jsr166y/forkjoin/LinkedAsyncAction.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 *
 */

package jsr166y.forkjoin;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;

/**
 * Resultless ForkJoinTasks with explicit completions, that may be
 * linked in parent-child relationships.  Unlike other kinds of tasks,
 * LinkedAsyncActions do not intrinisically complete upon exit from
 * their <tt>compute</tt> methods, but instead require explicit
 * invocation of their <tt>finish</tt> methods and completion of
 * subtasks.
 *
 * <p> Upon construction, an LinkedAsyncAction may register as a
 * subtask of a given parent task. In this case, completion of this
 * task will propagate to its parent. If the parent's pending subtask
 * completion count becomes zero, it too will finish.
 * LinkedAsyncActions rarely use methods <tt>join</tt> or
 * <tt>invoke</tt> but instead propagate completion to parents
 * implicitly via <tt>finish</tt>.  While typical, it is not necessary
 * for each task to <tt>finish</tt> itself. For example, it is
 * possible to treat one subtask as a continuation of the current task
 * by not registering it on construction.  In this case, a
 * <tt>finish</tt> of the subtask will trigger <tt>finish</tt> of the
 * parent without the parent explicitly doing so.
 *
 * <p> In addition to supporting these different computation styles
 * compared to Recursive tasks, LinkedAsyncActions may have smaller
 * stack space footprints while executing, but may have greater
 * per-task overhead.
 *
 * <p> <b>Sample Usage.</b> Here is a sketch of an LinkedAsyncAction
 * that visits all of the nodes of a graph. The details of the graph's
 * Node and Edge classes are omitted, but we assume each node contains
 * an <tt>AtomicBoolean</tt> mark that starts out false. To execute
 * this, you would create a GraphVisitor for the root node with null
 * parent, and <tt>invoke</tt> in a ForkJoinPool. Upon return, all
 * reachable nodes will have been visited.
 * 
 * <pre>
 * class GraphVisitor extends LinkedAsyncAction {
 *    final Node node;
 *    GraphVisitor(GraphVistor parent, Node node) {
 *      super(parent); this.node = node;
 *    }
 *    protected void compute() {
 *      if (node.mark.compareAndSet(false, true)) {
 *         for (Edge e : node.edges()) {
 *            Node dest = e.getDestination();
 *            if (!dest.mark.get())
 *               new GraphVisitor(this, dest).fork();
 *         }
 *         visit(node);
 *      }
 *      finish();
 *   }
 * }
 * </pre>
 *
 */
public abstract class LinkedAsyncAction extends ForkJoinTask<Void> {
    /**
     * Parent to notify on completion
     */
    private LinkedAsyncAction parent;

    /**
     * Count of outstanding subtask joins
     */
    private volatile int pendingCount;

    private static final AtomicIntegerFieldUpdater<LinkedAsyncAction> pendingCountUpdater =
        AtomicIntegerFieldUpdater.newUpdater(LinkedAsyncAction.class, "pendingCount");

    /**
     * Creates a new action with no parent. (You can add a parent
     * later (but before forking) via <tt>reinitialize</tt>).
     */
    protected LinkedAsyncAction() {
    }

    /**
     * Creates a new action with the given parent. If the parent is
     * non-null, this tasks registers with the parent, in which case,
     * the parent task cannot complete until this task completes.
     * @param parent the parent task, or null if none
     */
    protected LinkedAsyncAction(LinkedAsyncAction parent) {
        this.parent = parent;
        if (parent != null) 
            pendingCountUpdater.incrementAndGet(parent);
    }

    /**
     * Creates a new action with the given parent, optionally
     * registering with the parent. If the parent is non-null and
     * <tt>register</tt> is true, this tasks registers with the
     * parent, in which case, the parent task cannot complete until
     * this task completes.
     * @param parent the parent task, or null if none
     * @param register true if parent must wait for this task
     * to complete before it completes
     */
    protected LinkedAsyncAction(LinkedAsyncAction parent, boolean register) {
        this.parent = parent;
        if (parent != null && register) 
            pendingCountUpdater.incrementAndGet(parent);
    }

