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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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*/ |
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|
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import java.util.*; |
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import java.util.concurrent.*; |
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import java.util.concurrent.atomic.*; |
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|
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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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* 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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* 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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* 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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* 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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* 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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* 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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* 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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* } |
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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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|
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/** |
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* The maximum number of pending children |
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*/ |
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private static int maxPending = 0xffff; |
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|
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private volatile int controlState; |
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|
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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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|
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/** |
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* Parent to notify on completion |
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*/ |
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private LinkedAsyncAction parent; |
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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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*/ |
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protected LinkedAsyncAction() { |
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} |
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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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* 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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protected LinkedAsyncAction(LinkedAsyncAction parent) { |
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this.parent = parent; |
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if (parent != null) |
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parent.incrementControlState(); |
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} |
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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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* 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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* @param register true if parent must wait for this task |
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* to complete before it completes |
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*/ |
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protected LinkedAsyncAction(LinkedAsyncAction parent, boolean register) { |
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this.parent = parent; |
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if (parent != null && register) |
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parent.incrementControlState(); |
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} |
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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, 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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* 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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* register. |
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* @param parent the parent task, or null if none |
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* @param register true if parent must wait for this task |
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* to complete before it completes |
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* @param pending the pending join count |
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*/ |
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protected LinkedAsyncAction(LinkedAsyncAction parent, |
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boolean register, |
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int pending) { |
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this.parent = parent; |
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setControlState(pending); |
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if (parent != null && register) |
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parent.incrementControlState(); |
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} |
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|
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public final Void getRawResult() { return null; } |
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protected final void setRawResult(Void mustBeNull) { } |
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|
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/** |
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* Overridable callback action triggered by <tt>complete</tt>. 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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* default version of this method does nothing. But it may 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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protected void onCompletion() { |
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} |
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|
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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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* 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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* 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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* @return true if this task's exception should be propagated to |
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* this tasks 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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|
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/** Basic per-task complete */ |
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private void completeThis() { |
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complete(null); |
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} |
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|
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/** Basic per-task completeExceptionally */ |
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private void completeThisExceptionally(Throwable ex) { |
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super.completeExceptionally(ex); |
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} |
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|
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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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* invocation, that task and its ancestors |
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* <tt>completeExceptionally</tt>. |
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*/ |
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public final void complete() { |
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LinkedAsyncAction a = this; |
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while (a != null && !a.isDone()) { |
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int c = a.getControlState(); |
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if (c <= 0) { |
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try { |
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a.onCompletion(); |
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a.completeThis(); |
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} catch (Throwable rex) { |
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a.completeExceptionally(rex); |
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return; |
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} |
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a = a.parent; |
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} |
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else if (a.compareAndSetControlState(c, c-1)) |
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return; |
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} |
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} |
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|
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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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* 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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* 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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*/ |
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public final void completeExceptionally(Throwable ex) { |
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LinkedAsyncAction a = this; |
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while (!a.isCompletedAbnormally()) { |
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a.completeThisExceptionally(ex); |
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boolean up = a.onException(); // abort if this throws |
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a = a.parent; |
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if (a == null) |
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break; |
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if (!up) { |
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a.complete(); |
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break; |
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} |
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} |
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} |
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|
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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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*/ |
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public final LinkedAsyncAction getParent() { |
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return parent; |
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} |
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|
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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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*/ |
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public final int getPendingSubtaskCount() { |
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return getControlState(); |
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} |
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|
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/** |
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* Resets the internal bookkeeping state of this task, maintaining |
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* the current parent but clearing pending joins. |
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*/ |
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public void reinitialize() { |
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super.reinitialize(); |
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} |
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|
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/** |
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* Resets the internal bookkeeping state of this task, maintaining |
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* the current parent and setting pending joins to the given value. |
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* @param pending the number of pending joins |
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*/ |
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public void reinitialize(int pending) { |
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super.reinitialize(); |
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setControlState(pending); |
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} |
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|
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/** |
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* Reinitialize with the given parent, optionally registering. |
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* @param parent the parent task, or null if none |
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* @param register true if parent must wait for this task |
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* to complete before it completes |
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*/ |
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public void reinitialize(LinkedAsyncAction parent, boolean register) { |
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this.parent = parent; |
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super.reinitialize(); |
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if (parent != null && register) |
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parent.incrementControlState(); |
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} |
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|
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/** |
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* Reinitialize with the given parent, optionally registering |
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* and setting pending join count. |
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* @param parent the parent task, or null if none |
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* @param register true if parent must wait for this task |
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* to complete before it completes |
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* @param pending the pending join count |
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*/ |
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public void reinitialize(LinkedAsyncAction parent, |
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boolean register, |
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int pending) { |
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this.parent = parent; |
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super.reinitialize(); |
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setControlState(pending); |
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if (parent != null && register) |
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parent.incrementControlState(); |
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} |
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|
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/** |
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* Gets the control state, which is initially zero, or negative if |
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* this task has completed or cancelled. Once negative, the value |
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* cannot be changed. |
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* @return control state |
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*/ |
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protected final int getControlState() { |
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return controlState; |
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} |
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|
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/** |
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* Atomically sets the control state to the given updated value if |
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* the current value is and equal to the expected value. |
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* @param expect the expected value |
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* @param update the new value |
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* @return true if successful |
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*/ |
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protected final boolean compareAndSetControlState(int expect, |
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int update) { |
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return controlStateUpdater.compareAndSet(this, expect, update); |
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} |
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|
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/** |
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* Sets the control state to the given value, |
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* @param value the new value |
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*/ |
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protected final void setControlState(int value) { |
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controlState = value; |
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} |
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|
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/** |
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* Increment the control state |
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* @return true if successful |
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*/ |
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protected final void incrementControlState() { |
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controlStateUpdater.incrementAndGet(this); |
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} |
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|
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/** |
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* Decrement the control state |
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* @return true if successful |
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*/ |
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protected final void decrementControlState() { |
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controlStateUpdater.decrementAndGet(this); |
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} |
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|
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|
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} |
