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ForkJoinTask
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jsr166y.forkjoin</FONT>
<BR>
Class ForkJoinTask<V></H2>
<PRE>
java.lang.Object
<IMG SRC="../../resources/inherit.gif" ALT="extended by "><B>jsr166y.forkjoin.ForkJoinTask<V></B>
</PRE>
<DL>
<DT><B>Direct Known Subclasses:</B> <DD><A HREF="../../jsr166y/forkjoin/AsyncAction.html" title="class in jsr166y.forkjoin">AsyncAction</A>, <A HREF="../../jsr166y/forkjoin/CyclicAction.html" title="class in jsr166y.forkjoin">CyclicAction</A>, <A HREF="../../jsr166y/forkjoin/LinkedAsyncAction.html" title="class in jsr166y.forkjoin">LinkedAsyncAction</A>, <A HREF="../../jsr166y/forkjoin/RecursiveAction.html" title="class in jsr166y.forkjoin">RecursiveAction</A>, <A HREF="../../jsr166y/forkjoin/RecursiveTask.html" title="class in jsr166y.forkjoin">RecursiveTask</A></DD>
</DL>
<HR>
<DL>
<DT><PRE>public abstract class <B>ForkJoinTask<V></B><DT>extends java.lang.Object</DL>
</PRE>
<P>
Abstract base class for tasks that run within a ForkJoinPool. A
ForkJoinTask is a thread-like entity that is much lighter weight
than a normal thread. Huge numbers of tasks and subtasks may be
hosted by a small number of actual threads in a ForkJoinPool,
at the price of some usage limitations.
<p> The <tt>ForkJoinTask</tt> class is not directly subclassable
outside of this package. Instead, you can subclass one of the
supplied abstract classes that support the various styles of
fork/join processing. Normally, a concrete ForkJoinTask subclass
declares fields comprising its parameters, established in a
constructor, and then defines a <tt>protected compute</tt> method
that somehow uses the control methods supplied by this base
class. While these methods have <tt>public</tt> access, most may
only be called from within other ForkJoinTasks. Attempts to invoke
them in other contexts result in exceptions or errors including
ClassCastException. The only generally accessible methods are
those for cancellation and status checking. The only way to invoke
a "main" driver task is to submit it to a ForkJoinPool. Normally,
once started, this will in turn start other subtasks. Nearly all
of these base support methods are <tt>final</tt> because their
implementations are intrinsically tied to the underlying
lightweight task scheduling framework, and so cannot in general be
overridden.
<p> ForkJoinTasks play similar roles as <tt>Futures</tt> but
support a more limited range of use. The "lightness" of
ForkJoinTasks is due to a set of restrictions (that are only
partially statically enforceable) reflecting their intended use as
purely computational tasks -- calculating pure functions or
operating on purely isolated objects. The only coordination
mechanisms supported for ForkJoinTasks are <tt>fork</tt>, that
arranges asynchronous execution, and <tt>join</tt>, that doesn't
proceed until the task's result has been computed. (A simple form
of cancellation is also supported). The computation defined in the
<tt>compute</tt> method should not in general perform any other
form of blocking synchronization, should not perform IO, and should
be independent of other tasks. Minor breaches of these
restrictions, for example using shared output streams, may be
tolerable in practice, but frequent use will result in poor
performance, and the potential to indefinitely stall if the number
of threads not waiting for external synchronization becomes
exhausted. This usage restriction is in part enforced by not
permitting checked exceptions to be thrown. However, computations
may still encounter unchecked exceptions, that are rethrown to
callers attempting join them. These exceptions may additionally
include RejectedExecutionExceptions stemming from internal resource
exhaustion such as failure to allocate internal task queues.
