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package jsr166y; |
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|
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/** |
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* A resultless {@link ForkJoinTask} with a completion action |
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* performed when triggered and there are no remaining pending |
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* actions. Uses of CountedCompleter are similar to those of other |
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* completion based components (such as {@link |
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* java.nio.channels.CompletionHandler}) except that multiple |
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* <em>pending</em> completions may be necessary to trigger the {@link |
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* #onCompletion} action, not just one. Unless initialized otherwise, |
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* the {@link #getPendingCount pending count} starts at zero, but may |
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* be (atomically) changed using methods {@link #setPendingCount}, |
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* {@link #addToPendingCount}, and {@link |
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* #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 |
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* actions. CountedCompleters are in general more robust in the |
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* presence of subtask stalls and blockage than are other forms 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}, 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 most basic synchronization |
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* constructs, these methods affect only internal counts; they do not |
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* establish any further internal bookkeeping. In particular, the |
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* identities of pending tasks are not maintained. As illustrated |
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* below, you can create subclasses that do record some or all pended |
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* tasks or their results when needed. |
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* with its completer. As is the case with related synchronization |
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* components such as {@link java.util.concurrent.Phaser Phaser} and |
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* {@link java.util.concurrent.Semaphore Semaphore}, these methods |
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* affect only internal counts; they do not establish any further |
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* internal bookkeeping. In particular, the identities of pending |
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* tasks are not maintained. As illustrated below, you can create |
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* subclasses that do record some or all pending tasks or their |
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* results when needed. 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()} before returning. The class may also optionally |
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* override method {@link #onCompletion} to perform an action upon |
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* normal completion. |
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* #compute}, that should in most cases (as illustrated below), invoke |
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* {@code tryComplete()} once before returning. The class may also |
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* optionally override method {@link #onCompletion} to perform an |
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* action upon normal completion, and method {@link |
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* #onExceptionalCompletion} to perform an action upon any exception. |
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* |
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* <p>CountedCompleters most often do not bear results, in which case |
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> |
* they are normally declared as {@code CountedCompleter<Void>}, and |
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* will always return {@code null} as a result value. In other cases, |
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* you should override method {@link #getRawResult} to provide a |
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* result from {@code join(), invoke()}, and related methods. In |
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* general, this method should return the value of a field (or a |
