Possible Duplicate:
How do you dive into large code bases?

I have worked as a developer developing C/C++ applications for mobile platforms (Windows Mobile and Symbian) for about six years. About a year ago, however, I changed job and currently work with large(!) enterprise systems with high security and availability requirements, all developed in Java. My problem is that I am having a hard time getting a grip on the architecture of the systems, even after a year working with them, and understanding systems other people have built has never been my strong side. The fact that I haven't worked with enterprise systems before doesn't exactly help.

Does anyone have a good approach on how to learn and understand large systems? Are there any particular techniques and/or patterns I should read up on?