8 |
|
|
9 |
|
/** |
10 |
|
* A {@link ForkJoinTask} with a completion action performed when |
11 |
< |
* triggered and there are no remaining pending |
12 |
< |
* actions. CountedCompleters are in general more robust in the |
11 |
> |
* triggered and there are no remaining pending actions. |
12 |
> |
* CountedCompleters are in general more robust in the |
13 |
|
* presence of subtask stalls and blockage than are other forms of |
14 |
|
* ForkJoinTasks, but are less intuitive to program. Uses of |
15 |
|
* CountedCompleter are similar to those of other completion based |
16 |
|
* components (such as {@link java.nio.channels.CompletionHandler}) |
17 |
|
* except that multiple <em>pending</em> completions may be necessary |
18 |
< |
* to trigger the {@link #onCompletion} action, not just one. Unless |
19 |
< |
* initialized otherwise, the {@link #getPendingCount pending count} |
20 |
< |
* starts at zero, but may be (atomically) changed using methods |
21 |
< |
* {@link #setPendingCount}, {@link #addToPendingCount}, and {@link |
22 |
< |
* #compareAndSetPendingCount}. Upon invocation of {@link |
18 |
> |
* to trigger the completion action {@link #onCompletion(CountedCompleter)}, |
19 |
> |
* not just one. |
20 |
> |
* Unless initialized otherwise, the {@linkplain #getPendingCount pending |
21 |
> |
* count} starts at zero, but may be (atomically) changed using |
22 |
> |
* methods {@link #setPendingCount}, {@link #addToPendingCount}, and |
23 |
> |
* {@link #compareAndSetPendingCount}. Upon invocation of {@link |
24 |
|
* #tryComplete}, if the pending action count is nonzero, it is |
25 |
|
* decremented; otherwise, the completion action is performed, and if |
26 |
|
* this completer itself has a completer, the process is continued |
41 |
|
* <p>A concrete CountedCompleter class must define method {@link |
42 |
|
* #compute}, that should in most cases (as illustrated below), invoke |
43 |
|
* {@code tryComplete()} once before returning. The class may also |
44 |
< |
* optionally override method {@link #onCompletion} to perform an |
45 |
< |
* action upon normal completion, and method {@link |
46 |
< |
* #onExceptionalCompletion} to perform an action upon any exception. |
44 |
> |
* optionally override method {@link #onCompletion(CountedCompleter)} |
45 |
> |
* to perform an action upon normal completion, and method |
46 |
> |
* {@link #onExceptionalCompletion(Throwable, CountedCompleter)} to |
47 |
> |
* perform an action upon any exception. |
48 |
|
* |
49 |
|
* <p>CountedCompleters most often do not bear results, in which case |
50 |
|
* they are normally declared as {@code CountedCompleter<Void>}, and |
65 |
|
* only as an internal helper for other computations, so its own task |
66 |
|
* status (as reported in methods such as {@link ForkJoinTask#isDone}) |
67 |
|
* is arbitrary; this status changes only upon explicit invocations of |
68 |
< |
* {@link #complete}, {@link ForkJoinTask#cancel}, {@link |
69 |
< |
* ForkJoinTask#completeExceptionally} or upon exceptional completion |
70 |
< |
* of method {@code compute}. Upon any exceptional completion, the |
71 |
< |
* exception may be relayed to a task's completer (and its completer, |
72 |
< |
* and so on), if one exists and it has not otherwise already |
73 |
< |
* completed. Similarly, cancelling an internal CountedCompleter has |
74 |
< |
* only a local effect on that completer, so is not often useful. |
68 |
> |
* {@link #complete}, {@link ForkJoinTask#cancel}, |
69 |
> |
* {@link ForkJoinTask#completeExceptionally(Throwable)} or upon |
70 |
> |
* exceptional completion of method {@code compute}. Upon any |
71 |
> |
* exceptional completion, the exception may be relayed to a task's |
72 |
> |
* completer (and its completer, and so on), if one exists and it has |
