HAPI Source Code
HAPI is open source software, so naturally you have full access to the source code.
Browsing
If you just want to browse the latest released sources, the easiest way is to visit the JXR (Java XRef) viewer here.
If you want to see the version control history,
You may browse the HAPI source repository online using
Sourceforge's built in browser here:
https://sourceforge.net/p/hl7api/code/764/tree/
Checking out
HAPI uses Subversion as a version control system. You may check out the source code using the following repository (you will need a Sourceforge username and password to access the source code this way):