Re: [xwiki-devs] [Proposal] Change our Script API best practice regarding exception handling

Forget what I’ve said regarding the #try directive. Actually the way to use it is: #try() … wrap some script snipper, one or several calls… #end .. here only you can access the $exception binding. Thus: * Wrapping the whole content to evaluate won’t work * The Exception Catching Uberspector is really what we need. It’s more fine-grained than the #try directive (or it’s equivalent if you use #try() for each line of script calling methods). In addition: * We provide script APIs in xwiki-commons too so if we want this best practice of throwing exceptions in script APIs, we also need to make this work in xwiki-commons when used outside of XWiki. Thus we need a solution there too. ** A) One solution would be to use XWiki’s Context and make the last exception available with $xcontext.lastVelocityException but it’s not very natural, I hope we can find a better solution ** B) Another solution is to move VelocityManager to commons in xwiki-commons-velocity, offer a basic impl without skin support and override it in xwiki-platform to add the platform-specific stuff. I’ll start exploring B). Let me know if you don’t agree or if you have a better idea. 

Thanks -Vincent On 15 Jan 2016 at 12:15:41, vincent(a)massol.net (vincent@massol.net(mailto:vincent@massol.net)) wrote: > > [snip] > > > > > > > >> > Hi devs, > > > > > > >> > > > > > > > >> > Right now our strategy is for script services and script APIs in general > > > > > > >> > to catch exceptions, store them and offer a getLastError() method to get > > > > > > >> > them (see > > > > > > >> > http://extensions.xwiki.org/xwiki/bin/view/Extension/Script+Module#HBestPra… > > > > > > >> > ) > > > > > > >> > > > > > > > >> > However it would be much nicer to: > > > > > > >> > * Let our script services generate exceptions > > > > > > >> > * Offer a velocity script service to get the last exception raised by a > > > > > > >> > java call from velocity > > > > > > >> > * Implement this uberspector to catch the exceptions and to set them in > > > > > > >> > the execution context > > > > > > >> > > > > > > > >> > That should be quite easy to implement IMO. > > > > > > >> > > > > > > > >> > WDYT? > > > > > > >> > > > > > > > >> > > > > > > >> +1, it's a pain to call setLastError() everywhere there can be an exception > > > > > > >> thrown, and we almost always forget to do it (for this reason). > > > > > > >> > > > > > > >> Note that we also have the #try() directive now. > > > > > > > > > > > > > > Yes, I should have mentioned that there’s indeed also this possibility: > > > > > > > * Have script API throw Exceptions > > > > > > > * Force velocity script users to wrap their code with the try directive when they need to catch exceptions > > > > > > > > > > > > > > I still believe that the use of the Exception-catching uberspector is better. > > > > > > > > > > > > > > WDYT? > > > > > > > > > > > > Does it mean you plan to get rid of new #try directive ? Because it > > > > > > will be broken with this new uberspector. > > > > > > > > > > That’s a good point, I had not thought about the implementation at this stage. > > > > > > > > > > I think this could still work. When the #try directive is used I’d just have to setup some flag somewhere in Velocity and in the uberspector I could check if this flag is set and if so then don’t catch the exception. > > > > > > > > > > > > Actually, thinking more, I think you’re right and that the #try directive plays exactly the same role as an Exception-catching uberspector and I don’t see the need for the #try directive if we provide an uberspector. > > > > > > > > So I’m proposing to deprecate it but still keep it for backward compatibility for now (probably a full cycle). > > > > > > > > WDYT? > > > > > > Note that I’d like to change a bit the proposal and instead of making the exception available from a script service, I’d prefer to make it available as a known velocity binding such as $lastException. There’s no reason to use a script service since that would mean it would work for all scripts and in this case we only want it to work for Velocity. > > > > > > Since there’s no way to get the Velocity Context from within an uberspector, I’ll get it by using our Component Manager and get the VelocityManager component and call getVelocityContext()… If you know a better way, let me know. > > > > hmm… this would mean that I’d need to put this new uberspector in xwiki-platform since VelocityManager is in platform ATM… (@Thomas: our discussion of yesterday ;)). > > > There’s an alternative, which is to modify our implementation of VelocityEngine.evaluate() and decorate the source with a #try() directive so that it’s always called (and make sure that calling it nested won’t affect it for backward compat). > > This could be simpler to implement and doesn’t force us to move some velocity code to platform. > > WDYT? > > Thanks > -Vincent > > [snip]