Intersystems Iris with Java

The list of steps depend on how is your current application built (e.g. are you using direct global access vs. SQL). Depending on which direction you want to go you could simply wrap your existing objectscript code in either stored procs or services and access them from Java or rewrite all your existing logic in Java.

Again, the steps would depend on what exactly you're trying to accomplish and your current app makings.

Maybe this Getting Started guide is also a worthwhile reference