That’s what Spring claims but the truth is that using interfaces enables those things. Dependency injection might encourage the use of interfaces, but it also inserts an abstraction between tests and the things you want to test.

In my experience dependency injection frequently obfuscates system interactions and invalidates any assurances you might get from good unit tests.

Dependency injection is just a tool. It can be useful but is often misused and doesn’t perform magic.

Using a DI container is not required to implement the DI pattern, it's just a tool that facilitates some more complex DI.

I tend to prefer to keep my projects simple enough that manual DI is quite appropriate and readable.

There are good and bad implementations of dependency injection (or, perhaps, appropriate and inappropriate depending on requirements), but the principle of dependency inversion applies at an architectural level, before specifics of implementation and things like unit testing come into play