The way I learned Spring was basically by just being pushed into a Java project that was using it right after I finished uni. Tbh it was a bit overwhelming but I was able to slowly wrap my head around it in about a month or two. It was also the first "real" framework I ever used. Ever since then I started to just jump right into projects and try to grasp the basic concepts of the frameworks used, since they are mostly quite similar and try to expand my knowledge from there on. At least for me this worked well for NestJS, Flask, Django and somehow also for stuff like angular and Android development but there I had to put up with some formerly unknown concepts.

I ended up working for a company which had a large Spring project and learnt by working on that. It didn't seem too hard once I got my head around autowiring.

That was ten years ago, still using XML configuration files, and once I started working on projects with annotations instead things became a lot easier.

If you want a book that does a really good job of explaining the basics and laying the foundation of Spring from the ground up, then I recommend Spring Start Here. It's the book I wish I had when I started working on Spring (Boot) projects all those years ago.

If you aren’t familiar with dependency injection, that could prove a hurdle. Same for webflux/reactor. I would avoid using webflux until you feel more familiar with non-reactive spring. Otherwise it will feel overwhelming.

I've learned through Udemy courses, more precisely through Chad Darby courses.

As someone who needs to understand the concepts first (instead of simply learning by doing), the guides by Marco Behler really helped. And the official documentation is great too!

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.

As with any framework or language, it depends on how you like to learn things and how quickly you need to start producing code.

I like to just jump in and read docs where necessary along the way. That doesn't mean it's the best way for you.