    /**
     * Creates a new action with the given parent, optionally
     * registering with the parent, and setting the pending join count
     * to the given value. If the parent is non-null and
     * <tt>register</tt> is true, this tasks registers with the
     * parent, in which case, the parent task cannot complete until
     * this task completes. Setting the pending join count requires
     * care -- it is correct only if child tasks do not themselves
     * register.
     * @param parent the parent task, or null if none
     * @param register true if parent must wait for this task
     * to complete before it completes
     * @param pending the pending join count
     */
    protected LinkedAsyncAction(LinkedAsyncAction parent, 
                          boolean register,
                          int pending) {
        this.parent = parent;
        pendingCount = pending;
        if (parent != null && register) 
            pendingCountUpdater.incrementAndGet(parent);
    }

    /**
     * The asynchronous part of the computation performed by this
     * task.  While you must define this method, you should not in
     * general call it directly (although you can invoke immediately
     * via <tt>exec</tt>.) If this method throws a RuntimeException,
     * <tt>finishExceptionally</tt> is immediately invoked.
     */
    protected abstract void compute();

    /**
     * Overridable callback action triggered by <tt>finish</tt>.  Upon
     * invocation, all subtasks have completed.  After return, this
     * task <tt>isDone</tt> and is joinable by other tasks. The
     * default version of this method does nothing. But it may may be
     * overridden in subclasses to perform some action when this task
     * is about to complete.
     */
    protected void onCompletion() {
    }

    /**
     * Overridable callback action triggered by
     * <tt>finishExceptionally</tt>.  Upon invocation, this task has
     * aborted due to an exception (accessible via
     * <tt>getException</tt>). If this method returns <tt>true</tt>,
     * the exception propagates to the current task's
     * parent. Otherwise, normal completion is propagated.  The
     * default version of this method does nothing and returns
     * <tt>true</tt>.
     * @return true if this task's exception should be propagated to
     * this tasks parent.
     */
    protected boolean onException() {
        return true;
    }

    /**
     * Equivalent to <tt>finish(null)</tt>.
     */
    public final void finish() {
        finish(null);
    }

    /**
     * Completes this task. If the pending subtask completion count is
     * zero, invokes <tt>onCompletion</tt>, then causes this task to
     * be joinable (<tt>isDone</tt> becomes true), and then
     * recursively applies to this tasks's parent, if it exists. If an
     * exception is encountered in any <tt>onCompletion</tt>
     * invocation, that task and its ancestors
     * <tt>finishExceptionally</tt>.
     * 
     * @param result must be <tt>null</tt>.
     */
    public final void finish(Void result) {
        LinkedAsyncAction a = this;
        while (a != null && !a.isDone()) {
            int c = a.pendingCount;
            if (c == 0) {
                try {
                    a.onCompletion();
                } catch (RuntimeException rex) {
                    a.finishExceptionally(rex);
                    return;
                } 
                a.setDone();
                a = a.parent;
            }
            else if (pendingCountUpdater.compareAndSet(a, c, c-1))
                return;
        }
    }

    /**
     * Completes this task abnormally. Unless this task already
     * cancelled or aborted, upon invocation, this method invokes
     * <tt>onException</tt>, and then, depending on its return value,
     * finishes parent (if one exists) exceptionally or normally.  To
     * avoid unbounded exception loops, this method aborts if an
     * exception is encountered in any <tt>onException</tt>
     * invocation.
     * @param ex the exception to throw when joining this task
     * @throws NullPointerException if ex is null
     * @throws RuntimeException if any invocation of
     * <tt>onException</tt> does so.
     */
    public final void finishExceptionally(RuntimeException ex) {
        if (ex == null)
            throw new NullPointerException();
        LinkedAsyncAction a = this;
        while (a.casException(ex)) {
            boolean up = a.onException(); // abort if this throws
            a = a.parent;
            if (a == null)
                return;
            if (!up) {
                a.finish();
                return;
            }
        }
    }

    /**
     * Returns this task's parent, or null if none.
     * @return this task's parent, or null if none.
     */
    public final LinkedAsyncAction getParent() { 
        return parent; 
    }

    /**
     * Returns the number of subtasks that have not yet completed.
     * @return the number of subtasks that have not yet completed.
     */
    public final int getPendingSubtaskCount() { 
        return pendingCount; 
    }

    /**
     * Always returns null.
     * @return null
     */
    public final Void getResult() { 
        return null; 
    }

