a further improvement, notice that the left task need not even exist. |
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* Instead of creating a new one, we can iterate using the original task, |
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* and add a pending count for each fork. Additionally, because no task |
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* in this tree implements an {@link #onCompletion(CountedCompleter)} method, |
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* {@code tryComplete()} can be replaced with {@link #propagateCompletion}. |
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* |
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* <pre> {@code |
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* class ForEach<E> ... |
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* public void compute() { // version 3 |
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* int l = lo, h = hi; |
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* while (h - l >= 2) { |
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* int mid = (l + h) >>> 1; |
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* addToPendingCount(1); |
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* new ForEach(this, array, op, mid, h).fork(); // right child |
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* h = mid; |
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* } |
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* if (h > l) |
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* op.apply(array[l]); |
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* helpComplete(); |
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* public void compute() { // version 3 |
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* int l = lo, h = hi; |
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* while (h - l >= 2) { |
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* int mid = (l + h) >>> 1; |
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* addToPendingCount(1); |
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* new ForEach(this, array, op, mid, h).fork(); // right child |
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* h = mid; |
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* } |
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* if (h > l) |
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* op.apply(array[l]); |
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* propagateCompletion(); |
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* } |
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* }</pre> |
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* |
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* Additional improvements of such classes might entail precomputing |
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* instead of two per iteration, and using an adaptive threshold |
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* instead of always subdividing down to single elements. |
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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 |
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* java.util.concurrent.atomic.AtomicReference AtomicReference} as |
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* soon as one is found. The others can poll the result to avoid |
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* unnecessary work. (You could additionally {@linkplain #cancel |
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* cancel} other tasks, but it is usually simpler and more efficient |
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* to just let them notice that the result is set and if so skip |
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* further 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 (result.get() == null && h >= l) { |
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* if (h - l >= 2) { |
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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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* else { |
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* E x = array[l]; |
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* if (matches(x) && result.compareAndSet(null, x)) |
