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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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|
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/** |
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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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* 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. 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 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. 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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* used as a regular ForkJoinTask with this added functionality. |
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* However, any completer that in turn has another completer serves |
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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}, |
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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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* <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. 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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* 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 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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* 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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* leaf tasks, because they reduce inter-thread communication and |
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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). |
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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(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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* 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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* {@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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* ... |
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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 optimization, 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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* ... |
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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 optimizations of such classes might entail precomputing |
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* pending counts so that they can be established in constructors, |
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* specializing classes for leaf steps, subdividing by say, four, |
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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 |
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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 {@code compareAndSet} a common result, the |
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* trailing unconditional invocation of {@code tryComplete} could be |
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* made 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>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(CountedCompleter)}. As illustrated in the following |
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* class (that performs a simplified form of map-reduce where mappings |
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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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* 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<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<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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* super(p); |
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* this.array = array; this.mapper = mapper; |
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* this.reducer = reducer; this.lo = lo; this.hi = hi; |
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* } |
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* public void compute() { |
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* if (hi - lo >= 2) { |
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* int mid = (lo + hi) >>> 1; |
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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); |
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* left.sibling = right; |
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* right.sibling = left; |
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* setPendingCount(1); // only right is pending |
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* right.fork(); |
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* left.compute(); // directly execute left |
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* } |
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* else { |
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* if (hi > lo) |
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* result = mapper.apply(array[lo]); |
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* tryComplete(); |
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* } |
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* } |
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* public void onCompletion(CountedCompleter<?> caller) { |
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* if (caller != this) { |
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* MapReducer<E> child = (MapReducer<E>)caller; |
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* MapReducer<E> sib = child.sibling; |
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* if (sib == null || sib.result == null) |
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* result = child.result; |
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* else |
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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(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. Assuming 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>Completion Traversals</b>. If using {@code onCompletion} to |
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* process completions is inapplicable or inconvenient, you can use |
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* methods {@link #firstComplete} and {@link #nextComplete} to create |
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* custom traversals. For example, to define a MapReducer that only |
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* splits out right-hand tasks in the form of the third ForEach |
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* example, the completions must cooperatively reduce along |
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* unexhausted subtask links, which can be done as follows: |
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* |
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* <pre> {@code |
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* class MapReducer<E> extends CountedCompleter<E> { // version 2 |
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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<E> forks, next; // record subtask forks in list |
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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, MapReducer<E> next) { |
