Using Emacs or VI for Java is something for masochists. Helping them with refactoring support would take away their fun/pain. Doesn't use the Java LSP the Eclipse IDE? Maybe it would just be easier to configure your IDE like emacs.
If Emacs has access to a refactoring library and completions via LSP then it could have the strength of Emacs combined with the strong language support of IDEs. I'm saying this as a long time Emacs user who also used Android Studio a lot for Java programming, so I can appreciate the fortes of both sides of the fence.