18 |
|
* lighter weight than a normal thread. Huge numbers of tasks and |
19 |
|
* subtasks may be hosted by a small number of actual threads in a |
20 |
|
* ForkJoinPool, at the price of some usage limitations. |
21 |
< |
* |
21 |
> |
* |
22 |
|
* <p> A "main" ForkJoinTask begins execution when submitted to a |
23 |
|
* {@link ForkJoinPool}. Once started, it will usually in turn start |
24 |
|
* other subtasks. As indicated by the name of this class, many |
25 |
< |
* programs using ForkJoinTasks employ only methods <code>fork</code> |
26 |
< |
* and <code>join</code>, or derivatives such as |
27 |
< |
* <code>invokeAll</code>. However, this class also provides a number |
25 |
> |
* programs using ForkJoinTasks employ only methods {@code fork} |
26 |
> |
* and {@code join}, or derivatives such as |
27 |
> |
* {@code invokeAll}. However, this class also provides a number |
28 |
|
* of other methods that can come into play in advanced usages, as |
29 |
|
* well as extension mechanics that allow support of new forms of |
30 |
|
* fork/join processing. |
31 |
< |
* |
31 |
> |
* |
32 |
|
* <p>A ForkJoinTask is a lightweight form of {@link Future}. The |
33 |
|
* efficiency of ForkJoinTasks stems from a set of restrictions (that |
34 |
|
* are only partially statically enforceable) reflecting their |
36 |
|
* operating on purely isolated objects. The primary coordination |
37 |
|
* mechanisms are {@link #fork}, that arranges asynchronous execution, |
38 |
|
* and {@link #join}, that doesn't proceed until the task's result has |
39 |
< |
* been computed. Computations should avoid <code>synchronized</code> |
39 |
> |
* been computed. Computations should avoid {@code synchronized} |
40 |
|
* methods or blocks, and should minimize other blocking |
41 |
|
* synchronization apart from joining other tasks or using |
42 |
|
* synchronizers such as Phasers that are advertised to cooperate with |
48 |
|
* performance, and the potential to indefinitely stall if the number |
49 |
|
* of threads not waiting for IO or other external synchronization |
50 |
|
* becomes exhausted. This usage restriction is in part enforced by |
51 |
< |
* not permitting checked exceptions such as <code>IOExceptions</code> |
51 |
> |
* not permitting checked exceptions such as {@code IOExceptions} |
52 |
|
* to be thrown. However, computations may still encounter unchecked |
53 |
|
* exceptions, that are rethrown to callers attempting join |
54 |
|
* them. These exceptions may additionally include |
58 |
|
* <p>The primary method for awaiting completion and extracting |
59 |
|
* results of a task is {@link #join}, but there are several variants: |
60 |
|
* The {@link Future#get} methods support interruptible and/or timed |
61 |
< |
* waits for completion and report results using <code>Future</code> |
61 |
> |
* waits for completion and report results using {@code Future} |
62 |
|
* conventions. Method {@link #helpJoin} enables callers to actively |
63 |
|
* execute other tasks while awaiting joins, which is sometimes more |
64 |
|
* efficient but only applies when all subtasks are known to be |
65 |
|
* strictly tree-structured. Method {@link #invoke} is semantically |
66 |
< |
* equivalent to <code>fork(); join()</code> but always attempts to |
66 |
> |
* equivalent to {@code fork(); join()} but always attempts to |
67 |
|
* begin execution in the current thread. The "<em>quiet</em>" forms |
68 |
|
* of these methods do not extract results or report exceptions. These |
69 |
|
* may be useful when a set of tasks are being executed, and you need |
70 |
|
* to delay processing of results or exceptions until all complete. |
71 |
< |
* Method <code>invokeAll</code> (available in multiple versions) |
71 |
> |
* Method {@code invokeAll} (available in multiple versions) |
72 |
|
* performs the most common form of parallel invocation: forking a set |
73 |
|
* of tasks and joining them all. |
74 |
|
* |
76 |
|
* Instead, you subclass one of the abstract classes that support a |
77 |
|
* particular style of fork/join processing. Normally, a concrete |
78 |
|
* ForkJoinTask subclass declares fields comprising its parameters, |
79 |
< |
* established in a constructor, and then defines a <code>compute</code> |
79 |
> |
* established in a constructor, and then defines a {@code compute} |
80 |
|
* method that somehow uses the control methods supplied by this base |
81 |
< |