<P>
<P>
<HR>
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<CODE> void</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#cancel()">cancel</A></B>()</CODE>
<BR>
Asserts that the results of this task's computation will not be
used.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE>abstract java.lang.Throwable</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#exec()">exec</A></B>()</CODE>
<BR>
Immediately commences execution of this task by the current
worker thread unless already cancelled, returning any exception
thrown by its <tt>compute</tt> method.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE>abstract void</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#finish(V)">finish</A></B>(<A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A> result)</CODE>
<BR>
Completes this task, and if not already aborted or cancelled,
returning the given result upon <tt>join</tt> and related
operations.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE>abstract void</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#finishExceptionally(java.lang.Throwable)">finishExceptionally</A></B>(java.lang.Throwable ex)</CODE>
<BR>
Completes this task abnormally, and if not already aborted or
cancelled, causes it to throw the given exception upon
<tt>join</tt> and related operations.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> void</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#fork()">fork</A></B>()</CODE>
<BR>
Arranges to asynchronously execute this task, which will later
be directly or indirectly joined by the caller of this method.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE>abstract <A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A></CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#forkJoin()">forkJoin</A></B>()</CODE>
<BR>
Equivalent in effect to the sequence <tt>fork(); join();</tt>
but may be more efficient.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> java.lang.Throwable</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#getException()">getException</A></B>()</CODE>
<BR>
Returns the exception thrown by method <tt>compute</tt>, or a
CancellationException if cancelled, or null if none or if the
method has not yet completed.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> boolean</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#isCancelled()">isCancelled</A></B>()</CODE>
<BR>
Returns true if this task was cancelled.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> boolean</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#isDone()">isDone</A></B>()</CODE>
<BR>
Returns true if the computation performed by this task has
completed (or has been cancelled).</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> boolean</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#isStolen()">isStolen</A></B>()</CODE>
<BR>
Returns true if this task was stolen from some other worker in the
pool and has not yet completed.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> <A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A></CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#join()">join</A></B>()</CODE>
<BR>
Returns the result of the computation when it is ready.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> java.lang.Throwable</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#quietlyJoin()">quietlyJoin</A></B>()</CODE>
<BR>
Joins this task, without returning its result or throwing an
exception, but returning the exception that <tt>join</tt> would
throw.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE>abstract <A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A></CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#rawResult()">rawResult</A></B>()</CODE>
<BR>
Returns the result that would be returned by <tt>join</tt>, or
null if this task is not known to have been completed.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
<CODE> void</CODE></FONT></TD>
<TD><CODE><B><A HREF="../../jsr166y/forkjoin/ForkJoinTask.html#reinitialize()">reinitialize</A></B>()</CODE>
<BR>
Resets the internal bookkeeping state of this task, allowing a
subsequent <tt>fork</tt>.</TD>
</TR>
</TABLE>
<A NAME="methods_inherited_from_class_java.lang.Object"><!-- --></A>
<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
<TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor">
<TH ALIGN="left"><B>Methods inherited from class java.lang.Object</B></TH>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD><CODE>clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait</CODE></TD>
</TR>
</TABLE>
<P>
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<B>Method Detail</B></FONT></TH>
</TR>
</TABLE>
<A NAME="fork()"><!-- --></A><H3>
fork</H3>
<PRE>
public final void <B>fork</B>()</PRE>
<DL>
<DD>Arranges to asynchronously execute this task, which will later
be directly or indirectly joined by the caller of this method.
While it is not necessarily enforced, it is a usage error to
fork a task more than once unless it has completed and been
reinitialized. This method may be invoked only from within
other ForkJoinTask computations. Attempts to invoke in other
contexts result in exceptions or errors including
ClassCastException.
<P>
<DD><DL>
</DL>
</DD>
</DL>
<HR>
<A NAME="join()"><!-- --></A><H3>
join</H3>
<PRE>
public final <A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A> <B>join</B>()</PRE>
<DL>
<DD>Returns the result of the computation when it is ready.
Monitoring note: Callers of this method need not block, but may
instead assist in performing computations that may directly or
indirectly cause the result to be ready.
This method may be invoked only from within other ForkJoinTask
computations. Attempts to invoke in other contexts result
in exceptions or errors including ClassCastException.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>the computed result
<DT><B>Throws:</B>
<DD><CODE>java.lang.Throwable</CODE> - (a RuntimeException, Error, or unchecked
exception) if the underlying computation did so.</DL>
</DD>
</DL>
<HR>
<A NAME="forkJoin()"><!-- --></A><H3>
forkJoin</H3>
<PRE>
public abstract <A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A> <B>forkJoin</B>()</PRE>
<DL>
<DD>Equivalent in effect to the sequence <tt>fork(); join();</tt>
but may be more efficient.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>the computed result
<DT><B>Throws:</B>
<DD><CODE>java.lang.Throwable</CODE> - (a RuntimeException, Error, or unchecked
exception) if the underlying computation did so.</DL>
</DD>
</DL>
<HR>
<A NAME="isDone()"><!-- --></A><H3>
isDone</H3>
<PRE>
public final boolean <B>isDone</B>()</PRE>
<DL>
<DD>Returns true if the computation performed by this task has
completed (or has been cancelled). This method will eventually
return true upon task completion, but return values are not
otherwise guaranteed to be uniform across observers. Thus,
when task <tt>t</tt> finishes, concurrent invocations of
<tt>t.isDone()</tt> from other tasks or threads on may
transiently return different results, but on repeated
invocation, will eventually agree that <tt>t.isDone()</tt>.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>true if this computation has completed</DL>
</DD>
</DL>
<HR>
<A NAME="isCancelled()"><!-- --></A><H3>
isCancelled</H3>
<PRE>
public final boolean <B>isCancelled</B>()</PRE>
<DL>
<DD>Returns true if this task was cancelled.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>true if this task was cancelled</DL>
</DD>
</DL>
<HR>
<A NAME="cancel()"><!-- --></A><H3>
cancel</H3>
<PRE>
public void <B>cancel</B>()</PRE>
<DL>
<DD>Asserts that the results of this task's computation will not be
used. If a cancellation occurs before this task is processed,
then its <tt>compute</tt> method will not be executed,
<tt>isCancelled</tt> will report true, and <tt>join</tt> will
result in a CancellationException being thrown. Otherwise,
there are no guarantees about whether <tt>isCancelled</tt> will
report true, whether <tt>join</tt> will return normally or via
an exception, or whether these behaviors will remain consistent
upon repeated invocation. This method may be overridden in
subclasses, but if so, must still ensure that these minimal
properties hold.
