While nowadays I wouldn't touch Spring with a ten foot pole if I had the choice, back then (must have been mid 2000's as well), Spring helped me really develop as a software engineer; everything I know now from (Java-style) interfaces, contracts and unit testing comes from that one six month internship where I was left to my own devices to build an application similar to what Sonar is nowadays (or was a few years ago). I still have the code somewhere, so many comments <_<.