    /**
     * Resets the internal bookkeeping state of this task, maintaining
     * the current parent but clearing pending joins.
     */
    public void reinitialize() {
        super.reinitialize();
        if (pendingCount != 0)
            pendingCount = 0;
    }

    /**
     * Resets the internal bookkeeping state of this task, maintaining
     * the current parent and setting pending joins to the given value.
     * @param pending the number of pending joins
     */
    public void reinitialize(int pending) {
        super.reinitialize();
        pendingCount = pending;
    }

    /**
     * Reinitialize with the given parent, optionally registering.
     * @param parent the parent task, or null if none
     * @param register true if parent must wait for this task
     * to complete before it completes
     */
    public void reinitialize(LinkedAsyncAction parent, boolean register) {
        super.reinitialize();
        if (pendingCount != 0)
            pendingCount = 0;
        this.parent = parent;
        if (parent != null && register) 
            pendingCountUpdater.incrementAndGet(parent);
    }

    /**
     * Reinitialize with the given parent, optionally registering
     * and setting pending join count.
     * @param parent the parent task, or null if none
     * @param register true if parent must wait for this task
     * to complete before it completes
     * @param pending the pending join count
     */
    public void reinitialize(LinkedAsyncAction parent, 
                             boolean register,
                             int pending) {
        super.reinitialize();
        pendingCount = pending;
        this.parent = parent;
        if (parent != null && register) 
            pendingCountUpdater.incrementAndGet(parent);
    }


    public final Void invoke() {
        exec();
        return join();
    }

    public final RuntimeException exec() {
        if (exception == null) {
            try {
                compute();
            } catch(RuntimeException rex) {
                finishExceptionally(rex);
            }
        }
        return exception;
    }