* class. While these methods have <code>public</code> access (to allow |
81 |
> |
* class. While these methods have {@code public} access (to allow |
82 |
|
* instances of different task subclasses to call each others |
83 |
|
* methods), some of them may only be called from within other |
84 |
|
* ForkJoinTasks. Attempts to invoke them in other contexts result in |
85 |
< |
* exceptions or errors including ClassCastException. |
85 |
> |
* exceptions or errors possibly including ClassCastException. |
86 |
|
* |
87 |
< |
* <p>Most base support methods are <code>final</code> because their |
87 |
> |
* <p>Most base support methods are {@code final} because their |
88 |
|
* implementations are intrinsically tied to the underlying |
89 |
|
* lightweight task scheduling framework, and so cannot be overridden. |
90 |
|
* Developers creating new basic styles of fork/join processing should |
91 |
< |
* minimally implement <code>protected</code> methods |
92 |
< |
* <code>exec</code>, <code>setRawResult</code>, and |
93 |
< |
* <code>getRawResult</code>, while also introducing an abstract |
91 |
> |
* minimally implement {@code protected} methods |
92 |
> |
* {@code exec}, {@code setRawResult}, and |
93 |
> |
* {@code getRawResult}, while also introducing an abstract |
94 |
|
* computational method that can be implemented in its subclasses, |
95 |
< |
* possibly relying on other <code>protected</code> methods provided |
95 |
> |
* possibly relying on other {@code protected} methods provided |
96 |
|
* by this class. |
97 |
|
* |
98 |
|
* <p>ForkJoinTasks should perform relatively small amounts of |
102 |
|
* parellelism cannot improve throughput. If too small, then memory |
103 |
|
* and internal task maintenance overhead may overwhelm processing. |
104 |
|
* |
105 |
< |
* <p>ForkJoinTasks are <code>Serializable</code>, which enables them |
105 |
> |
* <p>ForkJoinTasks are {@code Serializable}, which enables them |
106 |
|
* to be used in extensions such as remote execution frameworks. It is |
107 |
|
* in general sensible to serialize tasks only before or after, but |
108 |
|
* not during execution. Serialization is not relied on during |
475 |
|
* than once unless it has completed and been reinitialized. This |
476 |
|
* method may be invoked only from within ForkJoinTask |
477 |
|
* computations. Attempts to invoke in other contexts result in |
478 |
< |
* exceptions or errors including ClassCastException. |
478 |
> |
* exceptions or errors possibly including ClassCastException. |
479 |
|
*/ |
480 |
|
public final void fork() { |
481 |
|
((ForkJoinWorkerThread)(Thread.currentThread())).pushTask(this); |
483 |
|
|
484 |
|
/** |
485 |
|
* Returns the result of the computation when it is ready. |
486 |
< |
* This method differs from <code>get</code> in that abnormal |
486 |
> |
* This method differs from {@code get} in that abnormal |
487 |
|
* completion results in RuntimeExceptions or Errors, not |
488 |
|
* ExecutionExceptions. |
489 |
|
* |
511 |
|
} |
512 |
|
|
513 |
|
/** |
514 |
< |
* Forks both tasks, returning when <code>isDone</code> holds for |
514 |
> |
* Forks both tasks, returning when {@code isDone} holds for |
515 |
|
* both of them or an exception is encountered. This method may be |
516 |
|
* invoked only from within ForkJoinTask computations. Attempts to |
517 |
|
* invoke in other contexts result in exceptions or errors |
518 |
< |
* including ClassCastException. |
518 |
> |
* possibly including ClassCastException. |
519 |
|
* @param t1 one task |
520 |
|
* @param t2 the other task |
521 |
|
* @throws NullPointerException if t1 or t2 are null |
528 |
|
} |
529 |
|
|
530 |
|
/** |
531 |
< |
* Forks the given tasks, returning when <code>isDone</code> holds |
531 |
> |
* Forks the given tasks, returning when {@code isDone} holds |
532 |
|
* for all of them. If any task encounters an exception, others |
533 |
|
* may be cancelled. This method may be invoked only from within |
534 |
|
* ForkJoinTask computations. Attempts to invoke in other contexts |
535 |
< |
* result in exceptions or errors including ClassCastException. |
535 |
> |
* result in exceptions or errors possibly including ClassCastException. |
536 |
|
* @param tasks the array of tasks |
537 |
|
* @throws NullPointerException if tasks or any element are null. |
538 |
|
* @throws RuntimeException or Error if any task did so. |
572 |
|
|
573 |
|
/** |
574 |
|
