16 |
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import java.util.WeakHashMap; |
17 |
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|
18 |
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/** |
19 |
< |
* Abstract base class for tasks that run within a {@link |
20 |
< |
* ForkJoinPool}. A ForkJoinTask is a thread-like entity that is much |
19 |
> |
* Abstract base class for tasks that run within a {@link ForkJoinPool}. |
20 |
> |
* A {@code ForkJoinTask} is a thread-like entity that is much |
21 |
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* lighter weight than a normal thread. Huge numbers of tasks and |
22 |
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* subtasks may be hosted by a small number of actual threads in a |
23 |
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* ForkJoinPool, at the price of some usage limitations. |
24 |
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* |
25 |
< |
* <p> A "main" ForkJoinTask begins execution when submitted to a |
26 |
< |
* {@link ForkJoinPool}. Once started, it will usually in turn start |
27 |
< |
* other subtasks. As indicated by the name of this class, many |
28 |
< |
* programs using ForkJoinTasks employ only methods {@code fork} and |
29 |
< |
* {@code join}, or derivatives such as {@code invokeAll}. However, |
30 |
< |
* this class also provides a number of other methods that can come |
31 |
< |
* into play in advanced usages, as well as extension mechanics that |
32 |
< |
* allow support of new forms of fork/join processing. |
25 |
> |
* <p>A "main" {@code ForkJoinTask} begins execution when submitted |
26 |
> |
* to a {@link ForkJoinPool}. Once started, it will usually in turn |
27 |
> |
* start other subtasks. As indicated by the name of this class, |
28 |
> |
* many programs using {@code ForkJoinTask} employ only methods |
29 |
> |
* {@link #fork} and {@link #join}, or derivatives such as {@link |
30 |
> |
* #invokeAll}. However, this class also provides a number of other |
31 |
> |
* methods that can come into play in advanced usages, as well as |
32 |
> |
* extension mechanics that allow support of new forms of fork/join |
33 |
> |
* processing. |
34 |
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* |
35 |
< |
* <p>A ForkJoinTask is a lightweight form of {@link Future}. The |
36 |
< |
* efficiency of ForkJoinTasks stems from a set of restrictions (that |
37 |
< |
* are only partially statically enforceable) reflecting their |
38 |
< |
* intended use as computational tasks calculating pure functions or |
39 |
< |
* operating on purely isolated objects. The primary coordination |
40 |
< |
* mechanisms are {@link #fork}, that arranges asynchronous execution, |
41 |
< |
* and {@link #join}, that doesn't proceed until the task's result has |
42 |
< |
* been computed. Computations should avoid {@code synchronized} |
43 |
< |
* methods or blocks, and should minimize other blocking |
44 |
< |
* synchronization apart from joining other tasks or using |
45 |
< |
* synchronizers such as Phasers that are advertised to cooperate with |
46 |
< |
* fork/join scheduling. Tasks should also not perform blocking IO, |
47 |
< |
* and should ideally access variables that are completely independent |
48 |
< |
* of those accessed by other running tasks. Minor breaches of these |
49 |
< |
* restrictions, for example using shared output streams, may be |
50 |
< |
* tolerable in practice, but frequent use may result in poor |
51 |
< |
* performance, and the potential to indefinitely stall if the number |
52 |
< |
* of threads not waiting for IO or other external synchronization |
53 |
< |
* becomes exhausted. This usage restriction is in part enforced by |
54 |
< |
* not permitting checked exceptions such as {@code IOExceptions} |
55 |
< |
* to be thrown. However, computations may still encounter unchecked |
56 |
< |
* exceptions, that are rethrown to callers attempting join |
57 |
< |
* them. These exceptions may additionally include |
58 |
< |
* RejectedExecutionExceptions stemming from internal resource |
59 |
< |
* exhaustion such as failure to allocate internal task queues. |
35 |
> |
* <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}. |
36 |
> |
* The efficiency of {@code ForkJoinTask}s stems from a set of |
37 |
> |
* restrictions (that are only partially statically enforceable) |
38 |
> |
* reflecting their intended use as computational tasks calculating |
39 |
> |
* pure functions or operating on purely isolated objects. The |
40 |
> |
* primary coordination mechanisms are {@link #fork}, that arranges |
41 |
> |