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* function of one or more fields) of the CountedCompleter object that |
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* holds the result upon completion. Method {@link #setRawResult} by |
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* default plays no role in CountedCompleters. It is possible, but |
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* 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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* {@link #complete}, {@link ForkJoinTask#cancel}, {@link |
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* ForkJoinTask#completeExceptionally} or upon exceptional completion |
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* of method {@code compute}. Upon any exceptional completion, the |
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* exception is relayed to a task's completer (and its completer, and |
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* so on), if one exists and it has not otherwise already completed. |
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* exception may be relayed to a task's completer (and its completer, |
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* and so on), if one exists and it has not otherwise already |
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* completed. Similarly, cancelling an internal CountedCompleter has |
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* only a local effect on that completer, so is not often useful. |
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* |
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* <p><b>Sample Usages.</b> |
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* |
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* <p><b>Parallel recursive decomposition.</b> CountedCompleters may |
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* be arranged in trees similar to those often used with {@link |
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* RecursiveAction}s, although the constructions involved in setting |
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* them up typically vary. Even though they entail a bit more |
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* them up typically vary. Here, the completer of each task is its |
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* parent in the computation tree. Even though they entail a bit more |
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* bookkeeping, CountedCompleters may be better choices when applying |
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* a possibly time-consuming operation (that cannot be further |
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* subdivided) to each element of an array or collection; especially |
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* continuations, other threads need not block waiting to perform |
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* them. |
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* |
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* <p> For example, here is an initial version of a class that uses |
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> |
* <p>For example, here is an initial version of a class that uses |
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* divide-by-two recursive decomposition to divide work into single |
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* pieces (leaf tasks). Even when work is split into individual calls, |
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* tree-based techniques are usually preferable to directly forking |
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* pair of subtasks to finish triggers completion of its parent |
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* (because no result combination is performed, the default no-op |
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* implementation of method {@code onCompletion} is not overridden). A |
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* static utility method sets up the base task and invokes it: |
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> |
* static utility method sets up the base task and invokes it |
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> |
* (here, implicitly using the {@link ForkJoinPool#commonPool()}). |
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* |
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* <pre> {@code |
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* class MyOperation<E> { void apply(E e) { ... } } |
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* |
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* class ForEach<E> extends CountedCompleter { |
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> |
* class ForEach<E> extends CountedCompleter<Void> { |
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* |
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* public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) { |