}
Revision:	1.1
Committed:	Wed Jul 4 23:28:03 2007 UTC (17 years ago) by dl
Branch:	MAIN
Log Message:	PlusScan, expose more Worker functionality
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	*
5	*/
6
7	package jsr166y.forkjoin;
8	import java.util.*;
9	import java.util.concurrent.*;
10	import java.util.concurrent.atomic.*;
11
12	/**
13	* Resultless ForkJoinTasks with explicit completions, that may be
14	* linked in parent-child relationships. Unlike other kinds of tasks,
15	* LinkedAsyncActions do not intrinisically complete upon exit from
16	* their <tt>compute</tt> methods, but instead require explicit
17	* invocation of their <tt>finish</tt> methods and completion of
18	* subtasks.
19	*
20	* <p> Upon construction, an LinkedAsyncAction may register as a
21	* subtask of a given parent task. In this case, completion of this
22	* task will propagate to its parent. If the parent's pending subtask
23	* completion count becomes zero, it too will finish.
24	* LinkedAsyncActions rarely use methods <tt>join</tt> or
25	* <tt>invoke</tt> but instead propagate completion to parents
26	* implicitly via <tt>finish</tt>. While typical, it is not necessary
27	* for each task to <tt>finish</tt> itself. For example, it is
28	* possible to treat one subtask as a continuation of the current task
29	* by not registering it on construction. In this case, a
30	* <tt>finish</tt> of the subtask will trigger <tt>finish</tt> of the
31	* parent without the parent explicitly doing so.
32	*
33	* <p> In addition to supporting these different computation styles
34	* compared to Recursive tasks, LinkedAsyncActions may have smaller
35	* stack space footprints while executing, but may have greater
36	* per-task overhead.
37	*
38	* <p> <b>Sample Usage.</b> Here is a sketch of an LinkedAsyncAction
39	* that visits all of the nodes of a graph. The details of the graph's
40	* Node and Edge classes are omitted, but we assume each node contains
41	* an <tt>AtomicBoolean</tt> mark that starts out false. To execute
42	* this, you would create a GraphVisitor for the root node with null
43	* parent, and <tt>invoke</tt> in a ForkJoinPool. Upon return, all
44	* reachable nodes will have been visited.
45	*
46	* <pre>
47	* class GraphVisitor extends LinkedAsyncAction {
48	* final Node node;
49	* GraphVisitor(GraphVistor parent, Node node) {
50	* super(parent); this.node = node;
51	* }
52	* protected void compute() {
53	* if (node.mark.compareAndSet(false, true)) {
54	* for (Edge e : node.edges()) {
55	* Node dest = e.getDestination();
56	* if (!dest.mark.get())
57	* new GraphVisitor(this, dest).fork();
58	* }
59	* visit(node);
60	* }
61	* finish();
62	* }
63	* }
64	* </pre>
65	*
66	*/
67	public abstract class LinkedAsyncAction extends ForkJoinTask<Void> {
68	/**
69	* Parent to notify on completion
70	*/
71	private LinkedAsyncAction parent;
72
73	/**
74	* Count of outstanding subtask joins
75	*/
76	private volatile int pendingCount;
77
78	private static final AtomicIntegerFieldUpdater<LinkedAsyncAction> pendingCountUpdater =
79	AtomicIntegerFieldUpdater.newUpdater(LinkedAsyncAction.class, "pendingCount");
80
81	/**
82	* Creates a new action with no parent. (You can add a parent
83	* later (but before forking) via <tt>reinitialize</tt>).
84	*/
85	protected LinkedAsyncAction() {
86	}
87
88	/**
89	* Creates a new action with the given parent. If the parent is
90	* non-null, this tasks registers with the parent, in which case,
91	* the parent task cannot complete until this task completes.
92	* @param parent the parent task, or null if none
93	*/
94	protected LinkedAsyncAction(LinkedAsyncAction parent) {
95	this.parent = parent;
96	if (parent != null)
97	pendingCountUpdater.incrementAndGet(parent);
98	}
99
100	/**
101	* Creates a new action with the given parent, optionally
102	* registering with the parent. If the parent is non-null and
103	* <tt>register</tt> is true, this tasks registers with the
104	* parent, in which case, the parent task cannot complete until
105	* this task completes.
106	* @param parent the parent task, or null if none
107	* @param register true if parent must wait for this task
108	* to complete before it completes
109	*/
110	protected LinkedAsyncAction(LinkedAsyncAction parent, boolean register) {
111	this.parent = parent;
112	if (parent != null && register)
113	pendingCountUpdater.incrementAndGet(parent);
114	}
115
116	/**
117	* Creates a new action with the given parent, optionally
118	* registering with the parent, and setting the pending join count
119	* to the given value. If the parent is non-null and
120	* <tt>register</tt> is true, this tasks registers with the
121	* parent, in which case, the parent task cannot complete until
122	* this task completes. Setting the pending join count requires
123	* care -- it is correct only if child tasks do not themselves
124	* register.
125	* @param parent the parent task, or null if none
126	* @param register true if parent must wait for this task
127	* to complete before it completes
128	* @param pending the pending join count
129	*/
130	protected LinkedAsyncAction(LinkedAsyncAction parent,
131	boolean register,
132	int pending) {
133	this.parent = parent;
134	pendingCount = pending;
135	if (parent != null && register)
136	pendingCountUpdater.incrementAndGet(parent);
137	}
138
139	/**
140	* The asynchronous part of the computation performed by this
141	* task. While you must define this method, you should not in
142	* general call it directly (although you can invoke immediately
143	* via <tt>exec</tt>.) If this method throws a RuntimeException,
