Neil wrote up an excellent and short article on how to embed OSGI into another Java based system with some basic API’s – check out his post.
A popular question on mailing lists and Stack Overflow is how to embed OSGi in a larger application. This is really easy to do, especially since a standard API for it was introduced in OSGi Release 4.1. Because of the standard, switching between Equinox, Felix and Knopflerfish can be as simple as a configuration change. – Neil Bartlet