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* pool.invoke(new ForEach<E>(null, array, op, 0, array.length)); |
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> |
* public static <E> void forEach(E[] array, MyOperation<E> op) { |
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> |
* new ForEach<E>(null, array, op, 0, array.length).invoke(); |
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> |
* } |
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> |
* |
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> |
* final E[] array; final MyOperation<E> op; final int lo, hi; |
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* ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) { |
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* 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 |
| 93 |
< |
* 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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* |
| 138 |
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* <pre> {@code |
| 139 |
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* class ForEach<E> ... |
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< |
* public void compute() { // version 2 |
| 141 |
< |
* if (hi - lo >= 2) { |
| 142 |
< |
* 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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< |
* } |
| 147 |
< |
* else { |
| 148 |
< |
* if (hi > lo) |
| 149 |
< |
* op.apply(array[lo]); |
| 150 |
< |
* tryComplete(); |
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< |
* } |
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> |
* public void compute() { // version 2 |
| 141 |
> |
* if (hi - lo >= 2) { |
| 142 |
> |
* int mid = (lo + hi) >>> 1; |
| 143 |
> |
* setPendingCount(1); // only one pending |
| 144 |
> |
* new ForEach(this, array, op, mid, hi).fork(); // right child |
| 145 |
> |
* new ForEach(this, array, op, lo, mid).compute(); // direct invoke |
| 146 |
> |
* } |
| 147 |
> |
* else { |
| 148 |
> |
* if (hi > lo) |
| 149 |
> |
* op.apply(array[lo]); |
| 150 |
> |
* tryComplete(); |
| 151 |
|
* } |
| 152 |
+ |
* } |
| 153 |
|
* }</pre> |
| 154 |
|
* |
| 155 |
|
* As a further improvement, notice that the left task need not even |
| 156 |
|
* exist. Instead of creating a new one, we can iterate using the |
| 157 |
< |
* original task, and add a pending count for each fork: |
| 157 |
> |
* original task, and add a pending count for each fork. Additionally, |
| 158 |
> |
* because no task in this tree implements an {@link #onCompletion} |
| 159 |
> |
* method, {@code tryComplete()} can be replaced with {@link |
| 160 |
> |
* #propagateCompletion}. |
| 161 |
|
* |
| 162 |
|
* <pre> {@code |
| 163 |
|
* class ForEach<E> ... |
| 164 |
< |
* public void compute() { // version 3 |
| 165 |
< |
* int l = lo, h = hi; |
| 166 |
< |
* while (h - l >= 2) { |
| 167 |
< |
* int mid = (l + h) >>> 1; |
| 168 |
< |
* addToPendingCount(1); |
| 169 |
< |
* new ForEach(this, array, op, mid, h).fork(); // right child |
| 170 |
< |
* h = mid; |
| 142 |
< |
* } |
| 143 |
< |
* if (h > l) |
| 144 |
< |
* op.apply(array[l]); |
| 145 |
< |
* tryComplete(); |
| 164 |
> |
* public void compute() { // version 3 |
| 165 |
> |
* int l = lo, h = hi; |
| 166 |
> |
* while (h - l >= 2) { |
| 167 |
> |
* int mid = (l + h) >>> 1; |
| 168 |
> |
* addToPendingCount(1); |
| 169 |
> |
* new ForEach(this, array, op, mid, h).fork(); // right child |
| 170 |
> |
* h = mid; |
| 171 |
|
* } |
| 172 |
+ |
* if (h > l) |
| 173 |
+ |
* op.apply(array[l]); |
| 174 |
+ |
* propagateCompletion(); |
| 175 |
+ |
* } |
| 176 |
|
* }</pre> |
| 177 |
|
* |
| 178 |
|
* Additional improvements of such classes might entail precomputing |
| 181 |
|
* instead of two per iteration, and using an adaptive threshold |
| 182 |
|
* instead of always subdividing down to single elements. |
| 183 |
|
* |
| 184 |
+ |
* <p><b>Searching.</b> A tree of CountedCompleters can search for a |
| 185 |
+ |
* value or property in different parts of a data structure, and |
| 186 |
+ |
* report a result in an {@link |
| 187 |
+ |
* java.util.concurrent.atomic.AtomicReference AtomicReference} as |
| 188 |
+ |
* soon as one is found. The others can poll the result to avoid |
| 189 |
+ |
* unnecessary work. (You could additionally {@linkplain #cancel |
| 190 |
+ |
* cancel} other tasks, but it is usually simpler and more efficient |
| 191 |
+ |
* to just let them notice that the result is set and if so skip |
| 192 |
+ |
* further processing.) Illustrating again with an array using full |
| 193 |
+ |
* partitioning (again, in practice, leaf tasks will almost always |
| 194 |
+ |
* process more than one element): |
| 195 |
+ |
* |
| 196 |
+ |
* <pre> {@code |
| 197 |
+ |
* class Searcher<E> extends CountedCompleter<E> { |
| 198 |
+ |
* final E[] array; final AtomicReference<E> result; final int lo, hi; |
| 199 |
+ |
* Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) { |
| 200 |
+ |
* super(p); |
| 201 |
+ |
* this.array = array; this.result = result; this.lo = lo; this.hi = hi; |
| 202 |
+ |
* } |
| 203 |
+ |
