So if your adapter is buggy and you get an exception, what do you do with it?
Carsten 2014-07-01 12:08 GMT+02:00 Jeff Young <j...@adobe.com>: > 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 > > -- Carsten Ziegeler Adobe Research Switzerland cziege...@apache.org