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* quietlyCompleteRoot(); // root task is now joinable |
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* break; |
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* } |
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* } |
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* tryComplete(); // normally complete whether or not found |
223 |
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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();}) |
235 |
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* because no further bookkeeping is required to manage completions |
236 |
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* once the root task completes. |
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* |
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* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine |
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* results of multiple subtasks usually need to access these results |
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* in method {@link #onCompletion}. As illustrated in the following |
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> |
* in method {@link #onCompletion(CountedCompleter)}. As illustrated in the following |
241 |
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* class (that performs a simplified form of map-reduce where mappings |
242 |
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* and reductions are all of type {@code E}), one way to do this in |
243 |
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* divide and conquer designs is to have each subtask record its |
251 |
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* class MyMapper<E> { E apply(E v) { ... } } |
252 |
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* class MyReducer<E> { E apply(E x, E y) { ... } } |
253 |
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* class MapReducer<E> extends CountedCompleter<E> { |
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* final E[] array; final MyMapper<E> mapper; |
255 |
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* final MyReducer<E> reducer; final int lo, hi; |
256 |
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* MapReducer<E> sibling; |
257 |
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* E result; |
258 |
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* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper, |
259 |
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* MyReducer<E> reducer, int lo, int hi) { |
260 |
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* super(p); |
261 |
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* this.array = array; this.mapper = mapper; |
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* this.reducer = reducer; this.lo = lo; this.hi = hi; |
254 |
> |
* final E[] array; final MyMapper<E> mapper; |
255 |
> |
* final MyReducer<E> reducer; final int lo, hi; |
256 |
> |
* MapReducer<E> sibling; |
257 |
> |
* E result; |
258 |
> |
* MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper, |
259 |
> |
* MyReducer<E> reducer, int lo, int hi) { |
260 |
> |
* super(p); |
261 |
> |
* this.array = array; this.mapper = mapper; |
262 |
> |
* this.reducer = reducer; this.lo = lo; this.hi = hi; |
263 |
> |
* } |
264 |
> |
* public void compute() { |
265 |
> |
* if (hi - lo >= 2) { |
266 |
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* int mid = (lo + hi) >>> 1; |
267 |
> |
* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid); |
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> |
* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi); |
269 |
> |
* left.sibling = right; |
270 |
> |
* right.sibling = left; |
271 |
> |
* setPendingCount(1); // only right is pending |
272 |
> |
* right.fork(); |
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> |
* left.compute(); // directly execute left |
274 |
|
* } |
275 |
< |
* public void compute() { |
276 |
< |
* if (hi - lo >= 2) { |
277 |
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* int mid = (lo + hi) >>> 1; |
278 |
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* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid); |