* public E getRawResult() { return result.get(); } |
| 204 |
+ |
* public void compute() { // similar to ForEach version 3 |
| 205 |
+ |
* int l = lo, h = hi; |
| 206 |
+ |
* while (result.get() == null && h >= l) { |
| 207 |
+ |
* if (h - l >= 2) { |
| 208 |
+ |
* int mid = (l + h) >>> 1; |
| 209 |
+ |
* addToPendingCount(1); |
| 210 |
+ |
* new Searcher(this, array, result, mid, h).fork(); |
| 211 |
+ |
* h = mid; |
| 212 |
+ |
* } |
| 213 |
+ |
* else { |
| 214 |
+ |
* E x = array[l]; |
| 215 |
+ |
* if (matches(x) && result.compareAndSet(null, x)) |
| 216 |
+ |
* quietlyCompleteRoot(); // root task is now joinable |
| 217 |
+ |
* break; |
| 218 |
+ |
* } |
| 219 |
+ |
* } |
| 220 |
+ |
* tryComplete(); // normally complete whether or not found |
| 221 |
+ |
* } |
| 222 |
+ |
* boolean matches(E e) { ... } // return true if found |
| 223 |
+ |
* |
| 224 |
+ |
* public static <E> E search(E[] array) { |
| 225 |
+ |
* return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke(); |
| 226 |
+ |
* } |
| 227 |
+ |
*}}</pre> |
| 228 |
+ |
* |
| 229 |
+ |
* In this example, as well as others in which tasks have no other |
| 230 |
+ |
* effects except to compareAndSet a common result, the trailing |
| 231 |
+ |
* unconditional invocation of {@code tryComplete} could be made |
| 232 |
+ |
* conditional ({@code if (result.get() == null) tryComplete();}) |
| 233 |
+ |
* because no further bookkeeping is required to manage completions |
| 234 |
+ |
* once the root task completes. |
| 235 |
+ |
* |
| 236 |
|
* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine |
| 237 |
|
* results of multiple subtasks usually need to access these results |
| 238 |
|
* in method {@link #onCompletion}. As illustrated in the following |
| 240 |
|
* and reductions are all of type {@code E}), one way to do this in |
| 241 |
|
* divide and conquer designs is to have each subtask record its |
| 242 |
|
* sibling, so that it can be accessed in method {@code onCompletion}. |
| 243 |
< |
* For clarity, this class uses explicit left and right subtasks, but |
| 244 |
< |
* variants of other streamlinings seen in the above example may also |
| 245 |
< |
* apply. |
| 243 |
> |
* This technique applies to reductions in which the order of |
| 244 |
> |
* combining left and right results does not matter; ordered |
| 245 |
> |
* reductions require explicit left/right designations. Variants of |
| 246 |
> |
* other streamlinings seen in the above examples may also apply. |
| 247 |
|
* |
| 248 |
|
* <pre> {@code |
| 249 |
|
* class MyMapper<E> { E apply(E v) { ... } } |
| 250 |
|
* class MyReducer<E> { E apply(E x, E y) { ... } } |
| 251 |
< |
* class MapReducer<E> extends CountedCompleter { |
| 252 |
< |
* final E[] array; final MyMapper<E> mapper; |
| 253 |
< |
* final MyReducer<E> reducer; final int lo, hi; |
| 254 |
< |
* MapReducer sibling; |
| 255 |
< |
* E result; |
| 256 |
< |
* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper, |
| 257 |
< |
* MyReducer<E> reducer, int lo, int hi) { |
| 258 |
< |
* super(p); |
| 259 |
< |
* this.array = array; this.mapper = mapper; |
| 260 |
< |
* this.reducer = reducer; this.lo = lo; this.hi = hi; |
| 251 |
> |
* class MapReducer<E> extends CountedCompleter<E> { |
| 252 |
> |
* final E[] array; final MyMapper<E> mapper; |
| 253 |
> |
* final MyReducer<E> reducer; final int lo, hi; |
| 254 |
> |
* MapReducer<E> sibling; |
| 255 |
> |
* E result; |
| 256 |
> |
* MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper, |
| 257 |
> |
* MyReducer<E> reducer, int lo, int hi) { |
| 258 |
> |
* super(p); |
| 259 |
> |
* this.array = array; this.mapper = mapper; |
| 260 |
> |
* this.reducer = reducer; this.lo = lo; this.hi = hi; |
| 261 |
> |
* } |
| 262 |
> |
* public void compute() { |
| 263 |
> |
* if (hi - lo >= 2) { |
| 264 |
> |
* int mid = (lo + hi) >>> 1; |
| 265 |
> |
* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid); |
| 266 |
> |
* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi); |
| 267 |
> |
* left.sibling = right; |
| 268 |
> |
* right.sibling = left; |
| 269 |
> |
* setPendingCount(1); // only right is pending |
| 270 |
> |
* right.fork(); |
| 271 |
> |
* left.compute(); // directly execute left |
| 272 |
|
* } |
| 273 |
< |
* public void compute() { |
| 274 |
< |
* if (hi - lo >= 2) { |
| 275 |
< |
* int mid = (lo + hi) >>> 1; |
| 276 |
< |
* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid); |
| 184 |
< |
* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi); |
| 185 |
< |
* left.sibling = right; |
| 186 |
< |
* right.sibling = left; |
| 187 |
< |
* setPendingCount(1); // only right is pending |
| 188 |
< |
* right.fork(); |
| 189 |
< |
* left.compute(); // directly execute left |
| 190 |
< |
* } |
| 191 |
< |
* else { |
| 192 |
< |
* if (hi > lo) |
| 193 |
< |
* result = mapper.apply(array[lo]); |
| 194 |
< |
* tryComplete(); |
| 195 |
< |
* } |
| 273 |
> |
* else { |
| 274 |
> |
* if (hi > lo) |
| 275 |
> |
* result = mapper.apply(array[lo]); |
| 276 |
> |
* tryComplete(); |
| 277 |
|
* } |
| 278 |
< |
* public void onCompletion(CountedCompleter caller) { |