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* super(p); |
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* this.array = array; this.mapper = mapper; |
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* this.reducer = reducer; this.lo = lo; this.hi = hi; |
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* this.next = next; |
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* } |
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* public void compute() { |
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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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* (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork(); |
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* h = mid; |
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* } |
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* if (h > l) |
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* result = mapper.apply(array[l]); |
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* // process completions by reducing along and advancing subtask links |
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* for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) { |
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* for (MapReducer t = (MapReducer)c, s = t.forks; s != null; s = t.forks = s.next) |
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* t.result = reducer.apply(t.result, s.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(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, null).invoke(); |
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* } |
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* }}</pre> |
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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 or 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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* 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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* } |
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* // sample use: |
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* PacketSender p = new PacketSender(); |
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* new HeaderBuilder(p, ...).fork(); |
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* new BodyBuilder(p, ...).fork();}</pre> |
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* |
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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<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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/** The number of pending tasks until completion */ |
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volatile int pending; |
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|
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/** |
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* Creates a new CountedCompleter with the given completer |
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* and initial pending count. |
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* |
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* @param completer this task's 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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int initialPendingCount) { |
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this.completer = completer; |
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this.pending = initialPendingCount; |
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} |
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|
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/** |
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* Creates a new CountedCompleter with the given completer |
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* and an initial pending count of zero. |
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* |
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* @param completer this task's completer, or {@code null} if none |
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*/ |
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protected CountedCompleter(CountedCompleter<?> completer) { |
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this.completer = completer; |
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} |
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|
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/** |
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* Creates a new CountedCompleter with no completer |
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* and an initial pending count of zero. |
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*/ |
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protected CountedCompleter() { |
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this.completer = null; |
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} |
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|
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/** |
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* The main computation performed by this task. |
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*/ |
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public abstract void compute(); |
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|
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/** |
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* Performs an action when method {@link #tryComplete} is invoked |
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* and the pending count is zero, or when the unconditional |
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* method {@link #complete} is invoked. By default, this method |
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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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public void onCompletion(CountedCompleter<?> caller) { |
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} |
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|
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/** |
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* Performs an action when method {@link |
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* #completeExceptionally(Throwable)} is invoked or method {@link |
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* #compute} throws an exception, and this task has not already |
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* otherwise completed normally. On entry to this method, this task |
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* {@link ForkJoinTask#isCompletedAbnormally}. The return value |
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* of this method controls further propagation: If {@code true} |
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* and this task has a completer that has not completed, then that |
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* completer is also completed exceptionally, with the same |
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* exception as this completer. The default implementation of |
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* this method does nothing except return {@code true}. |
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* |
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* @param ex the exception |
