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* published by the Free Software Foundation. Oracle designates this
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*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
/**
* A {@link ForkJoinTask} with a completion action performed when
* triggered and there are no remaining pending actions.
* CountedCompleters are in general more robust in the
* presence of subtask stalls and blockage than are other forms of
* ForkJoinTasks, but are less intuitive to program. Uses of
* CountedCompleter are similar to those of other completion based
* components (such as {@link java.nio.channels.CompletionHandler})
* except that multiple <em>pending</em> completions may be necessary
* to trigger the completion action {@link #onCompletion(CountedCompleter)},
* not just one.
* Unless initialized otherwise, the {@linkplain #getPendingCount pending
* count} starts at zero, but may be (atomically) changed using
* methods {@link #setPendingCount}, {@link #addToPendingCount}, and
* {@link #compareAndSetPendingCount}. Upon invocation of {@link
* #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 {@link java.util.concurrent.Phaser Phaser} and
* {@link java.util.concurrent.Semaphore 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 pending tasks or their
* results when needed. As illustrated below, utility methods
* supporting customization of completion traversals are also
* provided. However, because CountedCompleters provide only basic
* synchronization mechanisms, it may be useful to create further
* abstract subclasses that maintain linkages, fields, and additional
* support methods appropriate for a set of related usages.
*
* <p>A concrete CountedCompleter class must define method {@link
* #compute}, that should in most cases (as illustrated below), invoke
* {@code tryComplete()} once before returning. The class may also
* optionally override method {@link #onCompletion(CountedCompleter)}
* to perform an action upon normal completion, and method
* {@link #onExceptionalCompletion(Throwable, CountedCompleter)} to
* perform an action upon any exception.
*
* <p>CountedCompleters most often do not bear results, in which case
* they are normally declared as {@code CountedCompleter<Void>}, and
* will always return {@code null} as a result value. In other cases,
* you should override method {@link #getRawResult} to provide a
* result from {@code join(), invoke()}, and related methods. In
* general, this method should return the value of a field (or a
* function of one or more fields) of the CountedCompleter object that
* holds the result upon completion. Method {@link #setRawResult} by
* default plays no role in CountedCompleters. It is possible, but
* rarely applicable, to override this method to maintain other
* objects or fields holding result data.
*
* <p>A CountedCompleter that does not itself have a completer (i.e.,
* one for which {@link #getCompleter} returns {@code 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 {@link ForkJoinTask#isDone})
* is arbitrary; this status changes only upon explicit invocations of
* {@link #complete}, {@link ForkJoinTask#cancel},
* {@link ForkJoinTask#completeExceptionally(Throwable)} or upon
* exceptional completion of method {@code 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. Similarly, cancelling an internal
* CountedCompleter has only a local effect on that completer, so is
* not often useful.
*
* <p><b>Sample Usages.</b>
*
* <p><b>Parallel recursive decomposition.</b> CountedCompleters may
* be arranged in trees similar to those often used with {@link
* RecursiveAction}s, although the constructions involved in setting
* them up typically vary. Here, the completer of each task is its
* parent in the computation tree. 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 I/O) or auxiliary effects such as
* garbage collection. Because CountedCompleters provide their own
* continuations, other threads need not block waiting to perform
* them.
*
* <p>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 {@code onCompletion} is not overridden).
* A static utility method sets up the base task and invokes it
* (here, implicitly using the {@link ForkJoinPool#commonPool()}).
*
* <pre> {@code
* class MyOperation<E> { void apply(E e) { ... } }
*
* class ForEach<E> extends CountedCompleter<Void> {
*
* public static <E> void forEach(E[] array, MyOperation<E> op) {
* new ForEach<E>(null, array, op, 0, array.length).invoke();
* }
*
* 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();
* }
* }}</pre>
*
* 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
* {@code tryComplete}) the pending count is set to one:
*
* <pre> {@code
* 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();
* }
* }
* }</pre>
*
* 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. Additionally, because no task
* in this tree implements an {@link #onCompletion(CountedCompleter)} method,
* {@code tryComplete()} can be replaced with {@link #propagateCompletion}.