| 279 |
< |
* if (caller != this) { |
| 280 |
< |
* MapReducer<E> child = (MapReducer<E>)caller; |
| 281 |
< |
* MapReducer<E> sib = child.sibling; |
| 282 |
< |
* if (sib == null || sib.result == null) |
| 283 |
< |
* result = child.result; |
| 284 |
< |
* else |
| 285 |
< |
* result = reducer.apply(child.result, sib.result); |
| 286 |
< |
* } |
| 278 |
> |
* } |
| 279 |
> |
* public void onCompletion(CountedCompleter<?> caller) { |
| 280 |
> |
* if (caller != this) { |
| 281 |
> |
* MapReducer<E> child = (MapReducer<E>)caller; |
| 282 |
> |
* MapReducer<E> sib = child.sibling; |
| 283 |
> |
* if (sib == null || sib.result == null) |
| 284 |
> |
* result = child.result; |
| 285 |
> |
* else |
| 286 |
> |
* result = reducer.apply(child.result, sib.result); |
| 287 |
|
* } |
| 288 |
+ |
* } |
| 289 |
+ |
* public E getRawResult() { return result; } |
| 290 |
+ |
* |
| 291 |
+ |
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) { |
| 292 |
+ |
* return new MapReducer<E>(null, array, mapper, reducer, |
| 293 |
+ |
* 0, array.length).invoke(); |
| 294 |
+ |
* } |
| 295 |
+ |
* }}</pre> |
| 296 |
+ |
* |
| 297 |
+ |
* Here, method {@code onCompletion} takes a form common to many |
| 298 |
+ |
* completion designs that combine results. This callback-style method |
| 299 |
+ |
* is triggered once per task, in either of the two different contexts |
| 300 |
+ |
* in which the pending count is, or becomes, zero: (1) by a task |
| 301 |
+ |
* itself, if its pending count is zero upon invocation of {@code |
| 302 |
+ |
* tryComplete}, or (2) by any of its subtasks when they complete and |
| 303 |
+ |
* decrement the pending count to zero. The {@code caller} argument |
| 304 |
+ |
* distinguishes cases. Most often, when the caller is {@code this}, |
| 305 |
+ |
* no action is necessary. Otherwise the caller argument can be used |
| 306 |
+ |
* (usually via a cast) to supply a value (and/or links to other |
| 307 |
+ |
* values) to be combined. Assuming proper use of pending counts, the |
| 308 |
+ |
* actions inside {@code onCompletion} occur (once) upon completion of |
| 309 |
+ |
* a task and its subtasks. No additional synchronization is required |
| 310 |
+ |
* within this method to ensure thread safety of accesses to fields of |
| 311 |
+ |
* this task or other completed tasks. |
| 312 |
+ |
* |
| 313 |
+ |
* <p><b>Completion Traversals</b>. If using {@code onCompletion} to |
| 314 |
+ |
* process completions is inapplicable or inconvenient, you can use |
| 315 |
+ |
* methods {@link #firstComplete} and {@link #nextComplete} to create |
| 316 |
+ |
* custom traversals. For example, to define a MapReducer that only |
| 317 |
+ |
* splits out right-hand tasks in the form of the third ForEach |
| 318 |
+ |
* example, the completions must cooperatively reduce along |
| 319 |
+ |
* unexhausted subtask links, which can be done as follows: |
| 320 |
|
* |
| 321 |
< |
* public static <E> E mapReduce(ForkJoinPool pool, E[] array, |
| 322 |
< |
* MyMapper<E> mapper, MyReducer<E> reducer) { |
| 323 |
< |
* MapReducer<E> mr = new MapReducer<E>(null, array, mapper, |
| 324 |
< |
* reducer, 0, array.length); |
| 325 |
< |
* pool.invoke(mr); |
| 326 |
< |
* return mr.result; |
| 321 |
> |
* <pre> {@code |
| 322 |
> |
* class MapReducer<E> extends CountedCompleter<E> { // version 2 |
| 323 |
> |
* final E[] array; final MyMapper<E> mapper; |
| 324 |
> |
* final MyReducer<E> reducer; final int lo, hi; |
| 325 |
> |
* MapReducer<E> forks, next; // record subtask forks in list |
| 326 |
> |
* E result; |
| 327 |
> |
* MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper, |
| 328 |
> |
* MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) { |
| 329 |
> |
* super(p); |
| 330 |
> |
* this.array = array; this.mapper = mapper; |
| 331 |
> |
* this.reducer = reducer; this.lo = lo; this.hi = hi; |
| 332 |
> |
* this.next = next; |
| 333 |
> |
* } |
| 334 |
> |
* public void compute() { |
| 335 |
> |
* int l = lo, h = hi; |
| 336 |
> |
* while (h - l >= 2) { |
| 337 |
> |
* int mid = (l + h) >>> 1; |
| 338 |
> |
* addToPendingCount(1); |
| 339 |
> |
* (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork; |
| 340 |
> |
* h = mid; |
| 341 |
> |
* } |
| 342 |
> |
* if (h > l) |
| 343 |
> |
* result = mapper.apply(array[l]); |
| 344 |
> |
* // process completions by reducing along and advancing subtask links |
| 345 |
> |
* for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) { |
| 346 |
> |
* for (MapReducer t = (MapReducer)c, s = t.forks; s != null; s = t.forks = s.next) |
| 347 |
> |
* t.result = reducer.apply(t.result, s.result); |
| 348 |
|
* } |
| 349 |
< |
* } }</pre> |
| 349 |
> |
* } |
| 350 |
> |
* public E getRawResult() { return result; } |
| 351 |
> |
* |
| 352 |
> |
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) { |
| 353 |
> |
* return new MapReducer<E>(null, array, mapper, reducer, |
| 354 |
> |
* 0, array.length, null).invoke(); |
| 355 |
> |
* } |
| 356 |
> |
* }}</pre> |
| 357 |
|
* |
| 358 |
|
* <p><b>Triggers.</b> Some CountedCompleters are themselves never |