* asynchronous execution, and {@link #join}, that doesn't proceed |
42 |
> |
* until the task's result has been computed. Computations should |
43 |
> |
* avoid {@code synchronized} methods or blocks, and should minimize |
44 |
> |
* other blocking synchronization apart from joining other tasks or |
45 |
> |
* using synchronizers such as Phasers that are advertised to |
46 |
> |
* cooperate with fork/join scheduling. Tasks should also not perform |
47 |
> |
* blocking IO, and should ideally access variables that are |
48 |
> |
* completely independent of those accessed by other running |
49 |
> |
* tasks. Minor breaches of these restrictions, for example using |
50 |
> |
* shared output streams, may be tolerable in practice, but frequent |
51 |
> |
* use may result in poor performance, and the potential to |
52 |
> |
* indefinitely stall if the number of threads not waiting for IO or |
53 |
> |
* other external synchronization becomes exhausted. This usage |
54 |
> |
* restriction is in part enforced by not permitting checked |
55 |
> |
* exceptions such as {@code IOExceptions} to be thrown. However, |
56 |
> |
* computations may still encounter unchecked exceptions, that are |
57 |
> |
* rethrown to callers attempting join them. These exceptions may |
58 |
> |
* additionally include RejectedExecutionExceptions stemming from |
59 |
> |
* internal resource exhaustion such as failure to allocate internal |
60 |
> |
* task queues. |
61 |
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* |
62 |
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* <p>The primary method for awaiting completion and extracting |
63 |
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* results of a task is {@link #join}, but there are several variants: |
76 |
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* performs the most common form of parallel invocation: forking a set |
77 |
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* of tasks and joining them all. |
78 |
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* |
79 |
< |
* <p> The ForkJoinTask class is not usually directly subclassed. |
79 |
> |
* <p>The ForkJoinTask class is not usually directly subclassed. |
80 |
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* Instead, you subclass one of the abstract classes that support a |
81 |
< |
* particular style of fork/join processing. Normally, a concrete |
81 |
> |
* particular style of fork/join processing, typically {@link |
82 |
> |
* RecursiveAction} for computations that do not return results, or |
83 |
> |
* {@link RecursiveTask} for those that do. Normally, a concrete |
84 |
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* ForkJoinTask subclass declares fields comprising its parameters, |
85 |
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* established in a constructor, and then defines a {@code compute} |
86 |
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* method that somehow uses the control methods supplied by this base |
110 |
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* parallelism cannot improve throughput. If too small, then memory |
111 |
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* and internal task maintenance overhead may overwhelm processing. |
112 |
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* |
113 |
+ |
* <p>This class provides {@code adapt} methods for {@link |
114 |
+ |
* java.lang.Runnable} and {@link java.util.concurrent.Callable}, that |
115 |
+ |
* may be of use when mixing execution of ForkJoinTasks with other |
116 |
+ |
* kinds of tasks. When all tasks are of this form, consider using a |
117 |
+ |
* pool in {@link ForkJoinPool#setAsyncMode}. |
118 |
+ |
* |
119 |
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* <p>ForkJoinTasks are {@code Serializable}, which enables them |
120 |
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* to be used in extensions such as remote execution frameworks. It is |
121 |
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* in general sensible to serialize tasks only before or after, but |
257 |
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synchronized (this) { |
258 |
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try { |
259 |
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while (status >= 0) { |
260 |
< |
long nt = nanos - System.nanoTime() - startTime; |
260 |
> |
long nt = nanos - (System.nanoTime() - startTime); |
261 |
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if (nt <= 0) |
262 |
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break; |
263 |
|
wait(nt / 1000000, (int) (nt % 1000000)); |
541 |
