jsr166y
Class CountedCompleter

java.lang.Object
  jsr166y.ForkJoinTask<Void>
      jsr166y.CountedCompleter

All Implemented Interfaces:

Serializable, Future<Void>

public abstract class CountedCompleter
extends ForkJoinTask<Void>

A resultless ForkJoinTask with a completion action performed when triggered and there are no remaining pending actions. Uses of CountedCompleter are similar to those of other completion based components (such as CompletionHandler) except that multiple pending completions may be necessary to trigger the onCompletion(jsr166y.CountedCompleter) action, not just one. Unless initialized otherwise, the pending count starts at zero, but may be (atomically) changed using methods setPendingCount(int), addToPendingCount(int), and compareAndSetPendingCount(int, int). Upon invocation of tryComplete(), if the pending action count is nonzero, it is decremented; otherwise, the completion action is performed, and if this completer itself has a completer, the process is continued with its completer. As is the case with related synchronization components such as Phaser and Semaphore these methods affect only internal counts; they do not establish any further internal bookkeeping. In particular, the identities of pending tasks are not maintained. As illustrated below, you can create subclasses that do record some or all pended tasks or their results when needed.

A concrete CountedCompleter class must define method compute(), that should, in almost all use cases, invoke tryComplete() once before returning. The class may also optionally override method onCompletion(jsr166y.CountedCompleter) to perform an action upon normal completion, and method onExceptionalCompletion(java.lang.Throwable, jsr166y.CountedCompleter) to perform an action upon any exception.

A CountedCompleter that does not itself have a completer (i.e., one for which getCompleter() returns null) can be used as a regular ForkJoinTask with this added functionality. However, any completer that in turn has another completer serves only as an internal helper for other computations, so its own task status (as reported in methods such as ForkJoinTask.isDone()) is arbitrary; this status changes only upon explicit invocations of complete(java.lang.Void), ForkJoinTask.cancel(boolean), ForkJoinTask.completeExceptionally(java.lang.Throwable) or upon exceptional completion of method compute. Upon any exceptional completion, the exception may be relayed to a task's completer (and its completer, and so on), if one exists and it has not otherwise already completed.

Sample Usages.

Parallel recursive decomposition. CountedCompleters may be arranged in trees similar to those often used with RecursiveActions, although the constructions involved in setting them up typically vary. Even though they entail a bit more bookkeeping, CountedCompleters may be better choices when applying a possibly time-consuming operation (that cannot be further subdivided) to each element of an array or collection; especially when the operation takes a significantly different amount of time to complete for some elements than others, either because of intrinsic variation (for example IO) or auxiliary effects such as garbage collection. Because CountedCompleters provide their own continuations, other threads need not block waiting to perform them.

For example, here is an initial version of a class that uses divide-by-two recursive decomposition to divide work into single pieces (leaf tasks). Even when work is split into individual calls, tree-based techniques are usually preferable to directly forking leaf tasks, because they reduce inter-thread communication and improve load balancing. In the recursive case, the second of each pair of subtasks to finish triggers completion of its parent (because no result combination is performed, the default no-op implementation of method onCompletion is not overridden). A static utility method sets up the base task and invokes it:

 class MyOperation<E> { void apply(E e) { ... }  }

 class ForEach<E> extends CountedCompleter {

     public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) {
         pool.invoke(new ForEach<E>(null, array, op, 0, array.length));
     }

     final E[] array; final MyOperation<E> op; final int lo, hi;
     ForEach(CountedCompleter p, E[] array, MyOperation<E> op, int lo, int hi) {
         super(p);
         this.array = array; this.op = op; this.lo = lo; this.hi = hi;
     }

     public void compute() { // version 1
         if (hi - lo >= 2) {
             int mid = (lo + hi) >>> 1;
             setPendingCount(2); // must set pending count before fork
             new ForEach(this, array, op, mid, hi).fork(); // right child
             new ForEach(this, array, op, lo, mid).fork(); // left child
         }
         else if (hi > lo)
             op.apply(array[lo]);
         tryComplete();
     }
 } 

This design can be improved by noticing that in the recursive case, the task has nothing to do after forking its right task, so can directly invoke its left task before returning. (This is an analog of tail recursion removal.) Also, because the task returns upon executing its left task (rather than falling through to invoke tryComplete) the pending count is set to one:

 class ForEach<E> ...
     public void compute() { // version 2
         if (hi - lo >= 2) {
             int mid = (lo + hi) >>> 1;
             setPendingCount(1); // only one pending
             new ForEach(this, array, op, mid, hi).fork(); // right child
             new ForEach(this, array, op, lo, mid).compute(); // direct invoke
         }
         else {
             if (hi > lo)
                 op.apply(array[lo]);
             tryComplete();
         }
     }
 