455 |
* @param caller the task invoking this method (which may |
456 |
* be this task itself) |
457 |
* @return {@code true} if this exception should be propagated to this |
458 |
* task's completer, if one exists |
459 |
*/ |
460 |
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) { |
461 |
return true; |
462 |
} |
463 |
|
464 |
/** |
465 |
* Returns the completer established in this task's constructor, |
466 |
* or {@code null} if none. |
467 |
* |
468 |
* @return the completer |
469 |
*/ |
470 |
public final CountedCompleter<?> getCompleter() { |
471 |
return completer; |
472 |
} |
473 |
|
474 |
/** |
475 |
* Returns the current pending count. |
476 |
* |
477 |
* @return the current pending count |
478 |
*/ |
479 |
public final int getPendingCount() { |
480 |
return pending; |
481 |
} |
482 |
|
483 |
/** |
484 |
* Sets the pending count to the given value. |
485 |
* |
486 |
* @param count the count |
487 |
*/ |
488 |
public final void setPendingCount(int count) { |
489 |
pending = count; |
490 |
} |
491 |
|
492 |
/** |
493 |
* Adds (atomically) the given value to the pending count. |
494 |
* |
495 |
* @param delta the value to add |
496 |
*/ |
497 |
public final void addToPendingCount(int delta) { |
498 |
U.getAndAddInt(this, PENDING, delta); |
499 |
} |
500 |
|
501 |
/** |
502 |
* Sets (atomically) the pending count to the given count only if |
503 |
* it currently holds the given expected value. |
504 |
* |
505 |
* @param expected the expected value |
506 |
* @param count the new value |
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 |
* |
530 |
* @return the root of the current computation |
531 |
*/ |
532 |
public final CountedCompleter<?> getRoot() { |
533 |
CountedCompleter<?> a = this, p; |
534 |
while ((p = a.completer) != null) |
535 |
a = p; |
536 |
return a; |
537 |
} |
538 |
|
539 |
/** |
540 |
* If the pending count is nonzero, decrements the count; |
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; |
547 |
for (int c;;) { |
548 |
if ((c = a.pending) == 0) { |
549 |
a.onCompletion(s); |
550 |
if ((a = (s = a).completer) == null) { |
551 |
s.quietlyComplete(); |
552 |
return; |
553 |
} |
554 |
} |
555 |
else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) |
556 |
return; |
557 |
} |
558 |
} |
559 |
|
560 |
/** |
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 propagateCompletion() { |
570 |
CountedCompleter<?> a = this, s; |
571 |
for (int c;;) { |
572 |
if ((c = a.pending) == 0) { |
573 |
if ((a = (s = a).completer) == null) { |
574 |
s.quietlyComplete(); |
575 |
return; |
576 |
} |
577 |
} |
578 |
else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) |
579 |
return; |
580 |
} |
581 |
} |
582 |
|
583 |
/** |
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 {@link #quietlyCompleteRoot()}. |
599 |
* |
600 |
* @param rawResult the raw result |
601 |
*/ |
602 |
public void complete(T rawResult) { |
603 |
CountedCompleter<?> p; |
604 |
setRawResult(rawResult); |
605 |
onCompletion(this); |
606 |
quietlyComplete(); |
607 |
if ((p = completer) != null) |
608 |
p.tryComplete(); |
609 |
} |
610 |
|
611 |
/** |
612 |
* If this task's pending count is zero, returns this task; |
613 |
* otherwise decrements its pending count and returns {@code null}. |
614 |
* This method is designed to be used with {@link #nextComplete} in |
615 |
* completion traversal loops. |
616 |
* |
617 |
* @return this task, if pending count was zero, else {@code null} |
618 |
*/ |
619 |
public final CountedCompleter<?> firstComplete() { |
620 |
for (int c;;) { |
621 |
if ((c = pending) == 0) |
622 |
return this; |
623 |
else if (U.compareAndSwapInt(this, PENDING, c, c - 1)) |
624 |
return null; |
625 |
} |
626 |
} |
627 |
|
628 |
/** |
629 |
* If this task does not have a completer, invokes {@link |
630 |
* ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if |
631 |
* the completer's pending count is non-zero, decrements that |
632 |
* pending count and returns {@code null}. Otherwise, returns the |
633 |
* completer. This method can be used as part of a completion |
634 |
* traversal loop for homogeneous task hierarchies: |
635 |
* |
636 |
* <pre> {@code |
637 |
* for (CountedCompleter<?> c = firstComplete(); |
638 |
* c != null; |
639 |
* c = c.nextComplete()) { |
640 |
* // ... process c ... |
641 |
* }}</pre> |
642 |
* |
643 |
* @return the completer, or {@code null} if none |
644 |
*/ |
645 |
public final CountedCompleter<?> nextComplete() { |
646 |
CountedCompleter<?> p; |
647 |
if ((p = completer) != null) |
648 |
return p.firstComplete(); |
649 |
else { |
650 |
quietlyComplete(); |
651 |
return null; |
652 |
} |
653 |
} |
654 |
|
655 |
/** |
656 |
* Equivalent to {@code getRoot().quietlyComplete()}. |
657 |
*/ |
658 |
public final void quietlyCompleteRoot() { |
659 |
for (CountedCompleter<?> a = this, p;;) { |
660 |
if ((p = a.completer) == null) { |
661 |
a.quietlyComplete(); |
662 |
return; |
663 |
} |
664 |
a = p; |
665 |
} |
666 |
} |
667 |
|
668 |
/** |
669 |
* If this task has not completed, attempts to process at most the |
670 |
* given number of other unprocessed tasks for which this task is |
671 |
* on the completion path, if any are known to exist. |
672 |
* |
673 |
* @param maxTasks the maximum number of tasks to process. If |
674 |
* less than or equal to zero, then no tasks are |
675 |
* processed. |
676 |
*/ |
677 |
public final void helpComplete(int maxTasks) { |
678 |
Thread t; ForkJoinWorkerThread wt; |
679 |
if (maxTasks > 0 && status >= 0) { |
680 |
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) |
681 |
(wt = (ForkJoinWorkerThread)t).pool. |
682 |
helpComplete(wt.workQueue, this, maxTasks); |
683 |
else |
684 |
ForkJoinPool.common.externalHelpComplete(this, maxTasks); |
685 |
} |
686 |
} |
687 |
|
688 |
/** |
689 |
* Supports ForkJoinTask exception propagation. |
690 |
*/ |
691 |
void internalPropagateException(Throwable ex) { |
692 |
CountedCompleter<?> a = this, s = a; |
693 |
while (a.onExceptionalCompletion(ex, s) && |
694 |
(a = (s = a).completer) != null && a.status >= 0 && |
695 |
a.recordExceptionalCompletion(ex) == EXCEPTIONAL) |
696 |
; |
697 |
} |
698 |
|
699 |
/** |
700 |
* Implements execution conventions for CountedCompleters. |
701 |
*/ |
702 |
protected final boolean exec() { |
703 |
compute(); |
704 |
return false; |
705 |
} |
706 |
|
707 |
/** |
708 |
* Returns the result of the computation. By default, |
709 |
* returns {@code null}, which is appropriate for {@code Void} |
710 |
* actions, but in other cases should be overridden, almost |
711 |
* always to return a field or function of a field that |
712 |
* holds the result upon completion. |
713 |
* |
714 |
* @return the result of the computation |
715 |
*/ |
716 |
public T getRawResult() { return null; } |
717 |
|
718 |
/** |
719 |
* A method that result-bearing CountedCompleters may optionally |
720 |
* use to help maintain result data. By default, does nothing. |
721 |
* Overrides are not recommended. However, if this method is |
722 |
* overridden to update existing objects or fields, then it must |
723 |
* in general be defined to be thread-safe. |
724 |
*/ |
725 |
protected void setRawResult(T t) { } |
726 |
|
727 |
// Unsafe mechanics |
728 |
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
729 |
private static final long PENDING; |
730 |
static { |
731 |
try { |
732 |
PENDING = U.objectFieldOffset |
733 |
(CountedCompleter.class.getDeclaredField("pending")); |
734 |
} catch (ReflectiveOperationException e) { |
735 |
throw new Error(e); |
736 |
} |
737 |
} |
738 |
} |