OpenJDK Hotspot

The HotSpot code base has been worked on by dozens of people, over the course of 10 years, so far. (That's good and bad.) It's big. There are nearly 1500 C/C++ header and source files, comprising almost 250,000 lines of code. In addition to the expected class loader, bytecode interpreter, and supporting runtime routines, you get two runtime compilers from bytecode to native instructions, 3 (or so) garbage collectors, and a set of high-performance runtime libraries for synchronization, etc.