73 |
> |
* not otherwise already completed. Similarly, cancelling an internal |
74 |
> |
* CountedCompleter has only a local effect on that completer, so is |
75 |
> |
* not often useful. |
76 |
|
* |
77 |
|
* <p><b>Sample Usages.</b> |
78 |
|
* |
86 |
|
* subdivided) to each element of an array or collection; especially |
87 |
|
* when the operation takes a significantly different amount of time |
88 |
|
* to complete for some elements than others, either because of |
89 |
< |
* intrinsic variation (for example IO) or auxiliary effects such as |
89 |
> |
* intrinsic variation (for example I/O) or auxiliary effects such as |
90 |
|
* garbage collection. Because CountedCompleters provide their own |
91 |
|
* continuations, other threads need not block waiting to perform |
92 |
|
* them. |
99 |
|
* improve load balancing. In the recursive case, the second of each |
100 |
|
* pair of subtasks to finish triggers completion of its parent |
101 |
|
* (because no result combination is performed, the default no-op |
102 |
< |
* implementation of method {@code onCompletion} is not overridden). A |
103 |
< |
* static utility method sets up the base task and invokes it |
102 |
> |
* implementation of method {@code onCompletion} is not overridden). |
103 |
> |
* A static utility method sets up the base task and invokes it |
104 |
|
* (here, implicitly using the {@link ForkJoinPool#commonPool()}). |
105 |
|
* |
106 |
|
* <pre> {@code |
129 |
|
* op.apply(array[lo]); |
130 |
|
* tryComplete(); |
131 |
|
* } |
132 |
< |
* } }</pre> |
132 |
> |
* }}</pre> |
133 |
|
* |
134 |
|
* This design can be improved by noticing that in the recursive case, |
135 |
|
* the task has nothing to do after forking its right task, so can |
136 |
|
* directly invoke its left task before returning. (This is an analog |
137 |
|
* of tail recursion removal.) Also, because the task returns upon |
138 |
|
* executing its left task (rather than falling through to invoke |
139 |
< |
* tryComplete) the pending count is set to one: |
139 |
> |
* {@code tryComplete}) the pending count is set to one: |
140 |
|
* |
141 |
|
* <pre> {@code |
142 |
|
* class ForEach<E> ... |
155 |
|
* } |
156 |
|
* }</pre> |
157 |
|
* |
158 |
< |
* As a further improvement, notice that the left task need not even |
159 |
< |
* exist. Instead of creating a new one, we can iterate using the |
160 |
< |
* original task, and add a pending count for each fork. Additionally, |
161 |
< |
* because no task in this tree implements an {@link #onCompletion} |
162 |
< |
* method, {@code tryComplete()} can be replaced with {@link |
160 |
< |
* #propagateCompletion}. |
158 |
> |
* As a further improvement, notice that the left task need not even exist. |
159 |
> |
* Instead of creating a new one, we can iterate using the original task, |
160 |
> |
* and add a pending count for each fork. Additionally, because no task |
161 |
> |
* in this tree implements an {@link #onCompletion(CountedCompleter)} method, |
162 |
> |
* {@code tryComplete()} can be replaced with {@link #propagateCompletion}. |
163 |
|
* |
164 |
|
* <pre> {@code |
165 |
|
* class ForEach<E> ... |
185 |
|
* |
186 |
|
* <p><b>Searching.</b> A tree of CountedCompleters can search for a |
187 |
|
* value or property in different parts of a data structure, and |
188 |
< |
* report a result in an {@link java.util.concurrent.AtomicReference} |
189 |
< |
* as soon as one is found. The others can poll the result to avoid |
190 |
< |
* unnecessary work. (You could additionally {@link #cancel} other |
191 |
< |
* tasks, but it is usually simpler and more efficient to just let |
192 |
< |
* them notice that the result is set and if so skip further |
193 |
< |
* processing.) Illustrating again with an array using full |