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* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi); |
202 |
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* left.sibling = right; |
203 |
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* right.sibling = left; |
204 |
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* setPendingCount(1); // only right is pending |
205 |
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* right.fork(); |
206 |
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* left.compute(); // directly execute left |
207 |
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* } |
208 |
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* else { |
209 |
< |
* if (hi > lo) |
210 |
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* result = mapper.apply(array[lo]); |
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* tryComplete(); |
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* } |
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* else { |
276 |
> |
* if (hi > lo) |
277 |
> |
* result = mapper.apply(array[lo]); |
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> |
* tryComplete(); |
279 |
|
* } |
280 |
< |
* public void onCompletion(CountedCompleter caller) { |
281 |
< |
* if (caller != this) { |
282 |
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* MapReducer<E> child = (MapReducer<E>)caller; |
283 |
< |
* MapReducer<E> sib = child.sibling; |
284 |
< |
* if (sib == null || sib.result == null) |
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* result = child.result; |
286 |
< |
* else |
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* result = reducer.apply(child.result, sib.result); |
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* } |
280 |
> |
* } |
281 |
> |
* public void onCompletion(CountedCompleter<?> caller) { |
282 |
> |
* if (caller != this) { |
283 |
> |
* MapReducer<E> child = (MapReducer<E>)caller; |
284 |
> |
* MapReducer<E> sib = child.sibling; |
285 |
> |
* if (sib == null || sib.result == null) |
286 |
> |
* result = child.result; |
287 |
> |
* else |
288 |
> |
* result = reducer.apply(child.result, sib.result); |
289 |
|
* } |
290 |
< |
* public E getRawResult() { return result; } |
290 |
> |
* } |
291 |
> |
* public E getRawResult() { return result; } |
292 |
> |
* |
293 |
> |
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) { |
294 |
> |
* return new MapReducer<E>(null, array, mapper, reducer, |
295 |
> |
* 0, array.length).invoke(); |
296 |
> |
* } |
297 |
> |
* }}</pre> |
298 |
> |
* |
299 |
> |
* Here, method {@code onCompletion} takes a form common to many |
300 |
> |
* completion designs that combine results. This callback-style method |
301 |
> |
* is triggered once per task, in either of the two different contexts |
302 |
> |
* in which the pending count is, or becomes, zero: (1) by a task |
303 |
> |
* itself, if its pending count is zero upon invocation of {@code |
304 |
> |
* tryComplete}, or (2) by any of its subtasks when they complete and |
305 |
> |
* decrement the pending count to zero. The {@code caller} argument |
306 |
> |
* distinguishes cases. Most often, when the caller is {@code this}, |
307 |
> |
* no action is necessary. Otherwise the caller argument can be used |
308 |
> |
* (usually via a cast) to supply a value (and/or links to other |
309 |
> |
* values) to be combined. Assuming proper use of pending counts, the |
310 |
> |
* actions inside {@code onCompletion} occur (once) upon completion of |
311 |
> |
* a task and its subtasks. No additional synchronization is required |
312 |
> |
* within this method to ensure thread safety of accesses to fields of |
313 |
> |
* this task or other completed tasks. |
314 |
> |
* |
315 |
> |
* <p><b>Completion Traversals</b>. If using {@code onCompletion} to |
316 |
> |
* process completions is inapplicable or inconvenient, you can use |
317 |
> |
* methods {@link #firstComplete} and {@link #nextComplete} to create |
318 |
> |
* custom traversals. For example, to define a MapReducer that only |
319 |
> |
* splits out right-hand tasks in the form of the third ForEach |
320 |
> |
* example, the completions must cooperatively reduce along |
321 |
> |
* unexhausted subtask links, which can be done as follows: |
322 |
|
* |
323 |
< |
* public static <E> E mapReduce(ForkJoinPool pool, E[] array, |
324 |