<P>
<DD><DL>
</DL>
</DD>
</DL>
<HR>
<A NAME="getException()"><!-- --></A><H3>
getException</H3>
<PRE>
public final java.lang.Throwable <B>getException</B>()</PRE>
<DL>
<DD>Returns the exception thrown by method <tt>compute</tt>, or a
CancellationException if cancelled, or null if none or if the
method has not yet completed.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>the exception, or null if none</DL>
</DD>
</DL>
<HR>
<A NAME="rawResult()"><!-- --></A><H3>
rawResult</H3>
<PRE>
public abstract <A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A> <B>rawResult</B>()</PRE>
<DL>
<DD>Returns the result that would be returned by <tt>join</tt>, or
null if this task is not known to have been completed. This
method is designed to aid debugging, as well as to support
extensions. Its use in any other context is strongly
discouraged.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>the result, or null if not completed.</DL>
</DD>
</DL>
<HR>
<A NAME="reinitialize()"><!-- --></A><H3>
reinitialize</H3>
<PRE>
public void <B>reinitialize</B>()</PRE>
<DL>
<DD>Resets the internal bookkeeping state of this task, allowing a
subsequent <tt>fork</tt>. This method allows repeated reuse of
this task, but only if reuse occurs when this task has either
never been forked, or has been forked, then completed and all
outstanding joins of this task have also completed. Effects
under any other usage conditions are not guaranteed, and are
almost surely wrong. This method may be useful when repeatedly
executing pre-constructed trees of subtasks.
<P>
<DD><DL>
</DL>
</DD>
</DL>
<HR>
<A NAME="isStolen()"><!-- --></A><H3>
isStolen</H3>
<PRE>
public final boolean <B>isStolen</B>()</PRE>
<DL>
<DD>Returns true if this task was stolen from some other worker in the
pool and has not yet completed. This method should be called
only by the thread currently executing this task. The results
of calling this method in any other context are undefined.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>true is this task is stolen</DL>
</DD>
</DL>
<HR>
<A NAME="quietlyJoin()"><!-- --></A><H3>
quietlyJoin</H3>
<PRE>
public final java.lang.Throwable <B>quietlyJoin</B>()</PRE>
<DL>
<DD>Joins this task, without returning its result or throwing an
exception, but returning the exception that <tt>join</tt> would
throw. This method may be useful when processing collections of
tasks when some have been cancelled or otherwise known to have
aborted. This method may be invoked only from within other
ForkJoinTask computations. Attempts to invoke in other contexts
result in exceptions or errors including ClassCastException.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>the exception that would be thrown by <tt>join</tt>, or
null if this task completed normally.</DL>
</DD>
</DL>
<HR>
<A NAME="exec()"><!-- --></A><H3>
exec</H3>
<PRE>
public abstract java.lang.Throwable <B>exec</B>()</PRE>
<DL>
<DD>Immediately commences execution of this task by the current
worker thread unless already cancelled, returning any exception
thrown by its <tt>compute</tt> method.
<P>
<DD><DL>
<DT><B>Returns:</B><DD>exception thrown by compute (or via cancellation), or
null if none</DL>
</DD>
</DL>
<HR>
<A NAME="finish(java.lang.Object)"><!-- --></A><A NAME="finish(V)"><!-- --></A><H3>
finish</H3>
<PRE>
public abstract void <B>finish</B>(<A HREF="../../jsr166y/forkjoin/ForkJoinTask.html" title="type parameter in ForkJoinTask">V</A> result)</PRE>
<DL>
<DD>Completes this task, and if not already aborted or cancelled,
returning the given result upon <tt>join</tt> and related
operations. This method may be invoked only from within other
ForkJoinTask computations. This method may be used to provide
results, mainly for asynchronous tasks. Upon invocation, the
task itself, if running, must exit (return) in a small finite
number of steps.
<P>
<DD><DL>
<DT><B>Parameters:</B><DD><CODE>result</CODE> - the result to return</DL>
</DD>
</DL>
<HR>
<A NAME="finishExceptionally(java.lang.Throwable)"><!-- --></A><H3>
finishExceptionally</H3>
<PRE>
public abstract void <B>finishExceptionally</B>(java.lang.Throwable ex)</PRE>
<DL>
<DD>Completes this task abnormally, and if not already aborted or
cancelled, causes it to throw the given exception upon
<tt>join</tt> and related operations. Upon invocation, the task
itself, if running, must exit (return) in a small finite number
of steps.
<P>
<DD><DL>
<DT><B>Parameters:</B><DD><CODE>ex</CODE> - the exception to throw. While not necessarily
statically enforced, this must be a RuntimeException or Error.</DL>
</DD>
</DL>
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