As a further improvement, notice that the left task need not even exist. Instead of creating a new one, we can iterate using the original task, and add a pending count for each fork:

 class ForEach<E> ...
     public void compute() { // version 3
         int l = lo,  h = hi;
         while (h - l >= 2) {
             int mid = (l + h) >>> 1;
             addToPendingCount(1);
             new ForEach(this, array, op, mid, h).fork(); // right child
             h = mid;
         }
         if (h > l)
             op.apply(array[l]);
         tryComplete();
     }
 

Additional improvements of such classes might entail precomputing pending counts so that they can be established in constructors, specializing classes for leaf steps, subdividing by say, four, instead of two per iteration, and using an adaptive threshold instead of always subdividing down to single elements.

Recording subtasks. CountedCompleter tasks that combine results of multiple subtasks usually need to access these results in method onCompletion(jsr166y.CountedCompleter). As illustrated in the following class (that performs a simplified form of map-reduce where mappings and reductions are all of type E), one way to do this in divide and conquer designs is to have each subtask record its sibling, so that it can be accessed in method onCompletion. For clarity, this class uses explicit left and right subtasks, but variants of other streamlinings seen in the above example may also apply.

 class MyMapper<E> { E apply(E v) {  ...  } }
 class MyReducer<E> { E apply(E x, E y) {  ...  } }
 class MapReducer<E> extends CountedCompleter {
     final E[] array; final MyMapper<E> mapper;
     final MyReducer<E> reducer; final int lo, hi;
     MapReducer sibling;
     E result;
     MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
                MyReducer<E> reducer, int lo, int hi) {
         super(p);
         this.array = array; this.mapper = mapper;
         this.reducer = reducer; this.lo = lo; this.hi = hi;
     }
     public void compute() {
         if (hi - lo >= 2) {
             int mid = (lo + hi) >>> 1;
             MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
             MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
             left.sibling = right;
             right.sibling = left;
             setPendingCount(1); // only right is pending
             right.fork();
             left.compute();     // directly execute left
         }
         else {
             if (hi > lo)
                 result = mapper.apply(array[lo]);
             tryComplete();
         }
     }
     public void onCompletion(CountedCompleter caller) {
         if (caller != this) {
            MapReducer<E> child = (MapReducer<E>)caller;
            MapReducer<E> sib = child.sibling;
            if (sib == null || sib.result == null)
                result = child.result;
            else
                result = reducer.apply(child.result, sib.result);
         }
     }

     public static <E> E mapReduce(ForkJoinPool pool, E[] array,
                                   MyMapper<E> mapper, MyReducer<E> reducer) {
         MapReducer<E> mr = new MapReducer<E>(null, array, mapper,
                                              reducer, 0, array.length);
         pool.invoke(mr);
         return mr.result;
     }
 } 

Triggers. Some CountedCompleters are themselves never forked, but instead serve as bits of plumbing in other designs; including those in which the completion of one of more async tasks triggers another async task. For example:

 class HeaderBuilder extends CountedCompleter { ... }
 class BodyBuilder extends CountedCompleter { ... }
 class PacketSender extends CountedCompleter {
     PacketSender(...) { super(null, 1); ... } // trigger on second completion
     public void compute() { } // never called
     public void onCompletion(CountedCompleter caller) { sendPacket(); }
 }
 // sample use:
 PacketSender p = new PacketSender();
 new HeaderBuilder(p, ...).fork();
 new BodyBuilder(p, ...).fork();
 

Since:

1.8

Author:

Doug Lea

See Also:

Serialized Form

Constructor Summary

protected	CountedCompleter() Creates a new CountedCompleter with no completer and an initial pending count of zero.
protected	CountedCompleter(CountedCompleter completer) Creates a new CountedCompleter with the given completer and an initial pending count of zero.
protected	CountedCompleter(CountedCompleter completer, int initialPendingCount) Creates a new CountedCompleter with the given completer and initial pending count.

Method Summary

void	addToPendingCount(int delta) Adds (atomically) the given value to the pending count.
boolean	compareAndSetPendingCount(int expected, int count) Sets (atomically) the pending count to the given count only if it currently holds the given expected value.
void	complete(Void mustBeNull) Regardless of pending count, invokes onCompletion(jsr166y.CountedCompleter), marks this task as complete with a null return value, and further triggers tryComplete() on this task's completer, if one exists.
abstract void	compute() The main computation performed by this task.
protected boolean	exec() Implements execution conventions for CountedCompleters
CountedCompleter	getCompleter() Returns the completer established in this task's constructor, or null if none.
int	getPendingCount() Returns the current pending count.
Void	getRawResult() Always returns null.
void	onCompletion(CountedCompleter caller) Performs an action when method tryComplete() is invoked and there are no pending counts, or when the unconditional method complete(java.lang.Void) is invoked.
boolean	onExceptionalCompletion(Throwable ex, CountedCompleter caller) Performs an action when method ForkJoinTask.completeExceptionally(java.lang.Throwable) is invoked or method compute() throws an exception, and this task has not otherwise already completed normally.
void	setPendingCount(int count) Sets the pending count to the given value.
protected void	setRawResult(Void mustBeNull) Requires null completion value.
void	tryComplete() If the pending count is nonzero, decrements the count; otherwise invokes onCompletion(jsr166y.CountedCompleter) and then similarly tries to complete this task's completer, if one exists, else marks this task as complete.