* Forks all tasks in the collection, returning when |
575 |
< |
* <code>isDone</code> holds for all of them. If any task |
575 |
> |
* {@code isDone} holds for all of them. If any task |
576 |
|
* encounters an exception, others may be cancelled. This method |
577 |
|
* may be invoked only from within ForkJoinTask |
578 |
|
* computations. Attempts to invoke in other contexts resul!t in |
579 |
< |
* exceptions or errors including ClassCastException. |
579 |
> |
* exceptions or errors possibly including ClassCastException. |
580 |
|
* @param tasks the collection of tasks |
581 |
|
* @throws NullPointerException if tasks or any element are null. |
582 |
|
* @throws RuntimeException or Error if any task did so. |
640 |
|
/** |
641 |
|
* Asserts that the results of this task's computation will not be |
642 |
|
* used. If a cancellation occurs before atempting to execute this |
643 |
< |
* task, then execution will be suppressed, <code>isCancelled</code> |
644 |
< |
* will report true, and <code>join</code> will result in a |
645 |
< |
* <code>CancellationException</code> being thrown. Otherwise, when |
643 |
> |
* task, then execution will be suppressed, {@code isCancelled} |
644 |
> |
* will report true, and {@code join} will result in a |
645 |
> |
* {@code CancellationException} being thrown. Otherwise, when |
646 |
|
* cancellation races with completion, there are no guarantees |
647 |
< |
* about whether <code>isCancelled</code> will report true, whether |
648 |
< |
* <code>join</code> will return normally or via an exception, or |
647 |
> |
* about whether {@code isCancelled} will report true, whether |
648 |
> |
* {@code join} will return normally or via an exception, or |
649 |
|
* whether these behaviors will remain consistent upon repeated |
650 |
|
* invocation. |
651 |
|
* |
656 |
|
* <p> This method is designed to be invoked by <em>other</em> |
657 |
|
* tasks. To terminate the current task, you can just return or |
658 |
|
* throw an unchecked exception from its computation method, or |
659 |
< |
* invoke <code>completeExceptionally</code>. |
659 |
> |
* invoke {@code completeExceptionally}. |
660 |
|
* |
661 |
|
* @param mayInterruptIfRunning this value is ignored in the |
662 |
|
* default implementation because tasks are not in general |
695 |
|
/** |
696 |
|
* Completes this task abnormally, and if not already aborted or |
697 |
|
* cancelled, causes it to throw the given exception upon |
698 |
< |
* <code>join</code> and related operations. This method may be used |
698 |
> |
* {@code join} and related operations. This method may be used |
699 |
|
* to induce exceptions in asynchronous tasks, or to force |
700 |
|
* completion of tasks that would not otherwise complete. Its use |
701 |
|
* in other situations is likely to be wrong. This method is |
702 |
< |
* overridable, but overridden versions must invoke <code>super</code> |
702 |
> |
* overridable, but overridden versions must invoke {@code super} |
703 |
|
* implementation to maintain guarantees. |
704 |
|
* |
705 |
|
* @param ex the exception to throw. If this exception is |
714 |
|
|
715 |
|
/** |
716 |
|
* Completes this task, and if not already aborted or cancelled, |
717 |
< |
* returning a <code>null</code> result upon <code>join</code> and related |
717 |
> |
* returning a {@code null} result upon {@code join} and related |
718 |
|
* operations. This method may be used to provide results for |
719 |
|
* asynchronous tasks, or to provide alternative handling for |
720 |
|
* tasks that would not otherwise complete normally. Its use in |
721 |
|
* other situations is likely to be wrong. This method is |
722 |
< |
* overridable, but overridden versions must invoke <code>super</code> |
722 |
> |
* overridable, but overridden versions must invoke {@code super} |
723 |
|
* implementation to maintain guarantees. |
724 |
|
* |
725 |
|
* @param value the result value for this task. |
752 |
|
/** |
753 |
|
* Possibly executes other tasks until this task is ready, then |
754 |
|
* returns the result of the computation. This method may be more |
755 |
< |
* efficient than <code>join</code>, but is only applicable when |
755 |
> |
* efficient than {@code join}, but is only applicable when |
756 |
|
* there are no potemtial dependencies between continuation of the |
757 |
|
* current task and that of any other task that might be executed |
758 |
|
