I think the reality is that only large-ish companies have the slack time necessary to devote to mentoring. Companies have to get to a certain size first.
At a startup, you are running full tilt 24/7 by necessity.
At a startup, you are running full tilt 24/7 by necessity.
In a mid-sized organization you can work on a java webapp - as in your example - and possibly only on a part of that java webapp. In early stages of a startup, you're likely to have to know your way around the webapp's frontend, backend, mobile app/mobile web adaptation, handle SEO, plan for scaling, be a part DBA, handle server administration DevOps style, and do all kinds of internal business process automation unrelated to that product you're building. You'd need to be a jack of all trades at beginning - since there's noone around who's specializing in that other stuff - or spend half of your time becoming a jack of all trades.