Is your existing application monolithic or tiered in seperate processes/layers?
If tiered, you can convert the middle/app-tier to run in an OSGi container.
In my team's experience, we've found trying to do web-stuff in OSGi painful. Other pain points are Hibernate and Jakarta Commons Logging.
I find the OSGi specs pretty readable and I recommend you print out the flowchart that shows the algorithm for class loading. I'll guarantee you'll have moments of, "why am I getting a NoClassDefFoundError?": the flowchart will tell you why.