* while helping. (This usually holds for pure divide-and-conquer |
759 |
|
* tasks). This method may be invoked only from within |
760 |
|
* ForkJoinTask computations. Attempts to invoke in other contexts |
761 |
< |
* resul!t in exceptions or errors including ClassCastException. |
761 |
> |
* resul!t in exceptions or errors possibly including ClassCastException. |
762 |
|
* @return the computed result |
763 |
|
*/ |
764 |
|
public final V helpJoin() { |
772 |
|
* Possibly executes other tasks until this task is ready. This |
773 |
|
* method may be invoked only from within ForkJoinTask |
774 |
|
* computations. Attempts to invoke in other contexts resul!t in |
775 |
< |
* exceptions or errors including ClassCastException. |
775 |
> |
* exceptions or errors possibly including ClassCastException. |
776 |
|
*/ |
777 |
|
public final void quietlyHelpJoin() { |
778 |
|
if (status >= 0) { |
822 |
|
|
823 |
|
/** |
824 |
|
* Resets the internal bookkeeping state of this task, allowing a |
825 |
< |
* subsequent <code>fork</code>. This method allows repeated reuse of |
825 |
> |
* subsequent {@code fork}. This method allows repeated reuse of |
826 |
|
* this task, but only if reuse occurs when this task has either |
827 |
|
* never been forked, or has been forked, then completed and all |
828 |
|
* outstanding joins of this task have also completed. Effects |
855 |
|
* alternative local processing of tasks that could have been, but |
856 |
|
* were not, stolen. This method may be invoked only from within |
857 |
|
* ForkJoinTask computations. Attempts to invoke in other contexts |
858 |
< |
* result in exceptions or errors including ClassCastException. |
858 |
> |
* result in exceptions or errors possibly including ClassCastException. |
859 |
|
* @return true if unforked |
860 |
|
*/ |
861 |
|
public boolean tryUnfork() { |
893 |
|
// Extension methods |
894 |
|
|
895 |
|
/** |
896 |
< |
* Returns the result that would be returned by <code>join</code>, |
896 |
> |
* Returns the result that would be returned by {@code join}, |
897 |
|
* even if this task completed abnormally, or null if this task is |
898 |
|
* not known to have been completed. This method is designed to |
899 |
|
* aid debugging, as well as to support extensions. Its use in any |
918 |
|
* called otherwise. The return value controls whether this task |
919 |
|
* is considered to be done normally. It may return false in |
920 |
|
* asynchronous actions that require explicit invocations of |
921 |
< |
* <code>complete</code> to become joinable. It may throw exceptions |
921 |
> |
* {@code complete} to become joinable. It may throw exceptions |
922 |
|
* to indicate abnormal exit. |
923 |
|
* @return true if completed normally |
924 |
|
* @throws Error or RuntimeException if encountered during computation |
926 |
|
protected abstract boolean exec(); |
927 |
|
|
928 |
|
/** |
929 |
< |
* Returns, but does not unschedule or execute, the task most |
930 |
< |
* recently forked by the current thread but not yet executed, if |
931 |
< |
* one is available. There is no guarantee that this task will |
932 |
< |
* actually be polled or executed next. |
933 |
< |
* This method is designed primarily to support extensions, |
934 |
< |
* and is unlikely to be useful otherwise. |
929 |
> |
* Returns, but does not unschedule or execute, the task queued by |
930 |
> |
* the current thread but not yet executed, if one is |
931 |
> |
* available. There is no guarantee that this task will actually |
932 |
> |
* be polled or executed next. This method is designed primarily |
933 |
> |
* to support extensions, and is unlikely to be useful otherwise. |
934 |
> |
* This method may be invoked only from within ForkJoinTask |
935 |
> |
* computations. Attempts to invoke in other contexts result in |
936 |
> |
* exceptions or errors possibly including ClassCastException. |
937 |
|
* |
938 |
|
* @return the next task, or null if none are available |
939 |
|
*/ |
942 |
|
} |
943 |
|
|
944 |
|
/** |
945 |
< |
* Unschedules and returns, without executing, the task most |
946 |
< |
* recently forked by the current thread but not yet executed. |
947 |
< |
* This method is designed primarily to support extensions, |
948 |
< |
* and is unlikely to be useful otherwise. |
945 |
> |
* Unschedules and returns, without executing, the next task |
946 |
> |
* queued by the current thread but not yet executed. This method |
947 |
> |
