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import java.util.WeakHashMap; |
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/** |
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* Abstract base class for tasks that run within a {@link |
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* ForkJoinPool}. A ForkJoinTask is a thread-like entity that is much |
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* Abstract base class for tasks that run within a {@link ForkJoinPool}. |
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* A {@code ForkJoinTask} is a thread-like entity that is much |
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* lighter weight than a normal thread. Huge numbers of tasks and |
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* subtasks may be hosted by a small number of actual threads in a |
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* ForkJoinPool, at the price of some usage limitations. |
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* |
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* <p> A "main" ForkJoinTask begins execution when submitted to a |
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* {@link ForkJoinPool}. Once started, it will usually in turn start |
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* other subtasks. As indicated by the name of this class, many |
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* programs using ForkJoinTasks employ only methods {@code fork} and |
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* {@code join}, or derivatives such as {@code invokeAll}. However, |
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* this class also provides a number of other methods that can come |
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* into play in advanced usages, as well as extension mechanics that |
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* allow support of new forms of fork/join processing. |
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* <p>A "main" {@code ForkJoinTask} begins execution when submitted |
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* to a {@link ForkJoinPool}. Once started, it will usually in turn |
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* start other subtasks. As indicated by the name of this class, |
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* many programs using {@code ForkJoinTask} employ only methods |
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* {@link #fork} and {@link #join}, or derivatives such as {@link |
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> |
* #invokeAll}. However, this class also provides a number of other |
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> |
* methods that can come into play in advanced usages, as well as |
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* extension mechanics that allow support of new forms of fork/join |
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* processing. |
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* |
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* <p>A ForkJoinTask is a lightweight form of {@link Future}. The |
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< |
* efficiency of ForkJoinTasks stems from a set of restrictions (that |
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* are only partially statically enforceable) reflecting their |
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* intended use as computational tasks calculating pure functions or |
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* operating on purely isolated objects. The primary coordination |
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* mechanisms are {@link #fork}, that arranges asynchronous execution, |
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* and {@link #join}, that doesn't proceed until the task's result has |
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* been computed. Computations should avoid {@code synchronized} |
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* methods or blocks, and should minimize other blocking |
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* synchronization apart from joining other tasks or using |
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* synchronizers such as Phasers that are advertised to cooperate with |
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* fork/join scheduling. Tasks should also not perform blocking IO, |
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* and should ideally access variables that are completely independent |
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* of those accessed by other running tasks. Minor breaches of these |
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* restrictions, for example using shared output streams, may be |
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* tolerable in practice, but frequent use may result in poor |
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* performance, and the potential to indefinitely stall if the number |
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< |
* of threads not waiting for IO or other external synchronization |
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* becomes exhausted. This usage restriction is in part enforced by |
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< |
* not permitting checked exceptions such as {@code IOExceptions} |
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* to be thrown. However, computations may still encounter unchecked |
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* exceptions, that are rethrown to callers attempting join |
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* them. These exceptions may additionally include |
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< |
* RejectedExecutionExceptions stemming from internal resource |
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< |
* exhaustion such as failure to allocate internal task queues. |
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> |
* <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}. |
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> |
* The efficiency of {@code ForkJoinTask}s stems from a set of |
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> |
* restrictions (that are only partially statically enforceable) |
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> |
* reflecting their intended use as computational tasks calculating |
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> |
* pure functions or operating on purely isolated objects. The |
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> |
* primary coordination mechanisms are {@link #fork}, that arranges |
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> |
* asynchronous execution, and {@link #join}, that doesn't proceed |
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> |
* until the task's result has been computed. Computations should |
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> |
* avoid {@code synchronized} methods or blocks, and should minimize |
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> |
* other blocking synchronization apart from joining other tasks or |
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> |
* using synchronizers such as Phasers that are advertised to |
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> |
* cooperate with fork/join scheduling. Tasks should also not perform |
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> |
* blocking IO, and should ideally access variables that are |
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> |
* completely independent of those accessed by other running |
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> |
* tasks. Minor breaches of these restrictions, for example using |
