Approaches to generating and loading JCas classes for V3

There are two parts to JCas generation.  

• Generation of getters/setters and constructors and superclass specifications
• Generation of the storage for the data included in the type.

The JCas generation for the first kind of thing The JCas generation is particular to a merged type system; and potentially for the 2nd as well.  

A .  A single JVM may have multiple merged type systems running simultaneously; these can only(?) :

• running simultaneously, or sequentially, one after another in a single pipeline (with varying type systems, e.g. processing a sequence of deserialized CASs)
• these can be handled using separate class loaders per different merged type system

...

• , or via forming a Union (if possible) among the type systems and using that (but note it may not be possible, due to incompatible range types, and may be inefficient - storage is needed for the merged feature set which may be large).  
• 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

...

• , versus referencing "generically" via Type and Feature objects, in order to be able to "see" the instances.

Alternatives for storage generation of a type

One alternative is to generate particular fields for each feature.  The getters and setters then are simple references to those features.

An other alternative is to have an indirection via feature offsets into two different kinds of data: data the garbage collector follows (i.e., referents) and data which is primitive other than references.

• This alternative allows a close emulation of the capabilities and limitations of v2 JCas
  • it is possible to not have all features described in the JCas, but they can be present (for serialization/deserialization and for access via type system indirection).

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.
    • Affects class loading design, requires for some use cases change to user code
    • Supporting Customization via merging is more complex.
• preexisting - by the user externally, ahead of time, running JCasGen on a fully merged type system
  • Pros:
    • Maybe less complex
    • Classes can be customized - custom fields and methods can be added (possible with the V2 design)
  • 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

...

• 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
• To what extend can users in the new design customize JCas classes?
  • The intent is to continue to allow users to customize JCas classes.  See JCasClassLoadingUV3 JCasClassLoading#GeneratingandLoadingCascoverclass/mergingwithCustomization
  • This isn't prototyped yet.  But the expectation (hope?) is that the bytecode can be examined and merged as appropriate from the customized classes.
• One's expectations towards a more native JCas is the ability to store arbitrary objects in the CAS, potentially in customized JCas classes. Is this part of the new design? If yes, how to preserve such objects when a classloader changes and JCas is regenerated?
  • The main idea is that on regeneration, there's a "merge" step from any customizations that might be present in the classpath.
  • Another goal is that because the new V3 design has a new kind of range type for Features, called "JavaObject", which I hope will turn out to be able to hold arbitrary Java Objects (although we might limit this initially), this will allow storing arbitrary Java objects in the CAS as the values of particular features, without any customization.  

...