22 |
|
* subtasks may be hosted by a small number of actual threads in a |
23 |
|
* ForkJoinPool, at the price of some usage limitations. |
24 |
|
* |
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 |
|
* |
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 |
|
* |
62 |
|
* <p>The primary method for awaiting completion and extracting |
63 |
|
* results of a task is {@link #join}, but there are several variants: |
76 |
|
* performs the most common form of parallel invocation: forking a set |
77 |
|
* of tasks and joining them all. |
78 |
|
* |
79 |
< |
* <p> The ForkJoinTask class is not usually directly subclassed. |
79 |
> |
* <p>The ForkJoinTask class is not usually directly subclassed. |
80 |
|
* Instead, you subclass one of the abstract classes that support a |
81 |
|
* particular style of fork/join processing, typically {@link |
82 |
|
* RecursiveAction} for computations that do not return results, or |
698 |
|
* |
699 |
|
* <p>This method may be overridden in subclasses, but if so, must |
700 |
|
* still ensure that these minimal properties hold. In particular, |
701 |
< |
* the cancel method itself must not throw exceptions. |
701 |
> |
* the {@code 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}. |
727 |
|
|
728 |
|
/** |
729 |
|
* Returns the exception thrown by the base computation, or a |
730 |
< |
* CancellationException if cancelled, or null if none or if the |
731 |
< |
* method has not yet completed. |
730 |
> |
* {@code CancellationException} if cancelled, or {@code null} if |
731 |
> |
* none or if the method has not yet completed. |
732 |
|
* |
733 |
|
* @return the exception, or {@code null} if none |
734 |
|
*/ |
884 |
|
* this task, but only if reuse occurs when this task has either |
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 |
< |
* discouraged. This method may be useful when executing |
887 |
> |
* under any other usage conditions are not guaranteed. |
888 |
> |
* This method may be useful when executing |
889 |
|
* pre-constructed trees of subtasks in loops. |
890 |
|
*/ |
891 |
|
public void reinitialize() { |
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. * |
968 |
> |
* ClassCastException. |
969 |
|
* @return the surplus number of tasks, which may be negative |
970 |
|
*/ |
971 |
|
public static int getSurplusQueuedTaskCount() { |