Re: [henshin-user] Problem with NullPointerException in ChangeImpl.apply

[Daniel Strüber <strueber@xxxxxxxxxxxxxxxxxxxxxxxxx>]

Hi Joel,

Am 24.07.2016 um 04:15 schrieb Joel Greenyer:
> Hi all,
> I have a problem with a NullPointerException in 
> ChangeImpl.applyAndReverse().
> We use Henshin within our tool where execute and undo a rule. The 
> problem occurs during undo of a rule in which an attribute is set. See 
> stack trace below.
>
> The problem is that the field "attribute" is NULL when executing 
> "oldValue = object.eGet(attribute);" (line 4 of snippet below)
>
> 01        public void applyAndReverse() {
> 02            // Need to initialize?
> 03            if (!initialized) {
> 04                oldValue = object.eGet(attribute);
> 05                if ((oldValue==null && newValue==null) ||
> 06                    (oldValue!=null && oldValue.equals(newValue))) {
> 07                    attribute = null;
> 08                }
> 09            }
> 10            // Nothing to do?
> 11            if (attribute==null) {
> 12                return;
> 13            }
> 14            ...
>
> I think that the undo causes a second invocation of a change's 
> applyAndReverse() where the first invocation resulted in setting 
> "attribute = null;" (line 7)
>
> I'm not sure what the reason is to set "attribute" to null,
This looks a bit odd to me as well. My guess would be that setting 
"attribute" to null is used to permanently mark the change as an empty 
one, but then, this doesn't sound particularly clean, and also the 
handling of undo is broken, as you have pointed out.

> but it seems that when it is null it means that there is nothing to 
> change and reverse (line 10).
> So what works for me is changing line 4 to
>
> 04            if (!initialized && attribute != null) {
Makes sense to me. Can you commit a change to Gerrit (see [1] for brief 
explanations)? Otherwise, I can do so as well.

[1] https://wiki.eclipse.org/Gerrit#To_create_a_new_change

Regards,
Daniel

> Best regards
>
> Joel
>
>
>
>
> !ENTRY org.eclipse.core.jobs 4 2 2016-07-24 03:57:55.506
> !MESSAGE An internal error occurred during: "On-the-fly Controller 
> Synthesis from SML specification".
> !STACK 0
> java.lang.NullPointerException
>     at 
> org.eclipse.emf.ecore.impl.EClassImpl.getFeatureID(EClassImpl.java:1500)
>     at 
> org.eclipse.emf.ecore.impl.DynamicEObjectImpl.eDerivedStructuralFeatureID(DynamicEObjectImpl.java:218)
>     at 
> org.eclipse.emf.ecore.impl.BasicEObjectImpl.eGet(BasicEObjectImpl.java:1008)
>     at 
> org.eclipse.emf.ecore.impl.BasicEObjectImpl.eGet(BasicEObjectImpl.java:1003)
>     at 
> org.eclipse.emf.ecore.impl.BasicEObjectImpl.eGet(BasicEObjectImpl.java:998)
>     at 
> org.eclipse.emf.henshin.interpreter.impl.ChangeImpl$AttributeChangeImpl.applyAndReverse(ChangeImpl.java:134)
>     at 
> org.eclipse.emf.henshin.interpreter.impl.ChangeImpl$CompoundChangeImpl.applyAndReverse(ChangeImpl.java:466)
>     at 
> org.eclipse.emf.henshin.interpreter.impl.RuleApplicationImpl.undo(RuleApplicationImpl.java:126)
>     at 
> org.scenariotools.sml.runtime.henshin.logic.HenshinMessageEventsSideEffectsExecutorLogic.canExecuteSideEffects(HenshinMessageEventsSideEffectsExecutorLogic.java:133)
>     ...
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--
Dr. rer. nat. Daniel Strüber, Dipl.-Inf.
Software Engineering Research Group
Philipps-Universität Marburg, Germany