*
* <pre> {@code
* 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]);
* propagateCompletion();
* }
* }</pre>
*
* 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.
*
* <p><b>Searching.</b> A tree of CountedCompleters can search for a
* value or property in different parts of a data structure, and
* report a result in an {@link
* java.util.concurrent.atomic.AtomicReference AtomicReference} as
* soon as one is found. The others can poll the result to avoid
* unnecessary work. (You could additionally {@linkplain #cancel
* cancel} other tasks, but it is usually simpler and more efficient
* to just let them notice that the result is set and if so skip
* further processing.) Illustrating again with an array using full
* partitioning (again, in practice, leaf tasks will almost always
* process more than one element):
*
* <pre> {@code
* class Searcher<E> extends CountedCompleter<E> {
* final E[] array; final AtomicReference<E> result; final int lo, hi;
* Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
* super(p);
* this.array = array; this.result = result; this.lo = lo; this.hi = hi;
* }
* public E getRawResult() { return result.get(); }
* public void compute() { // similar to ForEach version 3
* int l = lo, h = hi;
* while (result.get() == null && h >= l) {
* if (h - l >= 2) {
* int mid = (l + h) >>> 1;
* addToPendingCount(1);
* new Searcher(this, array, result, mid, h).fork();
* h = mid;
* }
* else {
* E x = array[l];
* if (matches(x) && result.compareAndSet(null, x))
* quietlyCompleteRoot(); // root task is now joinable
* break;
* }
* }
* tryComplete(); // normally complete whether or not found
* }
* boolean matches(E e) { ... } // return true if found
*
* public static <E> E search(E[] array) {
* return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
* }
* }}</pre>
*
* In this example, as well as others in which tasks have no other
* effects except to compareAndSet a common result, the trailing
* unconditional invocation of {@code tryComplete} could be made
* conditional ({@code if (result.get() == null) tryComplete();})
* because no further bookkeeping is required to manage completions
* once the root task completes.
*
* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
* results of multiple subtasks usually need to access these results
* in method {@link #onCompletion(CountedCompleter)}. As illustrated in the following
* class (that performs a simplified form of map-reduce where mappings
* and reductions are all of type {@code 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 {@code onCompletion}.
* This technique applies to reductions in which the order of
* combining left and right results does not matter; ordered
* reductions require explicit left/right designations. Variants of
* other streamlinings seen in the above examples may also apply.
*
* <pre> {@code
* class MyMapper<E> { E apply(E v) { ... } }
* class MyReducer<E> { E apply(E x, E y) { ... } }
* class MapReducer<E> extends CountedCompleter<E> {
* final E[] array; final MyMapper<E> mapper;
* final MyReducer<E> reducer; final int lo, hi;
* MapReducer<E> 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 E getRawResult() { return result; }
*
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
* return new MapReducer<E>(null, array, mapper, reducer,
* 0, array.length).invoke();
* }
* }}</pre>
*
* Here, method {@code onCompletion} takes a form common to many
* completion designs that combine results. This callback-style method
* is triggered once per task, in either of the two different contexts
* in which the pending count is, or becomes, zero: (1) by a task
* itself, if its pending count is zero upon invocation of {@code
* tryComplete}, or (2) by any of its subtasks when they complete and
* decrement the pending count to zero. The {@code caller} argument
* distinguishes cases. Most often, when the caller is {@code this},
* no action is necessary. Otherwise the caller argument can be used
* (usually via a cast) to supply a value (and/or links to other
* values) to be combined. Assuming proper use of pending counts, the
* actions inside {@code onCompletion} occur (once) upon completion of
* a task and its subtasks. No additional synchronization is required
* within this method to ensure thread safety of accesses to fields of
* this task or other completed tasks.
