Approaches to generating and loading JCas classes for V3

The JCas generation is particular to a merged type system.  A single JVM may have multiple merged type systems running simultaneously; these can only(?) be handled using separate class loaders per different merged type system.  Furthermore, the class loader used to load the generate JCas classes must also be used for any Application, Annotator, or External resource code that references by name the JCas generated classes. (Note: UIMA provides an ability to reference "generically" via Type and Feature objects).

When to Generate

• Dynamic - at type system "commit"
  • Pros:
    • guaranteed exact match, no need for code to check this
    • can be done lazily - a large type system with most types unused need not generate most types
      • requires a custom class loader (I think)
  • Cons:
    • user code must commit the type system before any reference to a named JCas type.  This "error" can be checked for, if a special class loader is used.
• preexisting - by the user externally, ahead of time, running JCasGen on a fully merged type system
  • Pros:
    • Maybe less complex
  • Cons:
    • may be out of sync, need to have more runtime checks
• preexisting at JVM startup time (via -javaagent).
  • Cons:
    • -javaagent is a dependency outside of main Java
    • the merged type system computed might be different - users can write UIMA app code to do custom merging.
    • uses one classpath; in embedded apps there may be multiple classpaths. 
    • is not "lazy" - does whole type system generation
    • user requires extra attention when starting a UIMA-based application. Analytics application vendors cannot easily create standalone executable JARs or e.g. Groovy/Jython scripts

Some Q&A

• What happens when a type system changes?  Some ways this might occur:
  • CAS reinitialized due to restoring a persisted (binary/serialized) CAS.  
    • Two sub-cases:
      • The app/externalresource/annotator code has references by name to the generated JCas classes (e.g. they code "myFooInstance.getMyFeature()".
        • If this code was executed, the lazy loading and linking of Java would have linked the (e.g.) annotator reference to the class.  Java doesn't support "unlinking" in general, so the only way I see to handle this case is to create a new UIMATypeSystemClassLoader for the new type system (assuming it was different of course), and reload/relink new versions of the classes to connect with the new definitions of the JCas classes.
      • If there are no ref-by-name, then maybe (to be determined) we can redefine the class.  The APIs are there, it just depends if this is allowed.  This would be a case where the type system was thought to be "variable", so the code could actually not really have by-name references (other than, say, to built-in types, whose definition is imagined to be shared in any case). 
  • New types defined at runtime
    • either externally by an application (already possible), or internally by an AE while a pipeline is running (a potential new feature being discussed)
    • These would, I think, always be referenced via some kind of indirection, not by a Java-linked name.  For the moment, I was thinking that since the new JCas would support Java HashMaps, that this requirement could be met by implementing a map from a user-specified key (e.g., a string) to a value; these could be constrained using Java generics.  This would be an initial way to support this requirement; we could later investigate more native ways.
• is JCas the only option or is a basic CAS API still available?
  • The intent is to allow existing users of UIMA to run their pipelines in V3, as much as may be feasible.  So, yes, I think we should continue to support the following CAS and JCas api varieties:
    • JCas - creation via new operator, getters/setters for each field, array getters/setters for array-valued fields
    • So called low level JCas (makes use of the _Type class)
    • CAS Apis - use Feature objects to specify the feature, and have variety for the various range types.  (Probably will add "array" styles, which are currently missing I think).  This API doesn't use by-name java-linked references.  As of Java 8, there is a significant improvement to the compiler + JVM that makes this style just as efficient as the by-name linked style.
    • Low-level CAS Apis

Setup for Type System class loader

We can imagine a new version of UIMAClassLoader, called UIMATypeSystemLoader, which has no URL and only serves to lazily generate and load JCas types.  This loader would have a reference to a committed type system, and would need to be in the parent chain of anything that referenced the generated types by name.  This class loader would be associated with a TypeSystem instance.

This might be setup using UIMAFramework.withUIMATypeSystemContext("a top level class to load and run"); this would load that named class under an instance of UIMATypeSystemLoader.

It would be nice to also run if no such context was set up - in this case, only one type system might be supported (and exceptions thrown if it was redefined).  If no UIMATypeSystemLoader was in the parent path, then type system commit would need to batch generate and load (using that arbitrary class loader) all the types (lazy not supported).  

If there was an instance UIMATypeSystemLoader in the parent chain, then this would support lazy loading.

The UIMATypeSystemLoader has a ref to the committed type system, which is null before a commit.  Type system commit walks up the class loader chain looking for an instance of UIMATypeSystemLoader, and the first one found has its ref changed from null to the committed type system. If it was not null, compare the commited type system with the previous value - if equal, OK, and leave things as is.  If not, throw exception: can't change merged type system using same UIMATypeSystemLoader.

Approach - outside of UIMA framework

The UIMA framework could take an approach which says a particular UIMA application (imagine it running as a servlet) has its own classpath, set up and managed outside of the UIMA framework (e.g., by the J2EE servlet APIs).  Using the preexisting alternative, the user would generate the JCas classes, and include them in the servlet's classpath.

• supports lazy loading trivially, but not lazy generation
• restricts particular UIMA app to one type system.

Generation notes

Need to package ASM embedded within new prefix in project (ASM requirement?)

Need to insure package-name is defined before doing class definition

Need to avoid circular refs or see if they can be handled by delaying resolution in the class loader