|
FlexDoc/Javadoc 2.0 Demo Java Doc |
A "main" ForkJoinTask begins execution when it is explicitly submitted to a ForkJoinPool, or, if not already engaged in a ForkJoin computation, commenced in the ForkJoinPool.commonPool() via fork(), invoke(), or related methods. Once started, it will usually in turn start other subtasks. As indicated by the name of this class, many programs using ForkJoinTask employ only methods fork() and join(), or derivatives such as invokeAll. However, this class also provides a number of other methods that can come into play in advanced usages, as well as extension mechanics that allow support of new forms of fork/join processing.
A ForkJoinTask is a lightweight form of Future. The efficiency of ForkJoinTasks stems from a set of restrictions (that are only partially statically enforceable) reflecting their main use as computational tasks calculating pure functions or operating on purely isolated objects. The primary coordination mechanisms are fork(), that arranges asynchronous execution, and join(), that doesn't proceed until the task's result has been computed. Computations should ideally avoid synchronized methods or blocks, and should minimize other blocking synchronization apart from joining other tasks or using synchronizers such as Phasers that are advertised to cooperate with fork/join scheduling. Subdividable tasks should also not perform blocking I/O, and should ideally access variables that are completely independent of those accessed by other running tasks. These guidelines are loosely enforced by not permitting checked exceptions such as IOExceptions to be thrown. However, computations may still encounter unchecked exceptions, that are rethrown to callers attempting to join them. These exceptions may additionally include RejectedExecutionException stemming from internal resource exhaustion, such as failure to allocate internal task queues. Rethrown exceptions behave in the same way as regular exceptions, but, when possible, contain stack traces (as displayed for example using ex.printStackTrace()) of both the thread that initiated the computation as well as the thread actually encountering the exception; minimally only the latter.
It is possible to define and use ForkJoinTasks that may block, but doing so requires three further considerations: (1) Completion of few if any other tasks should be dependent on a task that blocks on external synchronization or I/O. Event-style async tasks that are never joined (for example, those subclassing CountedCompleter) often fall into this category. (2) To minimize resource impact, tasks should be small; ideally performing only the (possibly) blocking action. (3) Unless the ForkJoinPool.ManagedBlocker API is used, or the number of possibly blocked tasks is known to be less than the pool's ForkJoinPool.getParallelism() level, the pool cannot guarantee that enough threads will be available to ensure progress or good performance.
The primary method for awaiting completion and extracting results of a task is join(), but there are several variants: The Future.get() methods support interruptible and/or timed waits for completion and report results using Future conventions. Method invoke() is semantically equivalent to fork(); join() but always attempts to begin execution in the current thread. The "quiet" forms of these methods do not extract results or report exceptions. These may be useful when a set of tasks are being executed, and you need to delay processing of results or exceptions until all complete. Method invokeAll (available in multiple versions) performs the most common form of parallel invocation: forking a set of tasks and joining them all.
In the most typical usages, a fork-join pair act like a call (fork) and return (join) from a parallel recursive function. As is the case with other forms of recursive calls, returns (joins) should be performed innermost-first. For example, a.fork(); b.fork(); b.join(); a.join(); is likely to be substantially more efficient than joining a before b.
The execution status of tasks may be queried at several levels of detail: isDone() is true if a task completed in any way (including the case where a task was cancelled without executing); isCompletedNormally() is true if a task completed without cancellation or encountering an exception; isCancelled() is true if the task was cancelled (in which case getException(int) returns a CancellationException); and isCompletedAbnormally() is true if a task was either cancelled or encountered an exception, in which case getException(int) will return either the encountered exception or CancellationException.
The ForkJoinTask class is not usually directly subclassed. Instead, you subclass one of the abstract classes that support a particular style of fork/join processing, typically RecursiveAction for most computations that do not return results, RecursiveTask for those that do, and CountedCompleter for those in which completed actions trigger other actions. Normally, a concrete ForkJoinTask subclass declares fields comprising its parameters, established in a constructor, and then defines a compute method that somehow uses the control methods supplied by this base class.