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} |
542 |
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|
543 |
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/** |
544 |
< |
* Forks both tasks, returning when {@code isDone} holds for |
545 |
< |
* both of them or an exception is encountered. This method may be |
544 |
> |
* Forks the given tasks, returning when {@code isDone} holds for |
545 |
> |
* each task or an exception is encountered. This method may be |
546 |
|
* invoked only from within ForkJoinTask computations (as may be |
547 |
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* determined using method {@link #inForkJoinPool}). Attempts to |
548 |
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* invoke in other contexts result in exceptions or errors, |
549 |
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* possibly including ClassCastException. |
550 |
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* |
551 |
< |
* @param t1 one task |
552 |
< |
* @param t2 the other task |
553 |
< |
* @throws NullPointerException if t1 or t2 are null |
554 |
< |
* @throws RuntimeException or Error if either task did so |
551 |
> |
* @param t1 the first task |
552 |
> |
* @param t2 the second task |
553 |
> |
* @throws NullPointerException if any task is null |
554 |
> |
* @throws RuntimeException or Error if a task did so |
555 |
|
*/ |
556 |
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public static void invokeAll(ForkJoinTask<?>t1, ForkJoinTask<?> t2) { |
557 |
|
t2.fork(); |
560 |
|
} |
561 |
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|
562 |
|
/** |
563 |
< |
* Forks the given tasks, returning when {@code isDone} holds |
564 |
< |
* for all of them. If any task encounters an exception, others |
565 |
< |
* may be cancelled. This method may be invoked only from within |
563 |
> |
* Forks the given tasks, returning when {@code isDone} holds for |
564 |
> |
* each task or an exception is encountered. If any task |
565 |
> |
* encounters an exception, others may be, but are not guaranteed |
566 |
> |
* to be, cancelled. This method may be invoked only from within |
567 |
|
* ForkJoinTask computations (as may be determined using method |
568 |
|
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
569 |
|
* result in exceptions or errors, possibly including |
570 |
|
* ClassCastException. |
571 |
|
* |
572 |
< |
* @param tasks the array of tasks |
572 |
> |
* Overloadings of this method exist for the special cases |
573 |
> |
* of one to four arguments. |
574 |
> |
* |
575 |
> |
* @param tasks the tasks |
576 |
|
* @throws NullPointerException if tasks or any element are null |
577 |
|
* @throws RuntimeException or Error if any task did so |
578 |
|
*/ |
610 |
|
} |
611 |
|
|
612 |
|
/** |
613 |
< |
* Forks all tasks in the collection, returning when |
614 |
< |
* {@code isDone} holds for all of them. If any task |
615 |
< |
* encounters an exception, others may be cancelled. This method |
616 |
< |
* may be invoked only from within ForkJoinTask computations (as |
617 |
< |
* may be determined using method {@link |
618 |
< |
* #inForkJoinPool}). Attempts to invoke in other contexts result |
619 |
< |
* in exceptions or errors, possibly including ClassCastException. |
613 |
> |
* Forks all tasks in the collection, returning when {@code |
614 |
> |
* isDone} holds for each task or an exception is encountered. If |
615 |
> |
* any task encounters an exception, others may be, but are not |
616 |
> |
* guaranteed to be, cancelled. This method may be invoked only |
617 |
> |
* from within ForkJoinTask computations (as may be determined |
618 |
> |
* using method {@link #inForkJoinPool}). Attempts to invoke in |
619 |
> |
* other contexts result in exceptions or errors, possibly |
620 |
> |
* including ClassCastException. |
621 |
|
* |
622 |
|
* @param tasks the collection of tasks |
623 |
|
* @return the tasks argument, to simplify usage |
700 |
|
* still ensure that these minimal properties hold. In particular, |
701 |
|
* the cancel method itself must not throw exceptions. |
702 |
|
* |
703 |
< |
* <p> This method is designed to be invoked by <em>other</em> |
703 |
> |
* <p>This method is designed to be invoked by <em>other</em> |
704 |
|
* tasks. To terminate the current task, you can just return or |
705 |
|
* throw an unchecked exception from its computation method, or |
706 |
|
* invoke {@link #completeExceptionally}. |
747 |
|
* {@code join} and related operations. This method may be used |
748 |
|
* to induce exceptions in asynchronous tasks, or to force |
749 |
|
