Craig,
here's a summary of the discussion Michael and I had on the issue of
"queries and class registration" (as exposed by the 1.5 jvm). Please,
review and comment on our assumptions and solution approach.
1. JDO requires the application to register the "target" class and
relevant subclasses (!) before executing a polymorphic query,
navigation, getExtent (with subclasses), or getObjectById.
Now, at least this is what I think to remember from JDO1: Some JDO
implementations have no means to know the "class universe", i.e.,
all subclasses of a class or the PC class for an Oid class. So,
the getExtent method and queries are executed based on the set of
PC classes currently known to the runtime. This requires the
application to register (load+initialize) the relevant classes
before the above methods and queries are being used.
However, I couldn't find that provision in the jdo-2005-09-09.pdf
spec document (we grepped for a number of keywords).
Have I overlooked something or has this requirement upon the
application been dropped in the JDO2 spec?
2. On the other hand, a JDO implementation may actively load and
initialize PC classes.
Section 12.5 (Class Loaders) states explicitly:
JDO requires access to class instances in several situations
where the class instance is not provided explicitly. In these
cases, the only information available to the implementation is
the name of the class. To resolve class names to class
instances, JDO implementations will use
Class.forName(String name, boolean initialize, ClassLoader loader)
with up to three loaders. The initialize parameter can be
either true or false depending on the implementation.
So, in general, the user must be aware that a JDO implementation
may load and initialize classes, that is, execute static
initializers, in a sometimes, perhaps, unexpected order.
3. For classes/types used in queries, such as in navigational
expressions ("a.b.c"), the question arises if the application
is responsible for having them loaded before query compilation
or execution, or if a JDO implementation may load and initialize
them when compiling or executing a query.
I think it's OK for a JDO implementation to initialize those
classes, since this results from them being used in a query, that
is, the user has given a hint to use these classes.
This is the approach Michael has implemented, which solves the
JVM 1.5 class registration issue.
From a hotspot at London Heathrow,
Martin