| 359 |
|
* forked, but instead serve as bits of plumbing in other designs; |
| 361 |
|
* triggers another async task. For example: |
| 362 |
|
* |
| 363 |
|
* <pre> {@code |
| 364 |
< |
* class HeaderBuilder extends CountedCompleter { ... } |
| 365 |
< |
* class BodyBuilder extends CountedCompleter { ... } |
| 366 |
< |
* class PacketSender extends CountedCompleter { |
| 367 |
< |
* PacketSender(...) { super(null, 1); ... } // trigger on second completion |
| 368 |
< |
* public void compute() { } // never called |
| 369 |
< |
* public void onCompletion(CountedCompleter caller) { sendPacket(); } |
| 364 |
> |
* class HeaderBuilder extends CountedCompleter<...> { ... } |
| 365 |
> |
* class BodyBuilder extends CountedCompleter<...> { ... } |
| 366 |
> |
* class PacketSender extends CountedCompleter<...> { |
| 367 |
> |
* PacketSender(...) { super(null, 1); ... } // trigger on second completion |
| 368 |
> |
* public void compute() { } // never called |
| 369 |
> |
* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); } |
| 370 |
|
* } |
| 371 |
|
* // sample use: |
| 372 |
|
* PacketSender p = new PacketSender(); |
| 377 |
|
* @since 1.8 |
| 378 |
|
* @author Doug Lea |
| 379 |
|
*/ |
| 380 |
< |
public abstract class CountedCompleter extends ForkJoinTask<Void> { |
| 380 |
> |
public abstract class CountedCompleter<T> extends ForkJoinTask<T> { |
| 381 |
> |
private static final long serialVersionUID = 5232453752276485070L; |
| 382 |
> |
|
| 383 |
|
/** This task's completer, or null if none */ |
| 384 |
< |
final CountedCompleter completer; |
| 384 |
> |
final CountedCompleter<?> completer; |
| 385 |
|
/** The number of pending tasks until completion */ |
| 386 |
|
volatile int pending; |
| 387 |
|
|
| 389 |
|
* Creates a new CountedCompleter with the given completer |
| 390 |
|
* and initial pending count. |
| 391 |
|
* |
| 392 |
< |
* @param completer this tasks completer, or {@code null} if none |
| 392 |
> |
* @param completer this task's completer, or {@code null} if none |
| 393 |
|
* @param initialPendingCount the initial pending count |
| 394 |
|
*/ |
| 395 |
< |
protected CountedCompleter(CountedCompleter completer, |
| 395 |
> |
protected CountedCompleter(CountedCompleter<?> completer, |
| 396 |
|
int initialPendingCount) { |
| 397 |
|
this.completer = completer; |
| 398 |
|
this.pending = initialPendingCount; |
| 402 |
|
* Creates a new CountedCompleter with the given completer |
| 403 |
|
* and an initial pending count of zero. |
| 404 |
|
* |
| 405 |
< |
* @param completer this tasks completer, or {@code null} if none |
| 405 |
> |
* @param completer this task's completer, or {@code null} if none |
| 406 |
|
*/ |
| 407 |
< |
protected CountedCompleter(CountedCompleter completer) { |
| 407 |
> |
protected CountedCompleter(CountedCompleter<?> completer) { |
| 408 |
|
this.completer = completer; |
| 409 |
|
} |
| 410 |
|
|
| 422 |
|
public abstract void compute(); |
| 423 |
|
|
| 424 |
|
/** |
| 425 |
< |
* Executes the completion action when method {@link #tryComplete} |
| 426 |
< |
* is invoked and there are no pending counts, or when the |
| 427 |
< |
* unconditional method {@link #complete} is invoked. By default, |
| 428 |
< |
* this method does nothing. |
| 425 |
> |
* Performs an action when method {@link #tryComplete} is invoked |
| 426 |
> |
* and the pending count is zero, or when the unconditional |
| 427 |
> |
* method {@link #complete} is invoked. By default, this method |
| 428 |
> |
* does nothing. You can distinguish cases by checking the |
| 429 |
> |
* identity of the given caller argument. If not equal to {@code |
| 430 |
> |
* this}, then it is typically a subtask that may contain results |
| 431 |
> |
* (and/or links to other results) to combine. |
| 432 |
|
* |
| 433 |
|
* @param caller the task invoking this method (which may |
| 434 |
|
* be this task itself). |
| 435 |
|
*/ |
| 436 |
< |
public void onCompletion(CountedCompleter caller) { |
| 436 |
> |
public void onCompletion(CountedCompleter<?> caller) { |
| 437 |
> |
} |
| 438 |
> |
|
| 439 |
> |
/** |
| 440 |
> |
* Performs an action when method {@link #completeExceptionally} |
| 441 |
> |
* is invoked or method {@link #compute} throws an exception, and |
| 442 |
> |
* this task has not otherwise already completed normally. On |
| 443 |
> |
* entry to this method, this task {@link |
| 444 |
> |
* ForkJoinTask#isCompletedAbnormally}. The return value of this |
| 445 |
> |
* method controls further propagation: If {@code true} and this |
| 446 |
> |
* task has a completer, then this completer is also completed |
| 447 |
> |
* exceptionally. The default implementation of this method does |
| 448 |
> |
* nothing except return {@code true}. |
| 449 |
> |
* |
| 450 |
> |
* @param ex the exception |
| 451 |
> |
* @param caller the task invoking this method (which may |
| 452 |
> |
* be this task itself). |
| 453 |
> |
* @return true if this exception should be propagated to this |
| 454 |
> |
* task's completer, if one exists. |
| 455 |
> |
*/ |
| 456 |
> |
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) { |
