Trial by fire. We were in the infancy stages of a project and the alternative OTS options were really bad for our use case. We had a project manager that really believed in our group and pointed us to the Spring framework as one they had used in the past to solve a number of the same issues we were facing at the time. I had mainly worked in very heavy custom Java environments (no frameworks, and very few third-party dependencies) so dropping into a Framework with actual rules/guidelines was so nice and I'm currently noodling out how to rewrite some of our legacy apps in Spring because of how much it slots into our workflows.

I don't love programming books and find them very hard to follow, so I leaned heavily on Baeldung and StackOverflow for most of my questions. Like some of the other comments in this thread, I had a hard time understanding dependency injection since I had no prior experience with it, but man-oh-man is it useful.