I'm not sure if it's still true, but I believe back at Google the interop with C/C++ was even more integrated - if I'm not mistaken the "java" binary got recompiled from scratch (but thanks for caching, not much hit), and your C/C++ code got compiled in with it - so no ".so" loading - you still have single binary (and I think the .jar also went at the end of the file).

Or maybe that was for python (e.g. compile CPython along with any ffi modules as one) + .zip file with classes.

In any case, even if above was not 100% true, it's doable with system like bazel/buck/etc., and still allow you for smooth incremental development (or say in the default "fastbuild" .so files may still be used - locally, but in "opt" the full static link, or through a --config)