Registries, Reflection, Introspection Implementation for UNO
The UNO types and services bootstrapping code is very old, and concepts are tightly knit together. Whenever you want to change something you risk backwards incompatibility. The code causes mental pain, and whenever you need to touch it you want to run away screaming. One typically ends up doing minimally invasive changes. That way, you have a chance of surviving the process. But you also pile up guilt.
At the heart of the matter there is the old binary "store" file structure
and the XRegistry interface on top of it. At runtime, both all the UNO
type information (scattered across a number of binary .rdb files) and
all the UNO service information (scattered across a number of .rdb files
that used to be binary but have been mostly changed to XML now) are
represented by a single XRegistry instance each.
The way the respective information is represented in the XRegistry
interface simply corresponds to the way the information is stored in the
binary .rdb files. Those files are designed for storage of hierarchically
nested small blobs of information. Hence, for example information about
a UNO interface type com.sun.star.foo.XBar is stored in a nested "folder"
with path com - sun - star - foo - XBar, containing little blobs of
information about the type's ancestors, its methods, etc. Similarly
for information about instantiable services like com.sun.star.baz.Boz.
As there are typically multiple .rdb files containing types resp.
services (URE specific, LO specific, from extensions, ...), but they need
to be represented by a single XRegistry instance, so "nested registries"
were invented. They effectively form a linear list of chaining XRegistry
instances together. Whenever a path needs to be looked up in the top-level
registry, it effectively searches through the linear list of nested
registries. All with the cumbersome UNO XRegistry interface between
the individual parts. Horror.
When the XML service .rdbs were introduced, we chickened out (see above
for rationale) and put them behind an XRegistry facade, so that they
would seamlessly integrate with the existing mess. We postponed
systematic clean-up to the pie-in-the-sky days of LibreOffice 4 (or, "once we'll
become incompatible with OpenOffice.org," as the phrase used to be back then)