Hi Carsten, Sure, but Konrad has a point in that I think sometimes the client *does* care why the adaption failed. For instance, if it had to do with something entirely different from whether or not adaption would normally work.
Let's say that I have a resource that should adapt to XYZ, but that my adapter is currently buggy. I'd like to get an exception for that, but said exception is going to get eaten. I do agree that if I have a resource that should NOT adapt to XYZ, that getting back null is fine, and that I don't care why the adaption failed. Cheers, Jeff. On 01/07/2014 10:19, "Carsten Ziegeler" <cziege...@apache.org> wrote: >Sure :) For the adapter pattern, the client does not care why the adaption >failed, the client is just interested in the result (success or not) >Validation is a different beast, if validation fails you want to know >specific reasons why it failed - and this can be multiple. >I tried to explain in my first mail on this thread, that all other use >cases mentioned can be handled with the current implementation - with the >exception of validation. But I think validation requires a different >concept than the adapter pattern. > >Carsten > > >2014-07-01 11:09 GMT+02:00 Jeff Young <j...@adobe.com>: > >> Hi Carsten, >> >> Can you say more? (I'm not sure I understand what you're getting >>at....) >> >> Thanks, >> Jeff. >> >> >> On 01/07/2014 09:56, "Carsten Ziegeler" <cziege...@apache.org> wrote: >> >> >adaption and validation are different concerns >> > >> >Carsten >> > >> > >> >2014-07-01 10:55 GMT+02:00 Jeff Young <j...@adobe.com>: >> > >> >> We could solve that by defining a specific exception for >> >> adaptation-not-possible and then catch only that. >> >> >> >> Of course that would leak tons of exceptions from code written before >> >>that >> >> exception became available. Maybe do the catching based on some >>sort of >> >> version clue? >> >> >> >> Cheers, >> >> Jeff. >> >> >> >> >> >> On 01/07/2014 09:40, "Konrad Windszus" <konra...@gmx.de> wrote: >> >> >> >> >It is not (only) about throwing exceptions in case no suitable >>adapter >> >>is >> >> >available. It rather is about the fact, that today the adaptTo is a >> >> >barrier for all kinds of exceptions. In some cases the adaptation >>fails >> >> >for a specific reason (one example is Sling Models where injection >> >>fails, >> >> >another one is the issue mentioned in >> >> >https://issues.apache.org/jira/browse/SLING-2712 (ValueMap not >> >>supporting >> >> >primitives)). Both would be valid use cases where the client would >>be >> >> >interested to catch the exception here. >> >> > >> >> >On 01 Jul 2014, at 10:34, Carsten Ziegeler <cziege...@apache.org> >> >>wrote: >> >> > >> >> >> Adding a new interface would require us to implement it all over >>the >> >> >>place >> >> >> and as Felix points out, client code would always need to check >> >>whether >> >> >>the >> >> >> new interface is implemented or not Having to methods, like >> >>hasAdapter >> >> >>and >> >> >> adaptOrThrow does not work very well as between the two calls >>things >> >> >>might >> >> >> have changed already: while hasAdapter returns true, the >>underlying >> >> >>factory >> >> >> gets unregistered before adaptOrThrow is called. >> >> >> In many use cases, the current pattern works fine - the client >>does >> >>not >> >> >> care whether an exception is thrown within the adaption - it just >> >>cares >> >> >> whether an object is returned or not. And there are valid use >>cases, >> >> >>where >> >> >> client code does different things whether the adaption works or >>not >> >> >>(e.g. >> >> >> the post servlet checks for adaptTo(Node) and then does additional >> >> >>things >> >> >> if the resource is backed up by a node.) >> >> >> >> >> >> I see the point that there are also use cases where it would be >>fine >> >>to >> >> >> simpy throw an exception if adaptTo is not successful. This would >> >>make >> >> >>the >> >> >> client code easier. However as this most properly is a runtime >> >> >>exception, >> >> >> client code can today just call a method on the object and end up >> >>with a >> >> >> NPE - having the same result :) >> >> >> >> >> >> This leaves us with use cases where the client code explicitely >> >>wants to >> >> >> catch the exception and then do something depending on the >>exception. >> >> >>Maybe >> >> >> we should just add something for this explicit use case instead of >> >> >>bloating >> >> >> the general adaptTo mechanism? >> >> >> >> >> >> Regards >> >> >> Carsten >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> 2014-07-01 9:44 GMT+02:00 Konrad Windszus <konra...