*
* <p><b>Completion Traversals</b>. If using {@code onCompletion} to
* process completions is inapplicable or inconvenient, you can use
* methods {@link #firstComplete} and {@link #nextComplete} to create
* custom traversals. For example, to define a MapReducer that only
* splits out right-hand tasks in the form of the third ForEach
* example, the completions must cooperatively reduce along
* unexhausted subtask links, which can be done as follows:
*
* <pre> {@code
* class MapReducer<E> extends CountedCompleter<E> { // version 2
* final E[] array; final MyMapper<E> mapper;
* final MyReducer<E> reducer; final int lo, hi;
* MapReducer<E> forks, next; // record subtask forks in list
* E result;
* MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
* MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) {
* super(p);
* this.array = array; this.mapper = mapper;
* this.reducer = reducer; this.lo = lo; this.hi = hi;
* this.next = next;
* }
* public void compute() {
* int l = lo, h = hi;
* while (h - l >= 2) {
* int mid = (l + h) >>> 1;
* addToPendingCount(1);
* (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork();
* h = mid;
* }
* if (h > l)
* result = mapper.apply(array[l]);
* // process completions by reducing along and advancing subtask links
* for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
* for (MapReducer t = (MapReducer)c, s = t.forks; s != null; s = t.forks = s.next)
* t.result = reducer.apply(t.result, s.result);
* }
* }
* public E getRawResult() { return result; }
*
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
* return new MapReducer<E>(null, array, mapper, reducer,
* 0, array.length, null).invoke();
* }
* }}</pre>
*
* <p><b>Triggers.</b> Some CountedCompleters are themselves never
* forked, but instead serve as bits of plumbing in other designs;
* including those in which the completion of one or more async tasks
* triggers another async task. For example:
*
* <pre> {@code
* 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();
* }</pre>
*
* @since 1.8
* @author Doug Lea
*/
public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
private static final long serialVersionUID = 5232453752276485070L;
/** This task's completer, or null if none */
final CountedCompleter<?> completer;
/** The number of pending tasks until completion */
volatile int pending;
/**
* Creates a new CountedCompleter with the given completer
* and initial pending count.
*
* @param completer this task's completer, or {@code null} if none
* @param initialPendingCount the initial pending count
*/
protected CountedCompleter(CountedCompleter<?> completer,
int initialPendingCount) {
this.completer = completer;
this.pending = initialPendingCount;
}
/**
* Creates a new CountedCompleter with the given completer
* and an initial pending count of zero.
*
* @param completer this task's completer, or {@code null} if none
*/
protected CountedCompleter(CountedCompleter<?> completer) {
this.completer = completer;
}
/**
* Creates a new CountedCompleter with no completer
* and an initial pending count of zero.
*/
protected CountedCompleter() {
this.completer = null;
}
/**
* The main computation performed by this task.
*/
public abstract void compute();
/**
* Performs an action when method {@link #tryComplete} is invoked
* and the pending count is zero, or when the unconditional
* method {@link #complete} is invoked. By default, this method
* does nothing. You can distinguish cases by checking the
* identity of the given caller argument. If not equal to {@code
* this}, then it is typically a subtask that may contain results
* (and/or links to other results) to combine.
*
* @param caller the task invoking this method (which may
* be this task itself)
*/
public void onCompletion(CountedCompleter<?> caller) {
}
/**
* Performs an action when method {@link
* #completeExceptionally(Throwable)} is invoked or method {@link
* #compute} throws an exception, and this task has not already
* otherwise completed normally. On entry to this method, this task
* {@link ForkJoinTask#isCompletedAbnormally}. The return value
* of this method controls further propagation: If {@code true}
* and this task has a completer that has not completed, then that
* completer is also completed exceptionally, with the same
* exception as this completer. The default implementation of
* this method does nothing except return {@code true}.
*
* @param ex the exception
* @param caller the task invoking this method (which may
* be this task itself)
* @return {@code true} if this exception should be propagated to this
* task's completer, if one exists
*/
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
return true;
}
/**
* Returns the completer established in this task's constructor,
* or {@code null} if none.
*
* @return the completer
*/
public final CountedCompleter<?> getCompleter() {
return completer;
}
/**
* Returns the current pending count.
*
* @return the current pending count
*/
public final int getPendingCount() {
return pending;
}
/**
* Sets the pending count to the given value.
*
* @param count the count
*/
public final void setPendingCount(int count) {
pending = count;
}
/**
* Adds (atomically) the given value to the pending count.