| 457 |
> |
return true; |
| 458 |
|
} |
| 459 |
|
|
| 460 |
|
/** |
| 463 |
|
* |
| 464 |
|
* @return the completer |
| 465 |
|
*/ |
| 466 |
< |
public final CountedCompleter getCompleter() { |
| 466 |
> |
public final CountedCompleter<?> getCompleter() { |
| 467 |
|
return completer; |
| 468 |
|
} |
| 469 |
|
|
| 501 |
|
* |
| 502 |
|
* @param expected the expected value |
| 503 |
|
* @param count the new value |
| 504 |
< |
* @return true is successful |
| 504 |
> |
* @return true if successful |
| 505 |
|
*/ |
| 506 |
|
public final boolean compareAndSetPendingCount(int expected, int count) { |
| 507 |
|
return U.compareAndSwapInt(this, PENDING, expected, count); |
| 508 |
|
} |
| 509 |
|
|
| 510 |
|
/** |
| 511 |
+ |
* If the pending count is nonzero, (atomically) decrements it. |
| 512 |
+ |
* |
| 513 |
+ |
* @return the initial (undecremented) pending count holding on entry |
| 514 |
+ |
* to this method |
| 515 |
+ |
*/ |
| 516 |
+ |
public final int decrementPendingCountUnlessZero() { |
| 517 |
+ |
int c; |
| 518 |
+ |
do {} while ((c = pending) != 0 && |
| 519 |
+ |
!U.compareAndSwapInt(this, PENDING, c, c - 1)); |
| 520 |
+ |
return c; |
| 521 |
+ |
} |
| 522 |
+ |
|
| 523 |
+ |
/** |
| 524 |
+ |
* Returns the root of the current computation; i.e., this |
| 525 |
+ |
* task if it has no completer, else its completer's root. |
| 526 |
+ |
* |
| 527 |
+ |
* @return the root of the current computation |
| 528 |
+ |
*/ |
| 529 |
+ |
public final CountedCompleter<?> getRoot() { |
| 530 |
+ |
CountedCompleter<?> a = this, p; |
| 531 |
+ |
while ((p = a.completer) != null) |
| 532 |
+ |
a = p; |
| 533 |
+ |
return a; |
| 534 |
+ |
} |
| 535 |
+ |
|
| 536 |
+ |
/** |
| 537 |
|
* If the pending count is nonzero, decrements the count; |
| 538 |
|
* otherwise invokes {@link #onCompletion} and then similarly |
| 539 |
|
* tries to complete this task's completer, if one exists, |
| 540 |
|
* else marks this task as complete. |
| 541 |
|
*/ |
| 542 |
|
public final void tryComplete() { |
| 543 |
< |
for (CountedCompleter a = this, s = a;;) { |
| 544 |
< |
int c; |
| 543 |
> |
CountedCompleter<?> a = this, s = a; |
| 544 |
> |
for (int c;;) { |
| 545 |
|
if ((c = a.pending) == 0) { |
| 546 |
|
a.onCompletion(s); |
| 547 |
|
if ((a = (s = a).completer) == null) { |
| 555 |
|
} |
| 556 |
|
|
| 557 |
|
/** |
| 558 |
+ |
* Equivalent to {@link #tryComplete} but does not invoke {@link |
| 559 |
+ |
* #onCompletion} along the completion path: If the pending count |
| 560 |
+ |
* is nonzero, decrements the count; otherwise, similarly tries to |
| 561 |
+ |
* complete this task's completer, if one exists, else marks this |
| 562 |
+ |
* task as complete. This method may be useful in cases where |
| 563 |
+ |
* {@code onCompletion} should not, or need not, be invoked for |
| 564 |
+ |
* each completer in a computation. |
| 565 |
+ |
*/ |
| 566 |
+ |
public final void propagateCompletion() { |
| 567 |
+ |
CountedCompleter<?> a = this, s = a; |
| 568 |
+ |
for (int c;;) { |
| 569 |
+ |
if ((c = a.pending) == 0) { |
| 570 |
+ |
if ((a = (s = a).completer) == null) { |
| 571 |
+ |
s.quietlyComplete(); |
| 572 |
+ |
return; |
| 573 |
+ |
} |
| 574 |
+ |
} |
| 575 |
+ |
else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) |
| 576 |
+ |
return; |
| 577 |
+ |
} |
| 578 |
+ |
} |
| 579 |
+ |
|
| 580 |
+ |
/** |
| 581 |
|
* Regardless of pending count, invokes {@link #onCompletion}, |
| 582 |
< |
* marks this task as complete with a {@code null} return value, |
| 583 |
< |
* and further triggers {@link #tryComplete} on this task's |
| 584 |
< |
* completer, if one exists. This method may be useful when |
| 585 |
< |
* forcing completion as soon as any one (versus all) of several |
| 586 |
< |
* subtask results are obtained. |
| 587 |
< |
* |
| 588 |
< |
* @param mustBeNull the {@code null} completion value |
| 589 |
< |
*/ |
| 590 |
< |
public void complete(Void mustBeNull) { |
| 591 |
< |
CountedCompleter p; |
| 582 |
> |
* marks this task as complete and further triggers {@link |
| 583 |
> |
* #tryComplete} on this task's completer, if one exists. The |
| 584 |
> |
* given rawResult is used as an argument to {@link #setRawResult} |
| 585 |
> |
* before invoking {@link #onCompletion} or marking this task as |
| 586 |
> |
* complete; its value is meaningful only for classes overriding |
| 587 |
> |
* {@code setRawResult}. |
| 588 |
> |
* |
| 589 |
> |
* <p>This method may be useful when forcing completion as soon as |
| 590 |
> |
* any one (versus all) of several subtask results are obtained. |
| 591 |
> |
* However, in the common (and recommended) case in which {@code |
| 592 |
> |
* setRawResult} is not overridden, this effect can be obtained |
| 593 |
> |
* more simply using {@code quietlyCompleteRoot();}. |
| 594 |
> |
* |
| 595 |
> |
* @param rawResult the raw result |
| 596 |
> |
*/ |
| 597 |
> |
public void complete(T rawResult) { |
| 598 |
> |
CountedCompleter<?> p; |
| 599 |
> |
setRawResult(rawResult); |
| 600 |
|
onCompletion(this); |
| 601 |
|
quietlyComplete(); |
