In my opinion dependency injection solves a problem that doesn't need to exist, and does it by adding even more obfuscation and complexity.

The problem is that the original gang of four design patterns had very little to say about managing effects. In old java code things like network and file IO often happen deep inside the object graph, hidden behind multiple impenetrable abstractions such that it's impossible to run the logic without triggering the effect.

The wrong solution is to add even more obfuscation and abstraction, so that you can inject replacement classes deep inside the object graph where the effects happen. it solves the immediate problem of implementing tests, but makes everything else worse and more confusing.

The right solution is to surface all your effects at the top level of the call graph. The logic only generates data, and passes it back up to the top level of the program. The top level code then decides whether to feed this data into an effectful operation. Now all your code is easier to reason about, and in you can easily test the logic without triggering unwanted effects.

Yeah. Injection has a place in test patterns. Thankfully, it's usually possible to hide injection from strongly affecting anything else that matters, as long as the team hates injection deeply enough.