*
* @param delta the value to add
*/
public final void addToPendingCount(int delta) {
U.getAndAddInt(this, PENDING, delta);
}
/**
* Sets (atomically) the pending count to the given count only if
* it currently holds the given expected value.
*
* @param expected the expected value
* @param count the new value
* @return {@code true} if successful
*/
public final boolean compareAndSetPendingCount(int expected, int count) {
return U.compareAndSwapInt(this, PENDING, expected, count);
}
/**
* If the pending count is nonzero, (atomically) decrements it.
*
* @return the initial (undecremented) pending count holding on entry
* to this method
*/
public final int decrementPendingCountUnlessZero() {
int c;
do {} while ((c = pending) != 0 &&
!U.compareAndSwapInt(this, PENDING, c, c - 1));
return c;
}
/**
* Returns the root of the current computation; i.e., this
* task if it has no completer, else its completer's root.
*
* @return the root of the current computation
*/
public final CountedCompleter<?> getRoot() {
CountedCompleter<?> a = this, p;
while ((p = a.completer) != null)
a = p;
return a;
}
/**
* If the pending count is nonzero, decrements the count;
* otherwise invokes {@link #onCompletion(CountedCompleter)}
* and then similarly tries to complete this task's completer,
* if one exists, else marks this task as complete.
*/
public final void tryComplete() {
CountedCompleter<?> a = this, s = a;
for (int c;;) {
if ((c = a.pending) == 0) {
a.onCompletion(s);
if ((a = (s = a).completer) == null) {
s.quietlyComplete();
return;
}
}
else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
return;
}
}
/**
* Equivalent to {@link #tryComplete} but does not invoke {@link
* #onCompletion(CountedCompleter)} along the completion path:
* If the pending count is nonzero, decrements the count;
* otherwise, similarly tries to complete this task's completer, if
* one exists, else marks this task as complete. This method may be
* useful in cases where {@code onCompletion} should not, or need
* not, be invoked for each completer in a computation.
*/
public final void propagateCompletion() {
CountedCompleter<?> a = this, s = a;
for (int c;;) {
if ((c = a.pending) == 0) {
if ((a = (s = a).completer) == null) {
s.quietlyComplete();
return;
}
}
else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
return;
}
}
/**
* Regardless of pending count, invokes
* {@link #onCompletion(CountedCompleter)}, marks this task as
* complete and further triggers {@link #tryComplete} on this
* task's completer, if one exists. The given rawResult is
* used as an argument to {@link #setRawResult} before invoking
* {@link #onCompletion(CountedCompleter)} or marking this task
* as complete; its value is meaningful only for classes
* overriding {@code setRawResult}. This method does not modify
* the pending count.
*
* <p>This method may be useful when forcing completion as soon as
* any one (versus all) of several subtask results are obtained.
* However, in the common (and recommended) case in which {@code
* setRawResult} is not overridden, this effect can be obtained
* more simply using {@code quietlyCompleteRoot();}.
*
* @param rawResult the raw result
*/
public void complete(T rawResult) {
CountedCompleter<?> p;
setRawResult(rawResult);
onCompletion(this);
quietlyComplete();
if ((p = completer) != null)
p.tryComplete();
}
/**
* If this task's pending count is zero, returns this task;
* otherwise decrements its pending count and returns {@code
* null}. This method is designed to be used with {@link
* #nextComplete} in completion traversal loops.
*
* @return this task, if pending count was zero, else {@code null}
*/
public final CountedCompleter<?> firstComplete() {
for (int c;;) {
if ((c = pending) == 0)
return this;
else if (U.compareAndSwapInt(this, PENDING, c, c - 1))
return null;
}
}
/**
* If this task does not have a completer, invokes {@link
* ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if
* the completer's pending count is non-zero, decrements that
* pending count and returns {@code null}. Otherwise, returns the
* completer. This method can be used as part of a completion
* traversal loop for homogeneous task hierarchies:
*
* <pre> {@code
* for (CountedCompleter<?> c = firstComplete();
* c != null;
* c = c.nextComplete()) {
* // ... process c ...
* }}</pre>
*
* @return the completer, or {@code null} if none
*/
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