Java is 100-ton dinosaur to be working with using a simple editor. Spring, it dependencies, hibernate, junit alone make you want to work with an IDE.
There is an enormous amount of stuff that "conventional" IDEs to and emacs does not do.
Consider NetBeans: there are a lot of people that use the NetBeans platform to create stuff that have nothing to do with programming, the very same way emacs users create modes that have nothing to do with programming: https://platform.netbeans.org/screenshots.html
Seriously guys, grow the fk up.
In what programming language could Emacs possibly not be feasible option for a seasoned Emacs user?