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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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import java.util.*; |
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/** |
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* AsyncActions that may be linked in parent-child relationships. |
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* |
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* <p> Upon construction, an LinkedAsyncAction may register as a |
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* <p>Upon construction, a LinkedAsyncAction may register as a |
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* subtask of a given parent task. In this case, completion of this |
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* task will propagate to its parent. If the parent's pending subtask |
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* completion count becomes zero, it too will complete. |
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* LinkedAsyncActions rarely use methods <tt>join</tt> or |
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* <tt>invoke</tt> but instead propagate completion to parents |
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* implicitly via <tt>complete</tt>. While typical, it is not necessary |
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* for each task to <tt>complete</tt> itself. For example, it is |
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* LinkedAsyncActions rarely use methods {@code join} or |
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* {@code invoke} but instead propagate completion to parents |
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* implicitly via {@code complete}. While typical, it is not necessary |
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* for each task to {@code complete} itself. For example, it is |
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* possible to treat one subtask as a continuation of the current task |
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* by not registering it on construction. In this case, a |
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* <tt>complete</tt> of the subtask will trigger <tt>complete</tt> of the |
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* {@code complete} of the subtask will trigger {@code complete} of the |
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* parent without the parent explicitly doing so. |
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* |
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* <p> In addition to supporting these different computation styles |
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* <p>In addition to supporting these different computation styles |
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* compared to Recursive tasks, LinkedAsyncActions may have smaller |
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* stack space footprints while executing, but may have greater |
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* per-task overhead. |
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* |
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* <p> <b>Sample Usage.</b> Here is a sketch of an LinkedAsyncAction |
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* <p><b>Sample Usage.</b> Here is a sketch of a LinkedAsyncAction |
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* that visits all of the nodes of a graph. The details of the graph's |
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* Node and Edge classes are omitted, but we assume each node contains |
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* an <tt>AtomicBoolean</tt> mark that starts out false. To execute |
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* an {@code AtomicBoolean} mark that starts out false. To execute |
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* this, you would create a GraphVisitor for the root node with null |
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* parent, and <tt>invoke</tt> in a ForkJoinPool. Upon return, all |
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* parent, and {@code invoke} in a ForkJoinPool. Upon return, all |
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* reachable nodes will have been visited. |
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* |
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* <pre> |
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* class GraphVisitor extends LinkedAsyncAction { |
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* final Node node; |
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* GraphVisitor(GraphVistor parent, Node node) { |
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* super(parent); this.node = node; |
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* } |
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* protected void compute() { |
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* if (node.mark.compareAndSet(false, true)) { |
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* for (Edge e : node.edges()) { |
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* Node dest = e.getDestination(); |
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* if (!dest.mark.get()) |
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* new GraphVisitor(this, dest).fork(); |
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* } |
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* visit(node); |
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* } |
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* complete(); |
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* final Node node; |
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* GraphVisitor(GraphVisitor parent, Node node) { |
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* super(parent); this.node = node; |
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* } |
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* protected void compute() { |
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* if (node.mark.compareAndSet(false, true)) { |
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* for (Edge e : node.edges()) { |
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* Node dest = e.getDestination(); |
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* if (!dest.mark.get()) |
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* new GraphVisitor(this, dest).fork(); |
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* } |
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* visit(node); |
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* } |
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* complete(); |
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* } |
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* } |
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* </pre> |
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* |
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*/ |
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public abstract class LinkedAsyncAction extends ForkJoinTask<Void> { |
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static final AtomicIntegerFieldUpdater<LinkedAsyncAction> controlStateUpdater = |
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AtomicIntegerFieldUpdater.newUpdater(LinkedAsyncAction.class, "controlState"); |
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/** |
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* Parent to notify on completion |
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*/ |
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/** |
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* Creates a new action with no parent. (You can add a parent |
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* later (but before forking) via <tt>reinitialize</tt>). |
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* later (but before forking) via {@code reinitialize}). |
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*/ |
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protected LinkedAsyncAction() { |
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} |
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/** |
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* Creates a new action with the given parent. If the parent is |
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* non-null, this tasks registers with the parent, in which case, |
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* non-null, this task registers with the parent, in which case, |
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* the parent task cannot complete until this task completes. |
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* @param parent the parent task, or null if none |
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*/ |
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/** |
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* Creates a new action with the given parent, optionally |