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> |
* shared output streams, may be tolerable in practice, but frequent |
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> |
* use may result in poor performance, and the potential to |
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> |
* indefinitely stall if the number of threads not waiting for IO or |
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> |
* other external synchronization becomes exhausted. This usage |
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> |
* restriction is in part enforced by not permitting checked |
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> |
* exceptions such as {@code IOExceptions} to be thrown. However, |
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> |
* computations may still encounter unchecked exceptions, that are |
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> |
* rethrown to callers attempting join them. These exceptions may |
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> |
* additionally include RejectedExecutionExceptions stemming from |
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> |
* internal resource exhaustion such as failure to allocate internal |
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> |
* task queues. |
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* |
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* <p>The primary method for awaiting completion and extracting |
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* results of a task is {@link #join}, but there are several variants: |
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* performs the most common form of parallel invocation: forking a set |
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* of tasks and joining them all. |
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* |
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< |
* <p> The ForkJoinTask class is not usually directly subclassed. |
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> |
* <p>The ForkJoinTask class is not usually directly subclassed. |
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* Instead, you subclass one of the abstract classes that support a |
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< |
* particular style of fork/join processing. Normally, a concrete |
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> |
* particular style of fork/join processing, typically {@link |
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> |
* RecursiveAction} for computations that do not return results, or |
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> |
* {@link RecursiveTask} for those that do. Normally, a concrete |
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* ForkJoinTask subclass declares fields comprising its parameters, |
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* established in a constructor, and then defines a {@code compute} |
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* method that somehow uses the control methods supplied by this base |
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* parallelism cannot improve throughput. If too small, then memory |
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* and internal task maintenance overhead may overwhelm processing. |
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* |
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+ |
* <p>This class provides {@code adapt} methods for {@link |
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* java.lang.Runnable} and {@link java.util.concurrent.Callable}, that |
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* may be of use when mixing execution of ForkJoinTasks with other |
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* kinds of tasks. When all tasks are of this form, consider using a |
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* pool in {@link ForkJoinPool#setAsyncMode}. |
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* |
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* <p>ForkJoinTasks are {@code Serializable}, which enables them |
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* to be used in extensions such as remote execution frameworks. It is |
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* in general sensible to serialize tasks only before or after, but |
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} |
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|
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/** |
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< |
* Forks both tasks, returning when {@code isDone} holds for |
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* both of them or an exception is encountered. This method may be |
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> |
* Forks the given tasks, returning when {@code isDone} holds for |
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> |
* each task or an exception is encountered. This method may be |
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* invoked only from within ForkJoinTask computations (as may be |
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* determined using method {@link #inForkJoinPool}). Attempts to |
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* invoke in other contexts result in exceptions or errors, |
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* possibly including ClassCastException. |
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* |
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< |
* @param t1 one task |
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* @param t2 the other task |
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* @throws NullPointerException if t1 or t2 are null |
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* @throws RuntimeException or Error if either task did so |
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> |
* @param t1 the first task |
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> |
* @param t2 the second task |
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> |
* @throws NullPointerException if any task is null |
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> |
* @throws RuntimeException or Error if a task did so |
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*/ |
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public static void invokeAll(ForkJoinTask<?>t1, ForkJoinTask<?> t2) { |
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t2.fork(); |
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} |
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|
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/** |
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< |
* Forks the given tasks, returning when {@code isDone} holds |
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< |
* for all of them. If any task encounters an exception, others |
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< |
* may be cancelled. This method may be invoked only from within |
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> |
* Forks the given tasks, returning when {@code isDone} holds for |
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> |
* each task or an exception is encountered. If any task |
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> |
* encounters an exception, others may be, but are not guaranteed |
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> |
* to be, cancelled. This method may be invoked only from within |
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* ForkJoinTask computations (as may be determined using method |
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* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
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* result in exceptions or errors, possibly including |
