How to register custom PropertyAccessExceptionHandler implementation?

[donhaldo]

Hello,
trying to migrate from 0.13.0 version to 0.91 I'm able to set a custom CircularReferenceExceptionHandler using CircularReferenceConfigurer
but i didn't found a way to set my custom PropertyAccessExceptionHandler
on anyone of the available ObjectDifferBuilder configurers.
Is it work in progress?

Many thanks in advance
Roberto

[SQiShER]

Oh my, this actually fell victim to the 0.90 rewrite by accident. Currently the API doesn't provide a way to configure this. I'm already working on bringing it back, though.

Shouldn't take too long. Although I'm currently wondering whether the onPropertyWriteException handler method is really necessary, because such an exception can only occur when someone invokes DiffNode.canonicalSet or DiffNode.set, so one can simply catch the PropertyWriteException.

May I ask what your implementation does? Do you use the DiffNode that is passed to the handler methods? Because I'm thinking about removing it from the signature and just pass in the exception.

[donhaldo]

Hello,
when i first implemented an exception handler for the 0.13.0 release of java-object-diff i was fighting with lot of exceptions raised when comparing Hibernate Envers revision entities: i needed the proxied objects to get lazy associated entities to compare, so i couldn't detach them from session but, at the same time, the diffing tree invariably reached proxy attributes that raised errors when called to be compared.
The exception messages were not usefull to clearly identify failing attributes because the complete node paths was missing in the exception message, so i wrote a listener (originally a single interface for both circular and read/write errors) to log offending node paths.
In the meantime i reached a quite stable solution using attributes exclusion both with builder exclusions configuration and annotating entities with @ObjectDiffProperty, even if i don't like very much the annotations which pollute my persistent model with diff engine implementation details.
I think the DiffNode is indeed an essential information to pass to the exception listerners.

[SQiShER]

Thanks for the detailed description. That helped a lot! The new implementation is slightly different from the old one:

• The PropertyAccessExceptionHandler now only handles read exceptions (since that are the only ones, that need to be handled during diffing.)
• The handler method doesn't return a DiffNode anymore.
• When an exception is handled, the state of the affected node will get changed to the new state INACCESSIBLE and the differ will not attempt to go any deeper into that node.

If that all sounds ok to you, I'd be happy to prepare a new release for you. Let me know what you think!

Cheers
Daniel

[donhaldo]

Hello Daniel,
thank you a lot for your support!
When you say "When an exception is handled, the state of the affected node will get changed to the new state INACCESSIBLE" you mean that an handler must be registered in order to set the new state on the node or simply that every time the differ meets a method that rises an exception, in any case no further access is attempted from that node? In the latter case, the proposed solution sounds good for me!

Kind regards
Roberto

[SQiShER]

INACCESSIBLE will be set automatically. No handler required. However, the default exception handler will simply rethrow the exception, so the diffing will not continue. I think that's a sane default, because it requires developers to actively decide what to do about the problem. An alternative would be to simply ignore the affected node, perhaps log a warning and continue diffing. But I think that's risky, because it is not guaranteed that people notice the problem until it's too late.

When the exception handler has been triggered, it means that the value could not be accessed and therefore its impossible to compare. So, yes, no further access is attempted for that node.

Will this signature work for you?

public interface PropertyAccessExceptionHandler
{
    void onPropertyReadException(PropertyReadException exception, DiffNode node);
}

[donhaldo]

I agree: the default exception handler has to rethrow the exception.

The signature proposed for the PropertyAccessExceptionHandler method is good for me.

Thank you
Roberto

[SQiShER]

Alright, I just released version 0.91.1! It can take a couple hours until its available via Maven Central, though.