* completion of tasks that would not otherwise complete. Its use |
750 |
< |
* in other situations is likely to be wrong. This method is |
750 |
> |
* in other situations is discouraged. This method is |
751 |
|
* overridable, but overridden versions must invoke {@code super} |
752 |
|
* implementation to maintain guarantees. |
753 |
|
* |
767 |
|
* operations. This method may be used to provide results for |
768 |
|
* asynchronous tasks, or to provide alternative handling for |
769 |
|
* tasks that would not otherwise complete normally. Its use in |
770 |
< |
* other situations is likely to be wrong. This method is |
770 |
> |
* other situations is discouraged. This method is |
771 |
|
* overridable, but overridden versions must invoke {@code super} |
772 |
|
* implementation to maintain guarantees. |
773 |
|
* |
792 |
|
|
793 |
|
public final V get(long timeout, TimeUnit unit) |
794 |
|
throws InterruptedException, ExecutionException, TimeoutException { |
795 |
+ |
long nanos = unit.toNanos(timeout); |
796 |
|
ForkJoinWorkerThread w = getWorker(); |
797 |
|
if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke()) |
798 |
< |
awaitDone(w, unit.toNanos(timeout)); |
798 |
> |
awaitDone(w, nanos); |
799 |
|
return reportTimedFutureResult(); |
800 |
|
} |
801 |
|
|
867 |
|
* Possibly executes tasks until the pool hosting the current task |
868 |
|
* {@link ForkJoinPool#isQuiescent}. This method may be of use in |
869 |
|
* designs in which many tasks are forked, but none are explicitly |
870 |
< |
* joined, instead executing them until all are processed. |
870 |
> |
* joined, instead executing them until all are processed. This |
871 |
> |
* method may be invoked only from within ForkJoinTask |
872 |
> |
* computations (as may be determined using method {@link |
873 |
> |
* #inForkJoinPool}). Attempts to invoke in other contexts result |
874 |
> |
* in exceptions or errors, possibly including ClassCastException. |
875 |
|
*/ |
876 |
|
public static void helpQuiesce() { |
877 |
|
((ForkJoinWorkerThread) Thread.currentThread()) |
885 |
|
* never been forked, or has been forked, then completed and all |
886 |
|
* outstanding joins of this task have also completed. Effects |
887 |
|
* under any other usage conditions are not guaranteed, and are |
888 |
< |
* almost surely wrong. This method may be useful when executing |
888 |
> |
* discouraged. This method may be useful when executing |
889 |
|
* pre-constructed trees of subtasks in loops. |
890 |
|
*/ |
891 |
|
public void reinitialize() { |
898 |
|
* Returns the pool hosting the current task execution, or null |
899 |
|
* if this task is executing outside of any ForkJoinPool. |
900 |
|
* |
901 |
+ |
* @see #inForkJoinPool |
902 |
|
* @return the pool, or {@code null} if none |
903 |
|
*/ |
904 |
|
public static ForkJoinPool getPool() { |
941 |
|
* Returns an estimate of the number of tasks that have been |
942 |
|
* forked by the current worker thread but not yet executed. This |
943 |
|
* value may be useful for heuristic decisions about whether to |
944 |
< |
* fork other tasks. |
945 |
< |
* |
944 |
> |
* fork other tasks. This method may be invoked only from within |
945 |
> |
* ForkJoinTask computations (as may be determined using method |
946 |
> |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
947 |
> |
* result in exceptions or errors, possibly including |
948 |
> |
* ClassCastException. |
949 |
|
* @return the number of tasks |
950 |
|
*/ |
951 |
|
public static int getQueuedTaskCount() { |
961 |
|
* usages of ForkJoinTasks, at steady state, each worker should |
962 |
|
* aim to maintain a small constant surplus (for example, 3) of |
963 |
|
* tasks, and to process computations locally if this threshold is |
964 |
< |
* exceeded. |
965 |
< |
* |
964 |
> |
* exceeded. This method may be invoked only from within |
965 |
> |
* ForkJoinTask computations (as may be determined using method |
966 |
> |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
967 |
> |
* result in exceptions or errors, possibly including |
968 |
> |
* ClassCastException. * |
969 |
|
* @return the surplus number of tasks, which may be negative |
970 |
|
*/ |
971 |
|
public static int getSurplusQueuedTaskCount() { |
1010 |
|
protected abstract boolean exec(); |
1011 |
|
|
1012 |
|
/** |
1013 |
< |
* Returns, but does not unschedule or execute, the task queued by |
1014 |
< |
* the current thread but not yet executed, if one is |
1013 |
> |
* Returns, but does not unschedule or execute, a task queued by |
1014 |
> |
* the current thread but not yet executed, if one is immediately |
1015 |
|
* available. There is no guarantee that this task will actually |