144	* <tt>finishExceptionally</tt> is immediately invoked.
145	*/
146	protected abstract void compute();
147
148	/**
149	* Overridable callback action triggered by <tt>finish</tt>. Upon
150	* invocation, all subtasks have completed. After return, this
151	* task <tt>isDone</tt> and is joinable by other tasks. The
152	* default version of this method does nothing. But it may may be
153	* overridden in subclasses to perform some action when this task
154	* is about to complete.
155	*/
156	protected void onCompletion() {
157	}
158
159	/**
160	* Overridable callback action triggered by
161	* <tt>finishExceptionally</tt>. Upon invocation, this task has
162	* aborted due to an exception (accessible via
163	* <tt>getException</tt>). If this method returns <tt>true</tt>,
164	* the exception propagates to the current task's
165	* parent. Otherwise, normal completion is propagated. The
166	* default version of this method does nothing and returns
167	* <tt>true</tt>.
168	* @return true if this task's exception should be propagated to
169	* this tasks parent.
170	*/
171	protected boolean onException() {
172	return true;
173	}
174
175	/**
176	* Equivalent to <tt>finish(null)</tt>.
177	*/
178	public final void finish() {
179	finish(null);
180	}
181
182	/**
183	* Completes this task. If the pending subtask completion count is
184	* zero, invokes <tt>onCompletion</tt>, then causes this task to
185	* be joinable (<tt>isDone</tt> becomes true), and then
186	* recursively applies to this tasks's parent, if it exists. If an
187	* exception is encountered in any <tt>onCompletion</tt>
188	* invocation, that task and its ancestors
189	* <tt>finishExceptionally</tt>.
190	*
191	* @param result must be <tt>null</tt>.
192	*/
193	public final void finish(Void result) {
194	LinkedAsyncAction a = this;
195	while (a != null && !a.isDone()) {
196	int c = a.pendingCount;
197	if (c == 0) {
198	try {
199	a.onCompletion();
200	} catch (RuntimeException rex) {
201	a.finishExceptionally(rex);
202	return;
203	}
204	a.setDone();
205	a = a.parent;
206	}
207	else if (pendingCountUpdater.compareAndSet(a, c, c-1))
208	return;
209	}
210	}
211
212	/**
213	* Completes this task abnormally. Unless this task already
214	* cancelled or aborted, upon invocation, this method invokes
215	* <tt>onException</tt>, and then, depending on its return value,
216	* finishes parent (if one exists) exceptionally or normally. To
217	* avoid unbounded exception loops, this method aborts if an
218	* exception is encountered in any <tt>onException</tt>
219	* invocation.
220	* @param ex the exception to throw when joining this task
221	* @throws NullPointerException if ex is null
222	* @throws RuntimeException if any invocation of
223	* <tt>onException</tt> does so.
224	*/
225	public final void finishExceptionally(RuntimeException ex) {
226	if (ex == null)
227	throw new NullPointerException();
228	LinkedAsyncAction a = this;
229	while (a.casException(ex)) {
230	boolean up = a.onException(); // abort if this throws
231	a = a.parent;
232	if (a == null)
233	return;
234	if (!up) {
235	a.finish();
236	return;
237	}
238	}
239	}
240
241	/**
242	* Returns this task's parent, or null if none.
243	* @return this task's parent, or null if none.
244	*/
245	public final LinkedAsyncAction getParent() {
246	return parent;
247	}
248
249	/**
250	* Returns the number of subtasks that have not yet completed.
251	* @return the number of subtasks that have not yet completed.
252	*/
253	public final int getPendingSubtaskCount() {
254	return pendingCount;
255	}
256
257	/**
258	* Always returns null.
259	* @return null
260	*/
261	public final Void getResult() {
262	return null;
263	}
264
265	/**
266	* Resets the internal bookkeeping state of this task, maintaining
267	* the current parent but clearing pending joins.
268	*/
269	public void reinitialize() {
270	super.reinitialize();
271	if (pendingCount != 0)
272	pendingCount = 0;
273	}
274
275	/**
276	* Resets the internal bookkeeping state of this task, maintaining
277	* the current parent and setting pending joins to the given value.
278	* @param pending the number of pending joins
279	*/
280	public void reinitialize(int pending) {
281	super.reinitialize();
282	pendingCount = pending;
283	}
284
285	/**
286	* Reinitialize with the given parent, optionally registering.
287	* @param parent the parent task, or null if none
288	* @param register true if parent must wait for this task
289	* to complete before it completes
290	*/
291	public void reinitialize(LinkedAsyncAction parent, boolean register) {
292	super.reinitialize();
293	if (pendingCount != 0)
294	pendingCount = 0;
295	this.parent = parent;
296	if (parent != null && register)
297	pendingCountUpdater.incrementAndGet(parent);
298	}
299
300	/**
301	* Reinitialize with the given parent, optionally registering
302	* and setting pending join count.
303	* @param parent the parent task, or null if none
304	* @param register true if parent must wait for this task
305	* to complete before it completes
306	* @param pending the pending join count
307	*/
308	public void reinitialize(LinkedAsyncAction parent,
309	boolean register,
310	int pending) {
311	super.reinitialize();
312	pendingCount = pending;
313	this.parent = parent;
314	if (parent != null && register)
315	pendingCountUpdater.incrementAndGet(parent);
316	}
317
318
319	public final Void invoke() {
320	exec();
321	return join();
322	}
323
324	public final RuntimeException exec() {
325	if (exception == null) {
326	try {
327	compute();
328	} catch(RuntimeException rex) {
329	finishExceptionally(rex);
330	}
331	}
332	return exception;
333	}
334
335	}