In what ways are the abstractions leaky? @Tool or @GetMapping make no demands on how to implement “this is a tool” or “this is a GET REST endpoint.” That they’re coupled with Spring (or rather, Spring is the only implementation for the semantics of these annotations) doesn’t constitute a leaky abstraction.
I've been actively working with Spring since about 2008. About 3-4 times a year, I cuss and curse some strange side effects that occur during refactorings. And in some areas we've painted ourselves into a corner.
But all in all, it's a great set of frameworks in the enterprise Java/Kotlin space. I'd say it's that synergy, which makes it worth the while.
I'm curious, though. Is the use of dependency injection part of the portfolio of criticisms towards Spring?
This is fair. I think the complaint is that Spring is _beautiful_ in up to medium sized demos, but in any sufficiently large application you always seem to need to dig in and figure out what Spring is doing inside those annotations and do something unspeakable involving the giant stack of factory factory context thread local bean counter manager handler method proxy managers etc.
Also Spring is a kind of franchise or brand, and the individual projects under the umbrella vary a lot in quality.
Just about any tool will require a bunch of work at some point in the scale. Some front-load that, and some make it easy to get started but then you hit a point where you need to peek under the covers. Personally I prefer the latter, though I'm sure there's a lot of Stockholm syndrome involved in how I feel about Spring. And Spring's popularity means you're probably not the only one to hit any given problem.
The precise semantics usually aren’t that well specified, and debugging is difficult when something goes wrong. Annotation-based frameworks are generally more difficult to reason about than libraries you only call in to. One reason is that with frameworks you don’t know very well which parts of the framework code are all involved in calling your code, whereas with libraries the answer usually is “the parts you call in to”.
Spring has more “synergy” in a sense than using a bunch of separate libraries, but because of that it’s also a big ball of mud that your code sits on top of, but isn’t on top of it in the sense of being in control.