188 |
> |
* report a result in an {@link |
189 |
> |
* java.util.concurrent.atomic.AtomicReference AtomicReference} as |
190 |
> |
* soon as one is found. The others can poll the result to avoid |
191 |
> |
* unnecessary work. (You could additionally {@linkplain #cancel |
192 |
> |
* cancel} other tasks, but it is usually simpler and more efficient |
193 |
> |
* to just let them notice that the result is set and if so skip |
194 |
> |
* further processing.) Illustrating again with an array using full |
195 |
|
* partitioning (again, in practice, leaf tasks will almost always |
196 |
|
* process more than one element): |
197 |
|
* |
226 |
|
* public static <E> E search(E[] array) { |
227 |
|
* return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke(); |
228 |
|
* } |
229 |
< |
*}}</pre> |
229 |
> |
* }}</pre> |
230 |
|
* |
231 |
|
* In this example, as well as others in which tasks have no other |
232 |
|
* effects except to compareAndSet a common result, the trailing |
237 |
|
* |
238 |
|
* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine |
239 |
|
* results of multiple subtasks usually need to access these results |
240 |
< |
* in method {@link #onCompletion}. As illustrated in the following |
240 |
> |
* in method {@link #onCompletion(CountedCompleter)}. As illustrated in the following |
241 |
|
* class (that performs a simplified form of map-reduce where mappings |
242 |
|
* and reductions are all of type {@code E}), one way to do this in |
243 |
|
* divide and conquer designs is to have each subtask record its |
280 |
|
* } |
281 |
|
* public void onCompletion(CountedCompleter<?> caller) { |
282 |
|
* if (caller != this) { |
283 |
< |
* MapReducer<E> child = (MapReducer<E>)caller; |
284 |
< |
* MapReducer<E> sib = child.sibling; |
285 |
< |
* if (sib == null || sib.result == null) |
286 |
< |
* result = child.result; |
287 |
< |
* else |
288 |
< |
* result = reducer.apply(child.result, sib.result); |
283 |
> |
* MapReducer<E> child = (MapReducer<E>)caller; |
284 |
> |
* MapReducer<E> sib = child.sibling; |
285 |
> |
* if (sib == null || sib.result == null) |
286 |
> |
* result = child.result; |
287 |
> |
* else |
288 |
> |
* result = reducer.apply(child.result, sib.result); |
289 |
|
* } |
290 |
|
* } |
291 |
|
* public E getRawResult() { return result; } |
294 |
|
* return new MapReducer<E>(null, array, mapper, reducer, |
295 |
|
* 0, array.length).invoke(); |
296 |
|
* } |
297 |
< |
* } }</pre> |
297 |
> |
* }}</pre> |
298 |
|
* |
299 |
|
* Here, method {@code onCompletion} takes a form common to many |
300 |
|
* completion designs that combine results. This callback-style method |
306 |
|
* distinguishes cases. Most often, when the caller is {@code this}, |
307 |
|
* no action is necessary. Otherwise the caller argument can be used |
308 |
|
* (usually via a cast) to supply a value (and/or links to other |
309 |
< |
* values) to be combined. Asuuming proper use of pending counts, the |
309 |
> |
* values) to be combined. Assuming proper use of pending counts, the |
310 |
|
* actions inside {@code onCompletion} occur (once) upon completion of |
311 |
|
* a task and its subtasks. No additional synchronization is required |
312 |
|
* within this method to ensure thread safety of accesses to fields of |
338 |
|
* while (h - l >= 2) { |
339 |
|
* int mid = (l + h) >>> 1; |
340 |
|
* addToPendingCount(1); |
341 |
< |
* (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork; |
341 |
> |
* (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork(); |
342 |
|
* h = mid; |
343 |
|
* } |
344 |
|
* if (h > l) |
359 |
|
* |
360 |
|
* <p><b>Triggers.</b> Some CountedCompleters are themselves never |
361 |
|
* forked, but instead serve as bits of plumbing in other designs; |
