That's an interesting scenario you are describing. If somebody had asked me whether adding additional fields or lookup APIs into JCas cover classes, I'd probably have said that would not be a good idea.
I don't think that with the current CAS implementation much is lost if the scenario you are describing is remains unsupported. Will anything change regarding that scenario with v3? Cheers, -- Richard > On 19.08.2016, at 21:13, Marshall Schor <m...@schor.com> wrote: > > The switch doesn't modify any CAS - stored Feature Structure content. > > It does modify what JCas cover class is used when, for instance, iterating > over > some type in the CAS. > > Say, for example, you have implemented a JCas cover class for Token, which > just > has some standard getters and setters. > > Now, suppose, one of the pipelines implements a Pear which has an "enhanced" > version of the Token JCas class. For example, it might look up the "stem" > word > in some dictionary, and have a list of synonyms available, not stored in the > CAS, but via an extra API "fancysynonyms()". > > Now, suppose this CAS is read only, and there are two thread operating on it; > both with this Pear, where the code isn't going to modify the CAS, but within > the Pear, will iterate over the Tokens, and will call "fancysynonyms()". > > Now imagine the time-linear flow for two threads that looks like this: > > Thread 1: start -----------> enter Pear -----> access fancysynonyms > ---------> > exit Pear ------> etc. > > Thread 2: start -----------> enter Pear > --------------------------------------------------> access fancysynonyms > ---------> exit Pear ------> etc. > > Thread 1's "exit Pear" will have switched the CAS's jcas loaders back to the > "plain" one., so thread 2's iterator over Token would be returning the plain > version of the Token JCas class which would get a class-cast-exception when > the > code tried to cast it to the fancy version of the Token JCas class. > > At this point, I'm in favor of saying that you cannot use any classpath > switching with read-only shared CASs. This includes Pears, and the edge case > in > UIMA-5054 where the switching was to an "outer" class loading context. > -Marshall
