This is not a joke at all. The SIP protocol is (like all network protocols) based on a "formal grammar", which is described in an IETF RFC here: https://www.ietf.org/rfc/rfc3261.txt
Writing a protocol parser is not as simple as it seems, and ANTLR is a tool that eased the work tremendously (on top of making it reliable and testable).
Also, Java is required for COMPILATION, and not for RUNTIME (you'll see that installing belle-sip from your favorite package manager in linux doesn't pull the entire Java runtime).
Finally, let me remind you that bellesip and linphone are open source projects, and if you want to keep your computer Java-free, feel free to propose a patch removing the Java dependency.