362 |
< |
* including those in which the completion of one of more async tasks |
362 |
> |
* including those in which the completion of one or more async tasks |
363 |
|
* triggers another async task. For example: |
364 |
|
* |
365 |
|
* <pre> {@code |
391 |
|
* Creates a new CountedCompleter with the given completer |
392 |
|
* and initial pending count. |
393 |
|
* |
394 |
< |
* @param completer this tasks completer, or {@code null} if none |
394 |
> |
* @param completer this task's completer, or {@code null} if none |
395 |
|
* @param initialPendingCount the initial pending count |
396 |
|
*/ |
397 |
|
protected CountedCompleter(CountedCompleter<?> completer, |
404 |
|
* Creates a new CountedCompleter with the given completer |
405 |
|
* and an initial pending count of zero. |
406 |
|
* |
407 |
< |
* @param completer this tasks completer, or {@code null} if none |
407 |
> |
* @param completer this task's completer, or {@code null} if none |
408 |
|
*/ |
409 |
|
protected CountedCompleter(CountedCompleter<?> completer) { |
410 |
|
this.completer = completer; |
425 |
|
|
426 |
|
/** |
427 |
|
* Performs an action when method {@link #tryComplete} is invoked |
428 |
< |
* and there are no pending counts, or when the unconditional |
428 |
> |
* and the pending count is zero, or when the unconditional |
429 |
|
* method {@link #complete} is invoked. By default, this method |
430 |
|
* does nothing. You can distinguish cases by checking the |
431 |
|
* identity of the given caller argument. If not equal to {@code |
433 |
|
* (and/or links to other results) to combine. |
434 |
|
* |
435 |
|
* @param caller the task invoking this method (which may |
436 |
< |
* be this task itself). |
436 |
> |
* be this task itself) |
437 |
|
*/ |
438 |
|
public void onCompletion(CountedCompleter<?> caller) { |
439 |
|
} |
440 |
|
|
441 |
|
/** |
442 |
< |
* Performs an action when method {@link #completeExceptionally} |
443 |
< |
* is invoked or method {@link #compute} throws an exception, and |
444 |
< |
* this task has not otherwise already completed normally. On |
445 |
< |
* entry to this method, this task {@link |
446 |
< |
* ForkJoinTask#isCompletedAbnormally}. The return value of this |
447 |
< |
* method controls further propagation: If {@code true} and this |
448 |
< |
* task has a completer, then this completer is also completed |
449 |
< |
* exceptionally. The default implementation of this method does |
450 |
< |
* nothing except return {@code true}. |
442 |
> |
* Performs an action when method {@link |
443 |
> |
* #completeExceptionally(Throwable)} is invoked or method {@link |
444 |
> |
* #compute} throws an exception, and this task has not already |
445 |
> |
* otherwise completed normally. On entry to this method, this task |
446 |
> |
* {@link ForkJoinTask#isCompletedAbnormally}. The return value |
447 |
> |
* of this method controls further propagation: If {@code true} |
448 |
> |
* and this task has a completer that has not completed, then that |
449 |
> |
* completer is also completed exceptionally, with the same |
450 |
> |
* exception as this completer. The default implementation of |
451 |
> |
* this method does nothing except return {@code true}. |
452 |
|
* |
453 |
|
* @param ex the exception |
454 |
|
* @param caller the task invoking this method (which may |
455 |
< |
* be this task itself). |
456 |
< |
* @return true if this exception should be propagated to this |
457 |
< |
* tasks completer, if one exists. |
455 |
> |
* be this task itself) |
456 |
> |
* @return {@code true} if this exception should be propagated to this |
457 |
> |
* task's completer, if one exists |
458 |
|
*/ |
459 |
|
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) { |
460 |
|
