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The performance reason for using the low level accessors is not present in V3; in fact, these, if implemented, would be slower than the other APIs.

JCas Class sharing

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JCas classes are associated with a class loader.  Except for the built-in types which always have JCas Classes, other JCas classes are optional. Furthermore, JCas classes may define only a subset of the features of the fully merged type system. So, even when a JCas class is present, it may not have getters and setters for some features of the corresponding UIMA type. These features can be accessed of course using the plain APIs (see above). 

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It is possible to run multiple pipelines with non-compatible type systems and JCas classes by running each one under its own class loader; in this scenario, each pipeline will load its own copy of JCas classes from its own classloader's classpath.

JCas Class and UIMA Type conformance

JCas Classes have static final fields computed at load time. Each type system commit loads corresponding JCas classes (the load only happens the first time, per class loader).

A particular type system instance is being committed when a JCas class is loaded.  At load time, these rules are checked:

• Construct the supertype chain of the class being loaded.  It must be the case that, scanning upwards, there is a supertype that has a corresponding UIMA type.
  • It is OK if there are UIMA types between this and the found corresponding supertype - that just means there were no JCas types defined for those.
  • It is OK for the supertype chain to pass through supertypes which are not UIMA types, as long as the JCas supertypes are abstract (can't be instantiated)
• For each feature
  • the feature offset assigned to the class's static final value must match the feature offset
  • the feature's range must match
  • JCas-defined features which do not exist in the 1st type system loading this JCas class will result in invalid getters and setters for that feature, if an attempt is made in some code to get/set those features.

How JCas feature offsets are computed or validated at type-system-commit time

The type system is walked in subsumption order, and offsets are assigned to all features.  Then the JCas classes are loaded - the corresponding features are used to set the static final int offset values in the JCas class, if they are actually loaded.  If they are already loaded, the existing values are checked to insure that they match the type system assigned values. A mismatch can occur if multiple different type systems are being used. Mismatches (which cannot happen if only one type system is in use) result in a fatal error.

Connecting Instances with Type and Feature information

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At instance creation time, a lookup is done, using the instance of the type system, to get the actual type associated with the registry number.  This mechanism is encapsulated within the JCasRegistry class.

Locating the corresponding UIMA Feature when accessing a feature using JCas APIs

The generated getter or setter code for a JCas feature need to access needs the feature instance associated with stored-feature-offset-index information for the feature being set.  This cannot be statically compiled into the JCas class for the same reasons as above. A similar registry is used, and a unique incrementing integer is assigned to each feature referenced in a JCas class; these int values are kept as static final int values in the loaded class, and are similarly looked up when needed.

These are used in the trampoline methods of the generated getters and setters.  A getBegin() gets converted into a getIntValue(_getFeat(_FI_begin)), where getIntValue is the plain API call and _getFeat is a method taking the registered unique identifier for this feature and looking up the FeatureImpl instance for it.

Collections

accessed.  In the use-case of having multiple type systems for one JCas class set loaded under one class loader, each type system might have a different number for this; this design would make it necessary to have all accesses go thru one level of indirection to get the particular type system's offset for a feature.

This is avoided using the following technique that assigns the offsets to match already assigned ones:

• The first time a JCas class is loaded at type system commit time, it defines a final static int constant of the pre-computed offset.
• The 2nd time a JCas class is accessed at type system commit time, the first value stored is read and is used for the offset.

This requires that no JCas class access is done prior to type system commit, since the static final value can only be assigned once at resolution time.  This is normally the case, since it would be invalid to do something with a JCas class before the pipeline is set up.

Collections

UIMA v2 supports specially-named arrays of primitives (+ string), e.g. BooleanArray. 

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• limit (initially) generic spec to only simple type names, no support for extends, ?, etc.  Use TOP for "Object".

Strings

Keep special UIMA String type for compatibility and subtyping.

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