< |
* MyMapper<E> mapper, MyReducer<E> reducer) { |
325 |
< |
* return pool.invoke(new MapReducer<E>(null, array, mapper, |
326 |
< |
* reducer, 0, array.length)); |
323 |
> |
* <pre> {@code |
324 |
> |
* class MapReducer<E> extends CountedCompleter<E> { // version 2 |
325 |
> |
* final E[] array; final MyMapper<E> mapper; |
326 |
> |
* final MyReducer<E> reducer; final int lo, hi; |
327 |
> |
* MapReducer<E> forks, next; // record subtask forks in list |
328 |
> |
* E result; |
329 |
> |
* MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper, |
330 |
> |
* MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) { |
331 |
> |
* super(p); |
332 |
> |
* this.array = array; this.mapper = mapper; |
333 |
> |
* this.reducer = reducer; this.lo = lo; this.hi = hi; |
334 |
> |
* this.next = next; |
335 |
> |
* } |
336 |
> |
* public void compute() { |
337 |
> |
* int l = lo, h = hi; |
338 |
> |
* while (h - l >= 2) { |
339 |
> |
* int mid = (l + h) >>> 1; |
340 |
> |
* addToPendingCount(1); |
341 |
> |
* (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork(); |
342 |
> |
* h = mid; |
343 |
> |
* } |
344 |
> |
* if (h > l) |
345 |
> |
* result = mapper.apply(array[l]); |
346 |
> |
* // process completions by reducing along and advancing subtask links |
347 |
> |
* for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) { |
348 |
> |
* for (MapReducer t = (MapReducer)c, s = t.forks; s != null; s = t.forks = s.next) |
349 |
> |
* t.result = reducer.apply(t.result, s.result); |
350 |
|
* } |
351 |
< |
* } }</pre> |
351 |
> |
* } |
352 |
> |
* public E getRawResult() { return result; } |
353 |
> |
* |
354 |
> |
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) { |
355 |
> |
* return new MapReducer<E>(null, array, mapper, reducer, |
356 |
> |
* 0, array.length, null).invoke(); |
357 |
> |
* } |
358 |
> |
* }}</pre> |
359 |
|
* |
360 |
|
* <p><b>Triggers.</b> Some CountedCompleters are themselves never |
361 |
|
* forked, but instead serve as bits of plumbing in other designs; |
362 |
< |
* including those in which the completion of one of more async tasks |
362 |
> |
* including those in which the completion of one or more async tasks |
363 |
|
* triggers another async task. For example: |
364 |
|
* |
365 |
|
* <pre> {@code |
366 |
|
* class HeaderBuilder extends CountedCompleter<...> { ... } |
367 |
|
* class BodyBuilder extends CountedCompleter<...> { ... } |
368 |
|
* class PacketSender extends CountedCompleter<...> { |
369 |
< |
* PacketSender(...) { super(null, 1); ... } // trigger on second completion |
370 |
< |
* public void compute() { } // never called |
371 |
< |
* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); } |
369 |
> |
* PacketSender(...) { super(null, 1); ... } // trigger on second completion |
370 |
> |
* public void compute() { } // never called |
371 |
> |
* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); } |
372 |
|
* } |
373 |
|
* // sample use: |
374 |
|
* PacketSender p = new PacketSender(); |
391 |
|
* Creates a new CountedCompleter with the given completer |
392 |
|
* and initial pending count. |
393 |
|
* |
394 |
< |
* @param completer this tasks completer, or {@code null} if none |
394 |
> |
* @param completer this task's completer, or {@code null} if none |
395 |
|
* @param initialPendingCount the initial pending count |
396 |
|
*/ |
397 |
|
protected CountedCompleter(CountedCompleter<?> completer, |
404 |
|
* Creates a new CountedCompleter with the given completer |
405 |
|
* and an initial pending count of zero. |
406 |
|
* |
407 |
< |
* @param completer this tasks completer, or {@code null} if none |
407 |
> |
* @param completer this task's completer, or {@code null} if none |
408 |
|
*/ |
409 |
|
protected CountedCompleter(CountedCompleter<?> completer) { |
410 |
|
this.completer = completer; |
425 |
|
|
426 |
|
/** |
427 |
|
* Performs an action when method {@link #tryComplete} is invoked |
428 |
< |
* and there are no pending counts, or when the unconditional |
428 |
> |
* and the pending count is zero, or when the unconditional |
429 |
|
* method {@link #complete} is invoked. By default, this method |
430 |
< |
* does nothing. |
430 |
> |
* does nothing. You can distinguish cases by checking the |
431 |
> |
* identity of the given caller argument. If not equal to {@code |
432 |
> |