return true; |
494 |
|
* @param delta the value to add |
495 |
|
*/ |
496 |
|
public final void addToPendingCount(int delta) { |
497 |
< |
int c; // note: can replace with intrinsic in jdk8 |
497 |
> |
int c; |
498 |
|
do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta)); |
499 |
|
} |
500 |
|
|
504 |
|
* |
505 |
|
* @param expected the expected value |
506 |
|
* @param count the new value |
507 |
< |
* @return true if successful |
507 |
> |
* @return {@code true} if successful |
508 |
|
*/ |
509 |
|
public final boolean compareAndSetPendingCount(int expected, int count) { |
510 |
|
return U.compareAndSwapInt(this, PENDING, expected, count); |
513 |
|
/** |
514 |
|
* If the pending count is nonzero, (atomically) decrements it. |
515 |
|
* |
516 |
< |
* @return the initial (undecremented) pending count holding on entry |
516 |
> |
* @return the initial (undecremented) pending count holding on entry |
517 |
|
* to this method |
518 |
|
*/ |
519 |
|
public final int decrementPendingCountUnlessZero() { |
538 |
|
|
539 |
|
/** |
540 |
|
* If the pending count is nonzero, decrements the count; |
541 |
< |
* otherwise invokes {@link #onCompletion} and then similarly |
542 |
< |
* tries to complete this task's completer, if one exists, |
543 |
< |
* else marks this task as complete. |
541 |
> |
* otherwise invokes {@link #onCompletion(CountedCompleter)} |
542 |
> |
* and then similarly tries to complete this task's completer, |
543 |
> |
* if one exists, else marks this task as complete. |
544 |
|
*/ |
545 |
|
public final void tryComplete() { |
546 |
|
CountedCompleter<?> a = this, s = a; |
559 |
|
|
560 |
|
/** |
561 |
|
* Equivalent to {@link #tryComplete} but does not invoke {@link |
562 |
< |
* #onCompletion} along the completion path: If the pending count |
563 |
< |
* is nonzero, decrements the count; otherwise, similarly tries to |
564 |
< |
* complete this task's completer, if one exists, else marks this |
565 |
< |
* task as complete. This method may be useful in cases where |
566 |
< |
* {@code onCompletion} should not, or need not, be invoked for |
567 |
< |
* each completer in a computation. |
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 = a; |
581 |
|
} |
582 |
|
|
583 |
|
/** |
584 |
< |
* Regardless of pending count, invokes {@link #onCompletion}, |
585 |
< |
* marks this task as complete and further triggers {@link |
586 |
< |
* #tryComplete} on this task's completer, if one exists. The |
587 |
< |
* given rawResult is used as an argument to {@link #setRawResult} |
588 |
< |
* before invoking {@link #onCompletion} or marking this task as |
589 |
< |
* complete; its value is meaningful only for classes overriding |
590 |
< |
* {@code setRawResult}. |
584 |
> |
* Regardless of pending count, invokes |
585 |
> |
* {@link #onCompletion(CountedCompleter)}, marks this task as |
586 |
> |
* complete and further triggers {@link #tryComplete} on this |
587 |
> |
* task's completer, if one exists. The given rawResult is |
588 |
> |
* used as an argument to {@link #setRawResult} before invoking |
589 |
> |
* {@link #onCompletion(CountedCompleter)} or marking this task |
590 |
> |
* as complete; its value is meaningful only for classes |
591 |
> |
* overriding {@code setRawResult}. This method does not modify |
592 |
> |
* the pending count. |
593 |
|
* |
594 |
|
* <p>This method may be useful when forcing completion as soon as |
595 |
|
* any one (versus all) of several subtask results are obtained. |
629 |
|
/** |
630 |
|
* If this task does not have a completer, invokes {@link |
631 |
|
* ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if |
632 |
< |
* this task's pending count is non-zero, decrements its pending |
633 |
< |
* count and returns {@code null}. Otherwise, returns the |