1016 |
< |
* be polled or executed next. This method is designed primarily |
1017 |
< |
* to support extensions, and is unlikely to be useful otherwise. |
1018 |
< |
* This method may be invoked only from within ForkJoinTask |
1019 |
< |
* computations (as may be determined using method {@link |
1020 |
< |
* #inForkJoinPool}). Attempts to invoke in other contexts result |
1021 |
< |
* in exceptions or errors, possibly including ClassCastException. |
1016 |
> |
* be polled or executed next. Conversely, this method may return |
1017 |
> |
* null even if a task exists but cannot be accessed without |
1018 |
> |
* contention with other threads. This method is designed |
1019 |
> |
* primarily to support extensions, and is unlikely to be useful |
1020 |
> |
* otherwise. This method may be invoked only from within |
1021 |
> |
* ForkJoinTask computations (as may be determined using method |
1022 |
> |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
1023 |
> |
* result in exceptions or errors, possibly including |
1024 |
> |
* ClassCastException. |
1025 |
|
* |
1026 |
|
* @return the next task, or {@code null} if none are available |
1027 |
|
*/ |
1068 |
|
.pollTask(); |
1069 |
|
} |
1070 |
|
|
1071 |
< |
// adaptors |
1071 |
> |
/** |
1072 |
> |
* Adaptor for Runnables. This implements RunnableFuture |
1073 |
> |
* to be compliant with AbstractExecutorService constraints |
1074 |
> |
* when used in ForkJoinPool. |
1075 |
> |
*/ |
1076 |
> |
static final class AdaptedRunnable<T> extends ForkJoinTask<T> |
1077 |
> |
implements RunnableFuture<T> { |
1078 |
> |
final Runnable runnable; |
1079 |
> |
final T resultOnCompletion; |
1080 |
> |
T result; |
1081 |
> |
AdaptedRunnable(Runnable runnable, T result) { |
1082 |
> |
if (runnable == null) throw new NullPointerException(); |
1083 |
> |
this.runnable = runnable; |
1084 |
> |
this.resultOnCompletion = result; |
1085 |
> |
} |
1086 |
> |
public T getRawResult() { return result; } |
1087 |
> |
public void setRawResult(T v) { result = v; } |
1088 |
> |
public boolean exec() { |
1089 |
> |
runnable.run(); |
1090 |
> |
result = resultOnCompletion; |
1091 |
> |
return true; |
1092 |
> |
} |
1093 |
> |
public void run() { invoke(); } |
1094 |
> |
private static final long serialVersionUID = 5232453952276885070L; |
1095 |
> |
} |
1096 |
> |
|
1097 |
> |
/** |
1098 |
> |
* Adaptor for Callables |
1099 |
> |
*/ |
1100 |
> |
static final class AdaptedCallable<T> extends ForkJoinTask<T> |
1101 |
> |
implements RunnableFuture<T> { |
1102 |
> |
final Callable<? extends T> callable; |
1103 |
> |
T result; |
1104 |
> |
AdaptedCallable(Callable<? extends T> callable) { |
1105 |
> |
if (callable == null) throw new NullPointerException(); |
1106 |
> |
this.callable = callable; |
1107 |
> |
} |
1108 |
> |
public T getRawResult() { return result; } |
1109 |
> |
public void setRawResult(T v) { result = v; } |
1110 |
> |
public boolean exec() { |
1111 |
> |
try { |
1112 |
> |
result = callable.call(); |
1113 |
> |
return true; |
1114 |
> |
} catch (Error err) { |
1115 |
> |
throw err; |
1116 |
> |
} catch (RuntimeException rex) { |
1117 |
> |
throw rex; |
1118 |
> |
} catch (Exception ex) { |
1119 |
> |
throw new RuntimeException(ex); |
1120 |
> |
} |
1121 |
> |
} |
1122 |
> |
public void run() { invoke(); } |
1123 |
> |
private static final long serialVersionUID = 2838392045355241008L; |
1124 |
> |
} |
1125 |
|
|
1126 |
|
/** |
1127 |
|
* Returns a new ForkJoinTask that performs the {@code run} |
1131 |
|
* @param runnable the runnable action |
1132 |
|
* @return the task |
1133 |
|
*/ |
1134 |
< |
public static ForkJoinTask<Void> adapt(Runnable runnable) { |
1135 |
< |
return new ForkJoinPool.AdaptedRunnable<Void>(runnable, null); |
1134 |
> |
public static ForkJoinTask<?> adapt(Runnable runnable) { |
1135 |
> |
return new AdaptedRunnable<Void>(runnable, null); |
1136 |
|
} |
1137 |
|
|
1138 |
|
/** |
1145 |
|
* @return the task |
1146 |
|
*/ |
1147 |
|
public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) { |
1148 |
< |
return new ForkJoinPool.AdaptedRunnable<T>(runnable, result); |
1148 |
> |
return new AdaptedRunnable<T>(runnable, result); |
1149 |
|
} |
1150 |
|
|
1151 |
|
/** |
1157 |
|
* @param callable the callable action |
1158 |
|
* @return the task |
1159 |
|
*/ |
1160 |
< |
public static <T> ForkJoinTask<T> adapt(Callable<T> callable) { |
1161 |
< |
return new ForkJoinPool.AdaptedCallable<T>(callable); |
1160 |
> |
public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) { |
1161 |
> |
return new AdaptedCallable<T>(callable); |
1162 |
|
} |
1163 |
|
|
1164 |
|
// Serialization support |