* this}, then it is typically a subtask that may contain results |
433 |
> |
* (and/or links to other results) to combine. |
434 |
|
* |
435 |
|
* @param caller the task invoking this method (which may |
436 |
< |
* be this task itself). |
436 |
> |
* be this task itself) |
437 |
|
*/ |
438 |
|
public void onCompletion(CountedCompleter<?> caller) { |
439 |
|
} |
440 |
|
|
441 |
|
/** |
442 |
< |
* Performs an action when method {@link #completeExceptionally} |
443 |
< |
* is invoked or method {@link #compute} throws an exception, and |
444 |
< |
* this task has not otherwise already completed normally. On |
445 |
< |
* entry to this method, this task {@link |
446 |
< |
* ForkJoinTask#isCompletedAbnormally}. The return value of this |
447 |
< |
* method controls further propagation: If {@code true} and this |
448 |
< |
* task has a completer, then this completer is also completed |
449 |
< |
* exceptionally. The default implementation of this method does |
450 |
< |
* nothing except return {@code true}. |
442 |
> |
* Performs an action when method {@link |
443 |
> |
* #completeExceptionally(Throwable)} is invoked or method {@link |
444 |
> |
* #compute} throws an exception, and this task has not already |
445 |
> |
* otherwise completed normally. On entry to this method, this task |
446 |
> |
* {@link ForkJoinTask#isCompletedAbnormally}. The return value |
447 |
> |
* of this method controls further propagation: If {@code true} |
448 |
> |
* and this task has a completer that has not completed, then that |
449 |
> |
* completer is also completed exceptionally, with the same |
450 |
> |
* exception as this completer. The default implementation of |
451 |
> |
* this method does nothing except return {@code true}. |
452 |
|
* |
453 |
|
* @param ex the exception |
454 |
|
* @param caller the task invoking this method (which may |
455 |
< |
* be this task itself). |
456 |
< |
* @return true if this exception should be propagated to this |
457 |
< |
* tasks completer, if one exists. |
455 |
> |
* be this task itself) |
456 |
> |
* @return {@code true} if this exception should be propagated to this |
457 |
> |
* task's completer, if one exists |
458 |
|
*/ |
459 |
|
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) { |
460 |
|
return true; |
494 |
|
* @param delta the value to add |
495 |
|
*/ |
496 |
|
public final void addToPendingCount(int delta) { |
497 |
< |
int c; // note: can replace with intrinsic in jdk8 |
497 |
> |
int c; |
498 |
|
do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta)); |
499 |
|
} |
500 |
|
|
504 |
|
* |
505 |
|
* @param expected the expected value |
506 |
|
* @param count the new value |
507 |
< |
* @return true is successful |
507 |
> |
* @return {@code true} if successful |
508 |
|
*/ |
509 |
|
public final boolean compareAndSetPendingCount(int expected, int count) { |
510 |
|
return U.compareAndSwapInt(this, PENDING, expected, count); |
511 |
|
} |
512 |
|
|
513 |
|
/** |
514 |
+ |
* If the pending count is nonzero, (atomically) decrements it. |
515 |
+ |
* |
516 |
+ |
* @return the initial (undecremented) pending count holding on entry |
517 |
+ |
* to this method |
518 |
+ |
*/ |
519 |
+ |
public final int decrementPendingCountUnlessZero() { |
520 |
+ |
int c; |
521 |
+ |
do {} while ((c = pending) != 0 && |
522 |
+ |
!U.compareAndSwapInt(this, PENDING, c, c - 1)); |
523 |
+ |
return c; |
524 |
+ |
} |
525 |
+ |
|
526 |
+ |
/** |
527 |
|
* Returns the root of the current computation; i.e., this |
528 |
|
* task if it has no completer, else its completer's root. |
529 |
|
* |
538 |
|
|
539 |
|
/** |
540 |
|
* If the pending count is nonzero, decrements the count; |
541 |
< |
* otherwise invokes {@link #onCompletion} and then similarly |
542 |
< |
* tries to complete this task's completer, if one exists, |
543 |
< |
* else marks this task as complete. |
541 |
> |
* otherwise invokes {@link #onCompletion(CountedCompleter)} |
542 |
> |
* and then similarly tries to complete this task's completer, |
543 |
> |
* if one exists, else marks this task as complete. |
544 |
|
*/ |
545 |
|
public final void tryComplete() { |
546 |
|
CountedCompleter<?> a = this, s = a; |
558 |
|
} |
559 |
|
|
560 |
|
/** |
561 |
< |
* Identical to {@link #tryComplete}, but may additionally execute |
562 |
< |
* other tasks within the current computation (i.e., those |