* is designed primarily to support extensions, and is unlikely to |
948 |
> |
* be useful otherwise. This method may be invoked only from |
949 |
> |
* within ForkJoinTask computations. Attempts to invoke in other |
950 |
> |
* contexts result in exceptions or errors possibly including |
951 |
> |
* ClassCastException. |
952 |
|
* |
953 |
|
* @return the next task, or null if none are available |
954 |
|
*/ |
955 |
|
protected static ForkJoinTask<?> pollNextLocalTask() { |
956 |
< |
return ((ForkJoinWorkerThread)(Thread.currentThread())).popTask(); |
956 |
> |
return ((ForkJoinWorkerThread)(Thread.currentThread())).pollLocalTask(); |
957 |
|
} |
958 |
|
|
959 |
|
/** |
960 |
< |
* Unschedules and returns, without executing, the task most |
961 |
< |
* recently forked by the current thread but not yet executed, if |
962 |
< |
* one is available, or if not available, a task that was forked |
963 |
< |
* by some other thread, if available. Availability may be |
964 |
< |
* transient, so a <code>null</code> result does not necessarily |
965 |
< |
* imply quiecence of the pool this task is operating in. |
966 |
< |
* This method is designed primarily to support extensions, |
967 |
< |
* and is unlikely to be useful otherwise. |
968 |
< |
* |
960 |
> |
* Unschedules and returns, without executing, the next task |
961 |
> |
* queued by the current thread but not yet executed, if one is |
962 |
> |
* available, or if not available, a task that was forked by some |
963 |
> |
* other thread, if available. Availability may be transient, so a |
964 |
> |
* {@code null} result does not necessarily imply quiecence |
965 |
> |
* of the pool this task is operating in. This method is designed |
966 |
> |
* primarily to support extensions, and is unlikely to be useful |
967 |
> |
* otherwise. This method may be invoked only from within |
968 |
> |
* ForkJoinTask computations. Attempts to invoke in other contexts |
969 |
> |
* result in exceptions or errors possibly including |
970 |
> |
* ClassCastException. |
971 |
> |
* |
972 |
|
* @return a task, or null if none are available |
973 |
|
*/ |
974 |
|
protected static ForkJoinTask<?> pollTask() { |
975 |
|
return ((ForkJoinWorkerThread)(Thread.currentThread())). |
976 |
< |
pollLocalOrStolenTask(); |
976 |
> |
pollTask(); |
977 |
|
} |
978 |
|
|
979 |
|
// Serialization support |
1008 |
|
} |
1009 |
|
|
1010 |
|
// Temporary Unsafe mechanics for preliminary release |
1011 |
+ |
private static Unsafe getUnsafe() throws Throwable { |
1012 |
+ |
try { |
1013 |
+ |
return Unsafe.getUnsafe(); |
1014 |
+ |
} catch (SecurityException se) { |
1015 |
+ |
try { |
1016 |
+ |
return java.security.AccessController.doPrivileged |
1017 |
+ |
(new java.security.PrivilegedExceptionAction<Unsafe>() { |
1018 |
+ |
public Unsafe run() throws Exception { |
1019 |
+ |
return getUnsafePrivileged(); |
1020 |
+ |
}}); |
1021 |
+ |
} catch (java.security.PrivilegedActionException e) { |
1022 |
+ |
throw e.getCause(); |
1023 |
+ |
} |
1024 |
+ |
} |
1025 |
+ |
} |
1026 |
+ |
|
1027 |
+ |
private static Unsafe getUnsafePrivileged() |
1028 |
+ |
throws NoSuchFieldException, IllegalAccessException { |
1029 |
+ |
Field f = Unsafe.class.getDeclaredField("theUnsafe"); |
1030 |
+ |
f.setAccessible(true); |
1031 |
+ |
return (Unsafe) f.get(null); |
1032 |
+ |
} |
1033 |
+ |
|
1034 |
+ |
private static long fieldOffset(String fieldName) |
1035 |
+ |
throws NoSuchFieldException { |
1036 |
+ |
return _unsafe.objectFieldOffset |
1037 |
+ |
(ForkJoinTask.class.getDeclaredField(fieldName)); |
1038 |
+ |
} |
1039 |
|
|
1040 |
|
static final Unsafe _unsafe; |
1041 |
|
static final long statusOffset; |
1042 |
|
|
1043 |
|
static { |
1044 |
|
try { |
1045 |
< |
if (ForkJoinTask.class.getClassLoader() != null) { |
1046 |
< |
Field f = Unsafe.class.getDeclaredField("theUnsafe"); |
1047 |
< |
f.setAccessible(true); |
1048 |
< |
_unsafe = (Unsafe)f.get(null); |
1049 |
< |
} |
1014 |
< |
else |
1015 |
< |
_unsafe = Unsafe.getUnsafe(); |
1016 |
< |
statusOffset = _unsafe.objectFieldOffset |
1017 |
< |
(ForkJoinTask.class.getDeclaredField("status")); |
1018 |
< |
} catch (Exception ex) { throw new Error(ex); } |
1045 |
> |
_unsafe = getUnsafe(); |
1046 |
> |
statusOffset = fieldOffset("status"); |
1047 |
> |
} catch (Throwable e) { |
1048 |
> |
throw new RuntimeException("Could not initialize intrinsics", e); |
1049 |
> |
} |
1050 |
|
} |
1051 |
|
|
1052 |
|
} |