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* registering with the parent. If the parent is non-null and |
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* <tt>register</tt> is true, this tasks registers with the |
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* {@code register} is true, this task registers with the |
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* parent, in which case, the parent task cannot complete until |
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* this task completes. |
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* @param parent the parent task, or null if none |
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* Creates a new action with the given parent, optionally |
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* registering with the parent, and setting the pending join count |
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* to the given value. If the parent is non-null and |
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* <tt>register</tt> is true, this tasks registers with the |
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* {@code register} is true, this task registers with the |
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* parent, in which case, the parent task cannot complete until |
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* this task completes. Setting the pending join count requires |
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* care -- it is correct only if child tasks do not themselves |
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protected final void setRawResult(Void mustBeNull) { } |
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/** |
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* Overridable callback action triggered by <tt>complete</tt>. Upon |
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* Overridable callback action triggered by {@code complete}. Upon |
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* invocation, all subtasks have completed. After return, this |
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* task <tt>isDone</tt> and is joinable by other tasks. The |
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* task {@code isDone} and is joinable by other tasks. The |
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* default version of this method does nothing. But it may be |
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* overridden in subclasses to perform some action when this task |
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* is about to complete. |
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/** |
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* Overridable callback action triggered by |
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* <tt>completeExceptionally</tt>. Upon invocation, this task has |
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* {@code completeExceptionally}. Upon invocation, this task has |
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* aborted due to an exception (accessible via |
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* <tt>getException</tt>). If this method returns <tt>true</tt>, |
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* {@code getException}). If this method returns {@code true}, |
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* the exception propagates to the current task's |
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* parent. Otherwise, normal completion is propagated. The |
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* default version of this method does nothing and returns |
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* <tt>true</tt>. |
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* {@code true}. |
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* @return true if this task's exception should be propagated to |
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* this tasks parent. |
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* this task's parent |
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*/ |
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protected boolean onException() { |
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return true; |
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/** |
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* Completes this task. If the pending subtask completion count is |
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* zero, invokes <tt>onCompletion</tt>, then causes this task to |
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* be joinable (<tt>isDone</tt> becomes true), and then |
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* recursively applies to this tasks's parent, if it exists. If an |
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* exception is encountered in any <tt>onCompletion</tt> |
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* zero, invokes {@code onCompletion}, then causes this task to |
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* be joinable ({@code isDone} becomes true), and then |
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* recursively applies to this task's parent, if it exists. If an |
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* exception is encountered in any {@code onCompletion} |
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* invocation, that task and its ancestors |
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* <tt>completeExceptionally</tt>. |
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* {@code completeExceptionally}. |
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*/ |
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public final void complete() { |
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LinkedAsyncAction a = this; |
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/** |
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* Completes this task abnormally. Unless this task already |
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* cancelled or aborted, upon invocation, this method invokes |
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* <tt>onException</tt>, and then, depending on its return value, |
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* completees parent (if one exists) exceptionally or normally. To |
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* {@code onException}, and then, depending on its return value, |
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* completes parent (if one exists) exceptionally or normally. To |
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* avoid unbounded exception loops, this method aborts if an |
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* exception is encountered in any <tt>onException</tt> |
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* exception is encountered in any {@code onException} |
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* invocation. |
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* @param ex the exception to throw when joining this task |
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* @throws NullPointerException if ex is null |
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* @throws Throwable if any invocation of |
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* <tt>onException</tt> does so. |
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* {@code onException} does so |
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*/ |
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public final void completeExceptionally(Throwable ex) { |
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LinkedAsyncAction a = this; |
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/** |
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* Returns this task's parent, or null if none. |
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* @return this task's parent, or null if none. |
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* @return this task's parent, or null if none |
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*/ |
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public final LinkedAsyncAction getParent() { |
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return parent; |
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/** |
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* Returns the number of subtasks that have not yet completed. |
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* @return the number of subtasks that have not yet completed. |
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* @return the number of subtasks that have not yet completed |
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*/ |
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public final int getPendingSubtaskCount() { |
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return getControlState(); |
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controlStateUpdater.decrementAndGet(this); |
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} |
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– |
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} |
