From OSGi Community Wiki
Bundle-ClassPath specifies where to load classes from from the bundle. In an unzipped bundle this may refer to a directory (such as
target/classes). It may also refer to JARs as well (such as
lib.jar,other.jar). For unzipped bundles, referring to classes within a nested JAR has no performance constraints, but when accessing nested JARs inside JARs the OSGi runtime may have to extract the JAR into a temporary storage area, which may take additional time.
The default is '.' which allows classes to be loaded from the root of the bundle. For OSGi bundles which are also valid JARs this makes sense since that will be the default behaviour of other JARs.
Loading from nested JARs (whether packed or unpacked):
It is generally preferable to avoid nested JARs and either unpack-and-merge all the contents or extract them to external bundles.