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* ClassCastException. |
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* |
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< |
* @param tasks the array of tasks |
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> |
* Overloadings of this method exist for the special cases |
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> |
* of one to four arguments. |
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> |
* |
| 575 |
> |
* @param tasks the tasks |
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* @throws NullPointerException if tasks or any element are null |
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* @throws RuntimeException or Error if any task did so |
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*/ |
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} |
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|
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/** |
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< |
* Forks all tasks in the collection, returning when |
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< |
* {@code isDone} holds for all of them. If any task |
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< |
* encounters an exception, others may be cancelled. This method |
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< |
* may be invoked only from within ForkJoinTask computations (as |
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< |
* may be determined using method {@link |
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< |
* #inForkJoinPool}). Attempts to invoke in other contexts result |
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< |
* in exceptions or errors, possibly including ClassCastException. |
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> |
* Forks all tasks in the collection, returning when {@code |
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> |
* isDone} holds for each task or an exception is encountered. If |
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> |
* any task encounters an exception, others may be, but are not |
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> |
* guaranteed to be, cancelled. This method may be invoked only |
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> |
* from within ForkJoinTask computations (as may be determined |
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> |
* using method {@link #inForkJoinPool}). Attempts to invoke in |
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> |
* other contexts result in exceptions or errors, possibly |
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> |
* including ClassCastException. |
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* |
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* @param tasks the collection of tasks |
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* @return the tasks argument, to simplify usage |
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* |
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* <p>This method may be overridden in subclasses, but if so, must |
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* still ensure that these minimal properties hold. In particular, |
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< |
* the cancel method itself must not throw exceptions. |
| 701 |
> |
* the {@code cancel} method itself must not throw exceptions. |
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* |
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< |
* <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> |
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* tasks. To terminate the current task, you can just return or |
| 705 |
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* throw an unchecked exception from its computation method, or |
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* invoke {@link #completeExceptionally}. |
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|
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/** |
| 729 |
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* 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. |
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* |
| 733 |
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* @return the exception, or {@code null} if none |
| 734 |
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*/ |
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* {@code join} and related operations. This method may be used |
| 748 |
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* to induce exceptions in asynchronous tasks, or to force |
| 749 |
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* 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 |
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* implementation to maintain guarantees. |
| 753 |
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* |
| 767 |
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* operations. This method may be used to provide results for |
| 768 |
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* asynchronous tasks, or to provide alternative handling for |
| 769 |
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* 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 |
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* implementation to maintain guarantees. |
| 773 |
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* |
| 867 |
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* 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 |
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*/ |
| 876 |
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public static void helpQuiesce() { |
| 877 |
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((ForkJoinWorkerThread) Thread.currentThread()) |
| 884 |
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* 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 |
< |
* almost surely wrong. 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() { |
| 898 |
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* 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 |
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* @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 |
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* 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() { |
| 1099 |
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*/ |
| 1100 |
|
static final class AdaptedCallable<T> extends ForkJoinTask<T> |
| 1101 |
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implements RunnableFuture<T> { |
| 1102 |
< |
final Callable<T> callable; |
| 1102 |
> |
final Callable<? extends T> callable; |
| 1103 |
|
T result; |
| 1104 |
< |
AdaptedCallable(Callable<T> callable) { |
| 1104 |
> |
AdaptedCallable(Callable<? extends T> callable) { |
| 1105 |
|
if (callable == null) throw new NullPointerException(); |
| 1106 |
|
this.callable = callable; |
| 1107 |
|
} |
| 1131 |
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* @param runnable the runnable action |
| 1132 |
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* @return the task |
| 1133 |
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*/ |
| 1134 |
< |
public static ForkJoinTask<Void> adapt(Runnable runnable) { |
| 1134 |
> |
public static ForkJoinTask<?> adapt(Runnable runnable) { |
| 1135 |
|
return new AdaptedRunnable<Void>(runnable, null); |
| 1136 |
|
} |
| 1137 |
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|
| 1157 |
|
* @param callable the callable action |
| 1158 |
|
* @return the task |
| 1159 |
|
*/ |
| 1160 |
< |
public static <T> ForkJoinTask<T> adapt(Callable<T> callable) { |
| 1160 |
> |
public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) { |
| 1161 |
|
return new AdaptedCallable<T>(callable); |
| 1162 |
|
} |
| 1163 |
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|