Method join() and its variants are appropriate for use only when completion dependencies are acyclic; that is, the parallel computation can be described as a directed acyclic graph (DAG). Otherwise, executions may encounter a form of deadlock as tasks cyclically wait for each other. However, this framework supports other methods and techniques (for example the use of Phaser, helpQuiesce(), and complete(V)) that may be of use in constructing custom subclasses for problems that are not statically structured as DAGs. To support such usages, a ForkJoinTask may be atomically tagged with a short value using setForkJoinTaskTag(short) or compareAndSetForkJoinTaskTag(short, short) and checked using getForkJoinTaskTag(). The ForkJoinTask implementation does not use these protected methods or tags for any purpose, but they may be of use in the construction of specialized subclasses. For example, parallel graph traversals can use the supplied methods to avoid revisiting nodes/tasks that have already been processed. (Method names for tagging are bulky in part to encourage definition of methods that reflect their usage patterns.)
Most base support methods are final, to prevent overriding of implementations that are intrinsically tied to the underlying lightweight task scheduling framework. Developers creating new basic styles of fork/join processing should minimally implement protected methods exec(), setRawResult(V), and getRawResult(), while also introducing an abstract computational method that can be implemented in its subclasses, possibly relying on other protected methods provided by this class.
ForkJoinTasks should perform relatively small amounts of computation. Large tasks should be split into smaller subtasks, usually via recursive decomposition. As a very rough rule of thumb, a task should perform more than 100 and less than 10000 basic computational steps, and should avoid indefinite looping. If tasks are too big, then parallelism cannot improve throughput. If too small, then memory and internal task maintenance overhead may overwhelm processing.
This class provides adapt methods for Runnable and Callable, that may be of use when mixing execution of ForkJoinTasks with other kinds of tasks. When all tasks are of this form, consider using a pool constructed in asyncMode.
ForkJoinTasks are Serializable, which enables them to be used in extensions such as remote execution frameworks. It is sensible to serialize tasks only before or after, but not during, execution. Serialization is not relied on during execution itself.
Constructor Summary |
||
Constructor for subclasses to call.
|
Method Summary |
||
Returns a new ForkJoinTask that performs the call
method of the given Callable as its action, and returns
its result upon join(), translating any checked exceptions
encountered into RuntimeException.
|
||
static ForkJoinTask<?> |
Returns a new ForkJoinTask that performs the run
method of the given Runnable as its action, and returns
a null result upon join().
|
|
Returns a new ForkJoinTask that performs the run
method of the given Runnable as its action, and returns
the given result upon join().
|
||
boolean |
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
|
|
final boolean |
compareAndSetForkJoinTaskTag(short expect, short update)
Atomically conditionally sets the tag value for this task.
|
|
void |
Completes this task, and if not already aborted or cancelled,
returning the given value as the result of subsequent
invocations of join and related operations.
|
|
void |
Completes this task abnormally, and if not already aborted or
cancelled, causes it to throw the given exception upon
join and related operations.
|
|
protected abstract boolean |
exec()
Immediately performs the base action of this task and returns
true if, upon return from this method, this task is guaranteed
to have completed.
|
|
final ForkJoinTask<V> |
fork()
Arranges to asynchronously execute this task in the pool the
current task is running in, if applicable, or using the ForkJoinPool.commonPool() if not inForkJoinPool().
|
|
final V |
get()
Waits if necessary for the computation to complete, and then
retrieves its result.
|
|
final V |
Waits if necessary for at most the given time for the computation
to complete, and then retrieves its result, if available.
|
|
final Throwable |
Returns the exception thrown by the base computation, or a
CancellationException if cancelled, or null if
none or if the method has not yet completed.
|
|
final short |
Returns the tag for this task.
|
|
static ForkJoinPool |
getPool()
Returns the pool hosting the current thread, or null
if the current thread is executing outside of any ForkJoinPool.
|
|
static int |
Returns an estimate of the number of tasks that have been
forked by the current worker thread but not yet executed.
|
|
abstract V |
Returns the result that would be returned by join(), even
if this task completed abnormally, or null if this task
is not known to have been completed.
|
|
static int |
Returns an estimate of how many more locally queued tasks are
held by the current worker thread than there are other worker
threads that might steal them, or zero if this thread is not
operating in a ForkJoinPool.
|
|
static void |
Possibly executes tasks until the pool hosting the current task
is quiescent.
|
|
static boolean |
Returns true if the current thread is a ForkJoinWorkerThread executing as a ForkJoinPool computation.
|
|
final V |
invoke()
Commences performing this task, awaits its completion if
necessary, and returns its result, or throws an (unchecked)
RuntimeException or Error if the underlying
computation did so.
|
|
Forks all tasks in the specified collection, returning when
isDone holds for each task or an (unchecked) exception
is encountered, in which case the exception is rethrown.