563 |
< |
* with the same {@link #getRoot}. |
561 |
> |
* Equivalent to {@link #tryComplete} but does not invoke {@link |
562 |
> |
* #onCompletion(CountedCompleter)} along the completion path: |
563 |
> |
* If the pending count is nonzero, decrements the count; |
564 |
> |
* otherwise, similarly tries to complete this task's completer, if |
565 |
> |
* one exists, else marks this task as complete. This method may be |
566 |
> |
* useful in cases where {@code onCompletion} should not, or need |
567 |
> |
* not, be invoked for each completer in a computation. |
568 |
|
*/ |
569 |
< |
public final void helpComplete() { |
569 |
> |
public final void propagateCompletion() { |
570 |
|
CountedCompleter<?> a = this, s = a; |
571 |
|
for (int c;;) { |
572 |
|
if ((c = a.pending) == 0) { |
425 |
– |
a.onCompletion(s); |
573 |
|
if ((a = (s = a).completer) == null) { |
574 |
|
s.quietlyComplete(); |
575 |
|
return; |
576 |
|
} |
577 |
|
} |
578 |
< |
else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) { |
432 |
< |
if (!(Thread.currentThread() instanceof ForkJoinWorkerThread)) |
433 |
< |
ForkJoinPool.popAndExecCCFromCommonPool(a); |
578 |
> |
else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) |
579 |
|
return; |
435 |
– |
} |
580 |
|
} |
581 |
|
} |
582 |
|
|
583 |
|
/** |
584 |
< |
* Regardless of pending count, invokes {@link #onCompletion}, |
585 |
< |
* marks this task as complete and further triggers {@link |
586 |
< |
* #tryComplete} on this task's completer, if one exists. This |
587 |
< |
* method may be useful when forcing completion as soon as any one |
588 |
< |
* (versus all) of several subtask results are obtained. The |
589 |
< |
* given rawResult is used as an argument to {@link #setRawResult} |
590 |
< |
* before marking this task as complete; its value is meaningful |
591 |
< |
* only for classes overriding {@code setRawResult}. |
584 |
> |
* Regardless of pending count, invokes |
585 |
> |
* {@link #onCompletion(CountedCompleter)}, marks this task as |
586 |
> |
* complete and further triggers {@link #tryComplete} on this |
587 |
> |
* task's completer, if one exists. The given rawResult is |
588 |
> |
* used as an argument to {@link #setRawResult} before invoking |
589 |
> |
* {@link #onCompletion(CountedCompleter)} or marking this task |
590 |
> |
* as complete; its value is meaningful only for classes |
591 |
> |
* overriding {@code setRawResult}. This method does not modify |
592 |
> |
* the pending count. |
593 |
> |
* |
594 |
> |
* <p>This method may be useful when forcing completion as soon as |
595 |
> |
* any one (versus all) of several subtask results are obtained. |
596 |
> |
* However, in the common (and recommended) case in which {@code |
597 |
> |
* setRawResult} is not overridden, this effect can be obtained |
598 |
> |
* more simply using {@code quietlyCompleteRoot();}. |
599 |
|
* |
600 |
|
* @param rawResult the raw result |
601 |
|
*/ |
602 |
|
public void complete(T rawResult) { |
603 |
|
CountedCompleter<?> p; |
453 |
– |
onCompletion(this); |
604 |
|
setRawResult(rawResult); |
605 |
+ |
onCompletion(this); |
606 |
|
quietlyComplete(); |
607 |
|
if ((p = completer) != null) |
608 |
|
p.tryComplete(); |
609 |
|
} |
610 |
|
|
611 |
+ |
|
612 |
|
/** |
613 |
< |
* Support for FJT exception propagation |
613 |
> |
* If this task's pending count is zero, returns this task; |
614 |
> |
* otherwise decrements its pending count and returns {@code |
615 |
> |
* null}. This method is designed to be used with {@link |
616 |
> |
* #nextComplete} in completion traversal loops. |
617 |
> |
* |
618 |
> |
* @return this task, if pending count was zero, else {@code null} |
619 |
> |
*/ |
620 |
> |
public final CountedCompleter<?> firstComplete() { |
621 |
> |
for (int c;;) { |
622 |
> |
if ((c = pending) == 0) |
623 |
> |
return this; |
624 |
> |
else if (U.compareAndSwapInt(this, PENDING, c, c - 1)) |
625 |
> |
return null; |
626 |
> |
} |
627 |
> |
} |
628 |
> |
|
629 |
> |
/** |
630 |
> |
* If this task does not have a completer, invokes {@link |
631 |
> |
* ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if |
632 |
> |
* the completer's pending count is non-zero, decrements that |
633 |
> |
* pending count and returns {@code null}. Otherwise, returns the |
634 |
> |
* completer. This method can be used as part of a completion |
