/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
*
*/
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
/**
* AsyncActions that are always linked in binary parent-child
* relationships. Compared to Recursive tasks, BinaryAsyncActions may
* have smaller stack space footprints and faster completion mechanics
* but higher per-task footprints. Compared to LinkedAsyncActions,
* BinaryAsyncActions are simpler to use and have less overhead in
* typical uasges but are restricted to binary computation trees.
*
*
Upon construction, an BinaryAsyncAction does not bear any
* linkages. For non-root tasks, links must be established using
* method linkSubtasks before use.
*
*
Sample Usage. A version of Fibonacci:
*
* class Fib extends BinaryAsyncAction {
* final int n;
* int result;
* Fib(int n) { this.n = n; }
* protected void compute() {
* if (n > 1) {
* linkAndForkSubtasks(new Fib(n-1), new Fib(n-2));
* else {
* result = n; // fib(0)==0; fib(1)==1
* complete();
* }
* }
* protected void onComplete(BinaryAsyncAction x, BinaryAsyncAction y) {
* result = ((Fib)x).result + ((Fib)y).result;
* }
* }
*
* An alternative, and usually faster strategy is to instead use a
* loop to fork subtasks:
*
* protected void compute() {
* Fib f = this;
* while (f.n > 1) {
* Fib left = new Fib(f.n - 1);
* Fib right = new Fib(f.n - 2);
* f.linkSubtasks(left, right);
* right.fork(); // fork right
* f = left; // loop on left
* }
* f.result = f.n;
* f.complete();
* }
* }
*
*/
public abstract class BinaryAsyncAction extends ForkJoinTask {
private volatile int controlState;
static final AtomicIntegerFieldUpdater controlStateUpdater =
AtomicIntegerFieldUpdater.newUpdater(BinaryAsyncAction.class, "controlState");
/**
* Parent to propagate completion; nulled after completion to
* avoid retaining entire tree as garbage
*/
private BinaryAsyncAction parent;
/**
* Sibling to access on subtask joins, also nulled after completion.
*/
private BinaryAsyncAction sibling;
/**
* Creates a new action. Unless this is a root task, you will need
* to link it using method linkSubtasks before forking as
* a subtask.
*/
protected BinaryAsyncAction() {
}
public final Void getRawResult() { return null; }
protected final void setRawResult(Void mustBeNull) { }
/**
* Establishes links for the given tasks to have the current task
* as parent, and each other as siblings.
* @param x one subtask
* @param y the other subtask
* @throws NullPointerException if either argument is null.
*/
public final void linkSubtasks(BinaryAsyncAction x, BinaryAsyncAction y) {
x.parent = y.parent = this;
x.sibling = y;
y.sibling = x;
}
/**
* Overridable callback action triggered upon complete of
* subtasks. Upon invocation, both subtasks have completed.
* After return, this task isDone 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 (for example a reduction) when this task is
* completes.
* @param x one subtask
* @param y the other subtask
*/
protected void onComplete(BinaryAsyncAction x, BinaryAsyncAction y) {
}
/**
* Overridable callback action triggered by
* completeExceptionally. Upon invocation, this task has
* aborted due to an exception (accessible via
* getException). If this method returns true,
* the exception propagates to the current task's
* parent. Otherwise, normal completion is propagated. The
* default version of this method does nothing and returns
* true.
* @return true if this task's exception should be propagated to
* this tasks parent.
*/
protected boolean onException() {
return true;
}
/**
* Equivalent in effect to invoking linkSubtasks and then
* forking both tasks.
* @param x one subtask
* @param y the other subtask
*/
public void linkAndForkSubtasks(BinaryAsyncAction x, BinaryAsyncAction y) {
linkSubtasks(x, y);
y.fork();
x.fork();
}
/** Basic per-task complete */
private void completeThis() {
super.complete(null);
}
/** Basic per-task completeExceptionally */
private void completeThisExceptionally(Throwable ex) {
super.completeExceptionally(ex);
}
/*
* We use one bit join count on taskState. The first arriving
* thread CAS's from 0 to 1. The second ultimately sets status
* to signify completion.
*/
/**
* Completes this task, and if this task has a sibling that is
* also complete, invokes onComplete of parent task, and so
* on. If an exception is encountered, tasks instead
* completeExceptionally.
*/
public final void complete() {
// todo: Use tryUnfork without possibly blowing stack
BinaryAsyncAction a = this;
for (;;) {
BinaryAsyncAction s = a.sibling;
BinaryAsyncAction p = a.parent;
a.sibling = null;
a.parent = null;
a.completeThis();
if (p == null || p.compareAndSetControlState(0, 1))
break;
try {
p.onComplete(a, s);
} catch(Throwable rex) {
p.completeExceptionally(rex);
return;
}
a = p;
}
}
/**
* Completes this task abnormally. Unless this task already
* cancelled or aborted, upon invocation, this method invokes
* onException, and then, depending on its return value,
* completees parent (if one exists) exceptionally or normally. To
* avoid unbounded exception loops, this method aborts if an
* exception is encountered in any onException
* invocation.
* @param ex the exception to throw when joining this task
* @throws NullPointerException if ex is null
* @throws Throwable if any invocation of
* onException does so.
*/
public final void completeExceptionally(Throwable ex) {
BinaryAsyncAction a = this;
while (!a.isCompletedAbnormally()) {
a.completeThisExceptionally(ex);
BinaryAsyncAction s = a.sibling;
if (s != null)
s.cancel(false);
if (!a.onException() || (a = a.parent) == null)
break;
}
}
/**
* Returns this task's parent, or null if none or this task
* is already complete.
* @return this task's parent, or null if none.
*/
public final BinaryAsyncAction getParent() {
return parent;
}
/**
* Returns this task's sibling, or null if none or this task is
* already complete.
* @return this task's sibling, or null if none.
*/
public BinaryAsyncAction getSibling() {
return sibling;
}
/**
* Resets the internal bookkeeping state of this task, erasing
* parent and child linkages.
*/
public void reinitialize() {
parent = sibling = null;
super.reinitialize();
}
/**
* Gets the control state, which is initially zero, or negative if
* this task has completed or cancelled. Once negative, the value
* cannot be changed.
* @return control state
*/
protected final int getControlState() {
return controlState;
}
/**
* Atomically sets the control state to the given updated value if
* the current value is and equal to the expected value.
* @param expect the expected value
* @param update the new value
* @return true if successful
*/
protected final boolean compareAndSetControlState(int expect,
int update) {
return controlStateUpdater.compareAndSet(this, expect, update);
}
/**
* Attempts to set the control state to the given value, failing if
* this task is already completed or the control state value would be
* negative.
* @param value the new value
* @return true if successful
*/
protected final void setControlState(int value) {
controlState = value;
}
/**
* Sets the control state to the given value,
* @param value the new value
*/
protected final void incrementControlState() {
controlStateUpdater.incrementAndGet(this);
}
/**
* Decrement the control state
* @return true if successful
*/
protected final void decrementControlState() {
controlStateUpdater.decrementAndGet(this);
}
}