|
||
static void |
Forks the given tasks, returning when isDone holds for
each task or an (unchecked) exception is encountered, in which
case the exception is rethrown.
|
|
static void |
invokeAll(ForkJoinTask<?>... tasks)
Forks the given tasks, returning when isDone holds for
each task or an (unchecked) exception is encountered, in which
case the exception is rethrown.
|
|
final boolean |
Returns true if this task was cancelled before it completed
normally.
|
|
final boolean |
Returns true if this task threw an exception or was cancelled.
|
|
final boolean |
Returns true if this task completed without throwing an
exception and was not cancelled.
|
|
final boolean |
isDone()
Returns true if this task completed.
|
|
final V |
join()
Returns the result of the computation when it
is done.
|
|
protected static ForkJoinTask<?> |
Returns, but does not unschedule or execute, a task queued by
the current thread but not yet executed, if one is immediately
available.
|
|
protected static ForkJoinTask<?> |
Unschedules and returns, without executing, the next task
queued by the current thread but not yet executed, if the
current thread is operating in a ForkJoinPool.
|
|
protected static ForkJoinTask<?> |
If the current thread is operating in a ForkJoinPool,
unschedules and returns, without executing, a task externally
submitted to the pool, if one is available.
|
|
protected static ForkJoinTask<?> |
pollTask()
If the current thread is operating in a ForkJoinPool,
unschedules and returns, without executing, the next task
queued by the current thread but not yet executed, if one is
available, or if not available, a task that was forked by some
other thread, if available.
|
|
final void |
Completes this task normally without setting a value.
|
|
final void |
Commences performing this task and awaits its completion if
necessary, without returning its result or throwing its
exception.
|
|
final void |
Joins this task, without returning its result or throwing its
exception.
|
|
void |
Resets the internal bookkeeping state of this task, allowing a
subsequent fork.
|
|
final short |
setForkJoinTaskTag(short newValue)
Atomically sets the tag value for this task and returns the old value.
|
|
protected abstract void |
setRawResult(V value)
Forces the given value to be returned as a result.
|
|
boolean |
Tries to unschedule this task for execution.
|
Methods inherited from class java.lang.Object |
public ForkJoinTask |
() |
() |
public final V join |
() |
public final V invoke |
() |
public static void invokeAll |
public static void invokeAll |
(ForkJoinTask<?>... tasks) |
(Collection<T> tasks) |
public boolean cancel |
(boolean mayInterruptIfRunning) |
This method may be overridden in subclasses, but if so, must still ensure that these properties hold. In particular, the cancel method itself must not throw exceptions.
This method is designed to be invoked by other tasks. To terminate the current task, you can just return or throw an unchecked exception from its computation method, or invoke completeExceptionally(Throwable).
public final boolean isDone |
() |
public final boolean isCancelled |
() |
public final boolean isCompletedAbnormally |
() |
public final boolean isCompletedNormally |
() |
public final Throwable getException |
() |
public void completeExceptionally |
(Throwable ex) |
public void complete |
(V value) |
public final void quietlyComplete |
() |
public final V get |
() |
throws |
public final V get |
(long timeout, TimeUnit unit) |
throws |
public final void quietlyJoin |
() |
public final void quietlyInvoke |
() |
public static void helpQuiesce |
() |
public void reinitialize |
() |
Upon completion of this method, isDone() reports false, and getException() reports null. However, the value returned by getRawResult is unaffected. To clear this value, you can invoke setRawResult(null).
public static ForkJoinPool getPool |
() |
This method returns null if and only if inForkJoinPool() returns false.
public static boolean inForkJoinPool |
() |
public boolean tryUnfork |
() |
public static int getQueuedTaskCount |
() |
public static int getSurplusQueuedTaskCount |
() |
public abstract V getRawResult |
() |
protected abstract void setRawResult |
(V value) |
protected abstract boolean exec |
() |
protected static ForkJoinTask<?> peekNextLocalTask |
() |
protected static ForkJoinTask<?> pollNextLocalTask |
() |
protected static ForkJoinTask<?> pollTask |
() |
protected static ForkJoinTask<?> pollSubmission |
() |
public final short getForkJoinTaskTag |
() |
public final short setForkJoinTaskTag |
(short newValue) |
public final boolean compareAndSetForkJoinTaskTag |
(short expect, short update) |
public static ForkJoinTask<?> adapt |
(Runnable runnable) |
|
FlexDoc/Javadoc 2.0 Demo Java Doc |