635 |
> |
* traversal loop for homogeneous task hierarchies: |
636 |
> |
* |
637 |
> |
* <pre> {@code |
638 |
> |
* for (CountedCompleter<?> c = firstComplete(); |
639 |
> |
* c != null; |
640 |
> |
* c = c.nextComplete()) { |
641 |
> |
* // ... process c ... |
642 |
> |
* }}</pre> |
643 |
> |
* |
644 |
> |
* @return the completer, or {@code null} if none |
645 |
> |
*/ |
646 |
> |
public final CountedCompleter<?> nextComplete() { |
647 |
> |
CountedCompleter<?> p; |
648 |
> |
if ((p = completer) != null) |
649 |
> |
return p.firstComplete(); |
650 |
> |
else { |
651 |
> |
quietlyComplete(); |
652 |
> |
return null; |
653 |
> |
} |
654 |
> |
} |
655 |
> |
|
656 |
> |
/** |
657 |
> |
* Equivalent to {@code getRoot().quietlyComplete()}. |
658 |
> |
*/ |
659 |
> |
public final void quietlyCompleteRoot() { |
660 |
> |
for (CountedCompleter<?> a = this, p;;) { |
661 |
> |
if ((p = a.completer) == null) { |
662 |
> |
a.quietlyComplete(); |
663 |
> |
return; |
664 |
> |
} |
665 |
> |
a = p; |
666 |
> |
} |
667 |
> |
} |
668 |
> |
|
669 |
> |
/** |
670 |
> |
* Supports ForkJoinTask exception propagation. |
671 |
|
*/ |
672 |
|
void internalPropagateException(Throwable ex) { |
673 |
|
CountedCompleter<?> a = this, s = a; |
674 |
|
while (a.onExceptionalCompletion(ex, s) && |
675 |
< |
(a = (s = a).completer) != null && a.status >= 0) |
676 |
< |
a.recordExceptionalCompletion(ex); |
675 |
> |
(a = (s = a).completer) != null && a.status >= 0 && |
676 |
> |
a.recordExceptionalCompletion(ex) == EXCEPTIONAL) |
677 |
> |
; |
678 |
|
} |
679 |
|
|
680 |
|
/** |
681 |
< |
* Implements execution conventions for CountedCompleters |
681 |
> |
* Implements execution conventions for CountedCompleters. |
682 |
|
*/ |
683 |
|
protected final boolean exec() { |
684 |
|
compute(); |
688 |
|
/** |
689 |
|
* Returns the result of the computation. By default |
690 |
|
* returns {@code null}, which is appropriate for {@code Void} |
691 |
< |
* actions, but in other cases should be overridden. |
691 |
> |
* actions, but in other cases should be overridden, almost |
692 |
> |
* always to return a field or function of a field that |
693 |
> |
* holds the result upon completion. |
694 |
|
* |
695 |
|
* @return the result of the computation |
696 |
|
*/ |
699 |
|
/** |
700 |
|
* A method that result-bearing CountedCompleters may optionally |
701 |
|
* use to help maintain result data. By default, does nothing. |
702 |
+ |
* Overrides are not recommended. However, if this method is |
703 |
+ |
* overridden to update existing objects or fields, then it must |
704 |
+ |
* in general be defined to be thread-safe. |
705 |
|
*/ |
706 |
|
protected void setRawResult(T t) { } |
707 |
|
|
728 |
|
private static sun.misc.Unsafe getUnsafe() { |
729 |
|
try { |
730 |
|
return sun.misc.Unsafe.getUnsafe(); |
731 |
< |
} catch (SecurityException se) { |
732 |
< |
try { |
733 |
< |
return java.security.AccessController.doPrivileged |
734 |
< |
(new java.security |
735 |
< |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
736 |
< |
public sun.misc.Unsafe run() throws Exception { |
737 |
< |
java.lang.reflect.Field f = sun.misc |
738 |
< |
.Unsafe.class.getDeclaredField("theUnsafe"); |
739 |
< |
f.setAccessible(true); |
740 |
< |
return (sun.misc.Unsafe) f.get(null); |
741 |
< |
}}); |
742 |
< |
} catch (java.security.PrivilegedActionException e) { |
743 |
< |
throw new RuntimeException("Could not initialize intrinsics", |
744 |
< |
e.getCause()); |
745 |
< |
} |
731 |
> |
} catch (SecurityException tryReflectionInstead) {} |
732 |
> |
try { |
733 |
> |
return java.security.AccessController.doPrivileged |
734 |
> |
(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
735 |
> |
public sun.misc.Unsafe run() throws Exception { |
736 |
> |
Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class; |
737 |
> |
for (java.lang.reflect.Field f : k.getDeclaredFields()) { |
738 |
> |
f.setAccessible(true); |
739 |
> |
Object x = f.get(null); |
740 |
> |
if (k.isInstance(x)) |
741 |
> |
return k.cast(x); |
742 |
> |
} |
743 |
> |
throw new NoSuchFieldError("the Unsafe"); |
744 |
> |
}}); |
745 |
> |
} catch (java.security.PrivilegedActionException e) { |
746 |
> |
throw new RuntimeException("Could not initialize intrinsics", |
747 |
> |
e.getCause()); |
748 |
|
} |
749 |
|
} |
533 |
– |
|
750 |
|
} |