Methods inherited from class jsr166y.ForkJoinTask

adapt, adapt, adapt, cancel, compareAndSetForkJoinTaskTag, completeExceptionally, fork, get, get, getException, getForkJoinTaskTag, getPool, getQueuedTaskCount, getSurplusQueuedTaskCount, helpQuiesce, inForkJoinPool, invoke, invokeAll, invokeAll, invokeAll, isCancelled, isCompletedAbnormally, isCompletedNormally, isDone, join, peekNextLocalTask, pollNextLocalTask, pollTask, quietlyComplete, quietlyInvoke, quietlyJoin, reinitialize, setForkJoinTaskTag, tryUnfork

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CountedCompleter

protected CountedCompleter(CountedCompleter completer,
                           int initialPendingCount)

Creates a new CountedCompleter with the given completer and initial pending count.

Parameters:

completer - this tasks completer, or null if none

initialPendingCount - the initial pending count

CountedCompleter

protected CountedCompleter(CountedCompleter completer)

Creates a new CountedCompleter with the given completer and an initial pending count of zero.

Parameters:

completer - this tasks completer, or null if none

CountedCompleter

protected CountedCompleter()

Creates a new CountedCompleter with no completer and an initial pending count of zero.

Method Detail

compute

public abstract void compute()

The main computation performed by this task.

onCompletion

public void onCompletion(CountedCompleter caller)

Performs an action when method tryComplete() is invoked and there are no pending counts, or when the unconditional method complete(java.lang.Void) is invoked. By default, this method does nothing.

Parameters:

caller - the task invoking this method (which may be this task itself).

onExceptionalCompletion

public boolean onExceptionalCompletion(Throwable ex,
                                       CountedCompleter caller)

Performs an action when method ForkJoinTask.completeExceptionally(java.lang.Throwable) is invoked or method compute() throws an exception, and this task has not otherwise already completed normally. On entry to this method, this task ForkJoinTask.isCompletedAbnormally(). The return value of this method controls further propagation: If true and this task has a completer, then this completer is also completed exceptionally. The default implementation of this method does nothing except return true.

Parameters:

ex - the exception

caller - the task invoking this method (which may be this task itself).

Returns:

true if this exception should be propagated to this tasks completer, if one exists.

getCompleter

public final CountedCompleter getCompleter()

Returns the completer established in this task's constructor, or null if none.

Returns:

the completer

getPendingCount

public final int getPendingCount()

Returns the current pending count.

Returns:

the current pending count

setPendingCount

public final void setPendingCount(int count)

Sets the pending count to the given value.

Parameters:

count - the count

addToPendingCount

public final void addToPendingCount(int delta)

Adds (atomically) the given value to the pending count.

Parameters:

delta - the value to add

compareAndSetPendingCount

public final boolean compareAndSetPendingCount(int expected,
                                               int count)

Sets (atomically) the pending count to the given count only if it currently holds the given expected value.

Parameters:

expected - the expected value

count - the new value

Returns:

true is successful

tryComplete

public final void tryComplete()

If the pending count is nonzero, decrements the count; otherwise invokes onCompletion(jsr166y.CountedCompleter) and then similarly tries to complete this task's completer, if one exists, else marks this task as complete.

complete

public void complete(Void mustBeNull)

Regardless of pending count, invokes onCompletion(jsr166y.CountedCompleter), marks this task as complete with a null return value, and further triggers tryComplete() on this task's completer, if one exists. This method may be useful when forcing completion as soon as any one (versus all) of several subtask results are obtained.

Overrides:

complete in class ForkJoinTask<Void>

Parameters:

mustBeNull - the null completion value

exec

protected final boolean exec()

Implements execution conventions for CountedCompleters

Specified by:

exec in class ForkJoinTask<Void>

Returns:

true if this task is known to have completed normally

getRawResult

public final Void getRawResult()

Always returns null.

Specified by:

getRawResult in class ForkJoinTask<Void>

Returns:

null always

setRawResult

protected final void setRawResult(Void mustBeNull)

Requires null completion value.

Specified by:

setRawResult in class ForkJoinTask<Void>

Parameters:

mustBeNull - the value