632 |
> |
* the completer's pending count is non-zero, decrements that |
633 |
> |
* pending count and returns {@code null}. Otherwise, returns the |
634 |
|
* completer. This method can be used as part of a completion |
635 |
< |
* traversal loop for homogenous task hierarchies: |
635 |
> |
* traversal loop for homogeneous task hierarchies: |
636 |
|
* |
637 |
|
* <pre> {@code |
638 |
< |
* for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) { |
638 |
> |
* for (CountedCompleter<?> c = firstComplete(); |
639 |
> |
* c != null; |
640 |
> |
* c = c.nextComplete()) { |
641 |
|
* // ... process c ... |
642 |
|
* }}</pre> |
643 |
|
* |
667 |
|
} |
668 |
|
|
669 |
|
/** |
670 |
< |
* Support for FJT exception propagation |
670 |
> |
* Supports ForkJoinTask exception propagation. |
671 |
|
*/ |
672 |
|
void internalPropagateException(Throwable ex) { |
673 |
|
CountedCompleter<?> a = this, s = a; |
674 |
|
while (a.onExceptionalCompletion(ex, s) && |
675 |
< |
(a = (s = a).completer) != null && a.status >= 0) |
676 |
< |
a.recordExceptionalCompletion(ex); |
675 |
> |
(a = (s = a).completer) != null && a.status >= 0 && |
676 |
> |
a.recordExceptionalCompletion(ex) == EXCEPTIONAL) |
677 |
> |
; |
678 |
|
} |
679 |
|
|
680 |
|
/** |
681 |
< |
* Implements execution conventions for CountedCompleters |
681 |
> |
* Implements execution conventions for CountedCompleters. |
682 |
|
*/ |
683 |
|
protected final boolean exec() { |
684 |
|
compute(); |
686 |
|
} |
687 |
|
|
688 |
|
/** |
689 |
< |
* Returns the result of the computation. By default |
689 |
> |
* Returns the result of the computation. By default, |
690 |
|
* returns {@code null}, which is appropriate for {@code Void} |
691 |
|
* actions, but in other cases should be overridden, almost |
692 |
|
* always to return a field or function of a field that |
710 |
|
private static final long PENDING; |
711 |
|
static { |
712 |
|
try { |
713 |
< |
U = sun.misc.Unsafe.getUnsafe(); |
713 |
> |
U = getUnsafe(); |
714 |
|
PENDING = U.objectFieldOffset |
715 |
|
(CountedCompleter.class.getDeclaredField("pending")); |
716 |
|
} catch (Exception e) { |
728 |
|
private static sun.misc.Unsafe getUnsafe() { |
729 |
|
try { |
730 |
|
return sun.misc.Unsafe.getUnsafe(); |
731 |
< |
} catch (SecurityException se) { |
732 |
< |
try { |
733 |
< |
return java.security.AccessController.doPrivileged |
734 |
< |
(new java.security |
735 |
< |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
736 |
< |
public sun.misc.Unsafe run() throws Exception { |
737 |
< |
java.lang.reflect.Field f = sun.misc |
738 |
< |
.Unsafe.class.getDeclaredField("theUnsafe"); |
739 |
< |
f.setAccessible(true); |
740 |
< |
return (sun.misc.Unsafe) f.get(null); |
741 |
< |
}}); |
742 |
< |
} catch (java.security.PrivilegedActionException e) { |
743 |
< |
throw new RuntimeException("Could not initialize intrinsics", |
744 |
< |
e.getCause()); |
745 |
< |
} |
731 |
> |
} catch (SecurityException tryReflectionInstead) {} |
732 |
> |
try { |
733 |
> |
return java.security.AccessController.doPrivileged |
734 |
> |
(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
735 |
> |
public sun.misc.Unsafe run() throws Exception { |
736 |
> |
Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class; |
737 |
> |
for (java.lang.reflect.Field f : k.getDeclaredFields()) { |
738 |
> |
f.setAccessible(true); |
739 |
> |
Object x = f.get(null); |
740 |
> |
if (k.isInstance(x)) |
741 |
> |
return k.cast(x); |
742 |
> |
} |
743 |
> |
throw new NoSuchFieldError("the Unsafe"); |
744 |
> |
}}); |
745 |
> |
} catch (java.security.PrivilegedActionException e) { |
746 |
> |
throw new RuntimeException("Could not initialize intrinsics", |
747 |
> |
e.getCause()); |
748 |
|
} |
749 |
|
} |
750 |
|
} |