| 602 |
|
if ((p = completer) != null) |
| 603 |
|
p.tryComplete(); |
| 604 |
|
} |
| 605 |
|
|
| 606 |
+ |
|
| 607 |
+ |
/** |
| 608 |
+ |
* If this task's pending count is zero, returns this task; |
| 609 |
+ |
* otherwise decrements its pending count and returns {@code |
| 610 |
+ |
* null}. This method is designed to be used with {@link |
| 611 |
+ |
* #nextComplete} in completion traversal loops. |
| 612 |
+ |
* |
| 613 |
+ |
* @return this task, if pending count was zero, else {@code null} |
| 614 |
+ |
*/ |
| 615 |
+ |
public final CountedCompleter<?> firstComplete() { |
| 616 |
+ |
for (int c;;) { |
| 617 |
+ |
if ((c = pending) == 0) |
| 618 |
+ |
return this; |
| 619 |
+ |
else if (U.compareAndSwapInt(this, PENDING, c, c - 1)) |
| 620 |
+ |
return null; |
| 621 |
+ |
} |
| 622 |
+ |
} |
| 623 |
+ |
|
| 624 |
+ |
/** |
| 625 |
+ |
* If this task does not have a completer, invokes {@link |
| 626 |
+ |
* ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if |
| 627 |
+ |
* this task's pending count is non-zero, decrements its pending |
| 628 |
+ |
* count and returns {@code null}. Otherwise, returns the |
| 629 |
+ |
* completer. This method can be used as part of a completion |
| 630 |
+ |
* traversal loop for homogeneous task hierarchies: |
| 631 |
+ |
* |
| 632 |
+ |
* <pre> {@code |
| 633 |
+ |
* for (CountedCompleter<?> c = firstComplete(); |
| 634 |
+ |
* c != null; |
| 635 |
+ |
* c = c.nextComplete()) { |
| 636 |
+ |
* // ... process c ... |
| 637 |
+ |
* }}</pre> |
| 638 |
+ |
* |
| 639 |
+ |
* @return the completer, or {@code null} if none |
| 640 |
+ |
*/ |
| 641 |
+ |
public final CountedCompleter<?> nextComplete() { |
| 642 |
+ |
CountedCompleter<?> p; |
| 643 |
+ |
if ((p = completer) != null) |
| 644 |
+ |
return p.firstComplete(); |
| 645 |
+ |
else { |
| 646 |
+ |
quietlyComplete(); |
| 647 |
+ |
return null; |
| 648 |
+ |
} |
| 649 |
+ |
} |
| 650 |
+ |
|
| 651 |
+ |
/** |
| 652 |
+ |
* Equivalent to {@code getRoot().quietlyComplete()}. |
| 653 |
+ |
*/ |
| 654 |
+ |
public final void quietlyCompleteRoot() { |
| 655 |
+ |
for (CountedCompleter<?> a = this, p;;) { |
| 656 |
+ |
if ((p = a.completer) == null) { |
| 657 |
+ |
a.quietlyComplete(); |
| 658 |
+ |
return; |
| 659 |
+ |
} |
| 660 |
+ |
a = p; |
| 661 |
+ |
} |
| 662 |
+ |
} |
| 663 |
+ |
|
| 664 |
|
/** |
| 665 |
< |
* Implements execution conventions for CountedCompleters |
| 665 |
> |
* Supports ForkJoinTask exception propagation. |
| 666 |
> |
*/ |
| 667 |
> |
void internalPropagateException(Throwable ex) { |
| 668 |
> |
CountedCompleter<?> a = this, s = a; |
| 669 |
> |
while (a.onExceptionalCompletion(ex, s) && |
| 670 |
> |
(a = (s = a).completer) != null && a.status >= 0) |
| 671 |
> |
a.recordExceptionalCompletion(ex); |
| 672 |
> |
} |
| 673 |
> |
|
| 674 |
> |
/** |
| 675 |
> |
* Implements execution conventions for CountedCompleters. |
| 676 |
|
*/ |
| 677 |
|
protected final boolean exec() { |
| 678 |
|
compute(); |
| 680 |
|
} |
| 681 |
|
|
| 682 |
|
/** |
| 683 |
< |
* Always returns {@code null}. |
| 683 |
> |
* Returns the result of the computation. By default |
| 684 |
> |
* returns {@code null}, which is appropriate for {@code Void} |
| 685 |
> |
* actions, but in other cases should be overridden, almost |
| 686 |
> |
* always to return a field or function of a field that |
| 687 |
> |
* holds the result upon completion. |
| 688 |
|
* |
| 689 |
< |
* @return {@code null} always |
| 689 |
> |
* @return the result of the computation |
| 690 |
|
*/ |
| 691 |
< |
public final Void getRawResult() { return null; } |
| 691 |
> |
public T getRawResult() { return null; } |
| 692 |
|
|
| 693 |
|
/** |
| 694 |
< |
* Requires null completion value. |
| 694 |
> |
* A method that result-bearing CountedCompleters may optionally |
| 695 |
> |
* use to help maintain result data. By default, does nothing. |
| 696 |
> |
* Overrides are not recommended. However, if this method is |
| 697 |
> |
* overridden to update existing objects or fields, then it must |
| 698 |
> |
* in general be defined to be thread-safe. |
| 699 |
|
*/ |
| 700 |
< |
protected final void setRawResult(Void mustBeNull) { } |
| 401 |
< |
|
| 402 |
< |
/** |
| 403 |
< |
* Support for FJT exception propagation |
| 404 |
< |
*/ |
| 405 |
< |
final ForkJoinTask<?> internalGetCompleter() { return completer; } |
| 700 |
> |
protected void setRawResult(T t) { } |
| 701 |
|
|
| 702 |
|
// Unsafe mechanics |
| 703 |
|
private static final sun.misc.Unsafe U; |
| 704 |
|
private static final long PENDING; |
| 705 |
|
static { |
| 706 |
|
try { |
| 707 |
< |
U = getUnsafe(); |
| 707 |
> |
U = sun.misc.Unsafe.getUnsafe(); |
| 708 |
|
PENDING = U.objectFieldOffset |
| 709 |
|
(CountedCompleter.class.getDeclaredField("pending")); |
| 710 |
|
} catch (Exception e) { |
| 712 |
|
} |
| 713 |
|
} |
| 714 |
|
|
| 420 |
– |
|
| 715 |
|
/** |
| 716 |
|
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
| 717 |
|
* Replace with a simple call to Unsafe.getUnsafe when integrating |
| 739 |
|
} |
| 740 |
|
} |
| 741 |
|
} |
| 448 |
– |
|
| 742 |
|
} |