@gmx.de>: >> >> >> >> >> >>> Regarding 1) Having such a Result class would mean that all >>consumer >> >> >>>would >> >> >>> need to unwrap the exception first. So instead of being forced of >> >> >>> implementing a null-check (as with the old solution) one would >>need >> >>to >> >> >>> implement another check. I want to prevent such a burden to the >> >> >>>consumers. >> >> >>> Regarding 2) Since the client code knows on which object the >> >> >>> adaptToOrThrow is called I don¹t get why an instanceof check is >> >> >>>necessary. >> >> >>> Whether this object implements Adaptable2 is known at >>compile-time, >> >>so >> >> >>>you >> >> >>> do have the full IDE-support here. >> >> >>> Regarding 3) In that case I would no longer allow a null value >>to be >> >> >>> returned. One drawback is, that all the null checks are no longer >> >> >>>effective. >> >> >>> >> >> >>> IMHO solution 2) is the best. At the same time I would deprecate >>the >> >> >>>old >> >> >>> Adaptable, because I cannot come up with a real use-case where >> >> >>>returning >> >> >>> has an advantage. I would rather implement another method boolean >> >> >>> hasAdapter(Class<AdapterType> type) on the Adaptable2 interface. >> >> >>> Regards, >> >> >>> Konrad >> >> >>> >> >> >>> On 01 Jul 2014, at 09:07, Felix Meschberger <fmesc...@adobe.com> >> >> wrote: >> >> >>> >> >> >>>> Hi >> >> >>>> >> >> >>>> There currently are two issues floating around dealing with the >> >> >>>>question >> >> >>> of returning more information than just null from the >> >> >>> Adaptable.adaptTo(Class) method: >> >> >>> https://issues.apache.org/jira/browse/SLING-3714 and >> >> >>> https://issues.apache.org/jira/browse/SLING-3709. I think these >> >> >>>requests >> >> >>> warrant some discussion on the list. >> >> >>>> >> >> >>>> Background: adaptTo can be implemented by Adaptable >>implementations >> >> >>> themselves or, by extending from SlingAdaptable, they may defer >>to >> >>an >> >> >>> AdapterMananager. AdapterManager itself is extended by >> >>AdapterFactory >> >> >>> services. All these interfaces define an adaptTo method. All >>these >> >> >>>methods >> >> >>> return null if adaption is not possible and don't declare or >> >>document >> >> >>>to >> >> >>> throw an exception. >> >> >>>> >> >> >>>> While not explicitly documented as such, the intention is and >>was >> >>that >> >> >>> adaptTo never throws on the grounds that adaption may fail which >>is >> >> >>> considered a valid result and thus exceptions are not to be >>expected >> >> >>>and >> >> >>> handled. >> >> >>>> >> >> >>>> Hence all implementations of the methods generally >> >> >>> catch-and-log-but-don't-throw. Interestingly >>SlingAdaptable.adaptTo >> >>and >> >> >>> AdapterManagerImpl.getAdapter don't catch ‹ so any >>RuntimeException >> >> >>>thrown >> >> >>> from an AdapterFactory would be forwarded. >> >> >>>> >> >> >>>> Having said this there are options available: >> >> >>>> >> >> >>>> (1) Add support for a new Result<?> class. We would probably >> >>implement >> >> >>> this in the AdapterManager.getAdapter implementation explicitly >> >> >>>handling >> >> >>> this case because it would entail catching the adaptTo/getAdapter >> >> >>>calls to >> >> >>> get the exception (the Result.getError should return Throwable >> >> >>>probably not >> >> >>> Error) >> >> >>>> >> >> >>>> Use would be limited to new AdapterFactory implementations >>throwing >> >> >>> RuntimeExcpetion. For Sling Models this would be the case. >> >> >>>> >> >> >>>> (2) Add a new adaptToOrThrow method, which is declared to throw >>a >> >> >>> RuntimeException and never return null: Either it can adapt or it >> >> >>>throws. >> >> >>> This would require a new interface Adaptable2 (probably) to not >> >>break >> >> >>> existing Adaptable implementations. The SlingAdaptable base class >> >>would >> >> >>> implement the new method of course, probably something like this: >> >> >>>> >> >> >>>>> SlingAdaptable implements Adaptable2 { >> >> >>>>> Š >> >> >>>>> public <AdapterType> AdapterType >>adaptToOrThrow(Class<AdapterType> >> >> >>> type) { >> >> >>>>> AdapterType result = this.adaptTo(type); >> >> >>>>> if (result != null) { >> >> >>>>> return result; >> >> >>>>> } >> >> >>>>> throw new CannotAdaptException(Š); >> >> >>>>> } >> >> >>>>> } >> >> >>>>> >> >> >>>> >> >> >>>> Use is problematic because you would have to know whether you >>can >> >>call >> >> >>> the new method: So instead of an null check you now have an >> >>instanceof >> >> >>> check Š Except for the Resource interface which would be >>extended to >> >> >>>extend >> >> >>> from Adaptable2 as well. >> >> >>>> >> >> >>>> (3) Document, that Adaptable.adaptTo may throw a >>RuntimeException. >> >> >>>> >> >> >>>> The problem here is, that this may conceptually break existing >> >>callers >> >> >>> of Adaptable.adaptTo which don't expect an exception at all ‹ >> >> >>>presumably >> >> >>> this is a minor nuisance because technically a RuntimeException >>may >> >> >>>always >> >> >>> be thrown. >> >> >>>> >> >> >>>> Regards >> >> >>>> Felix >> >> >>> >> >> >>> >> >> >> >> >> >> >> >> >> -- >> >> >> Carsten Ziegeler >> >> >> Adobe Research Switzerland >> >> >> cziege...@apache.org >> >> > >> >> >> >> >> > >> > >> >-- >> >Carsten Ziegeler >> >Adobe Research Switzerland >> >cziege...@apache.org >> >> > > >-- >Carsten Ziegeler >Adobe Research Switzerland >cziege...@apache.org
