What is meant by 'Inversion' in Dependency inversion

Question

I am learning spring. I understood dependency injection. In some place I also see it called dependency inversion. I got why it is termed as injection but what is meant by "inversion"? Which dependency is it actually inverting?

Answer 1

Good question - the word inversion is somewhat surprising (since, after applying the DIP, the lower level dependency module obviously doesn't now depend on the higher level caller module, either - both caller and dependency are now just more loosely coupled through an additional abstraction).

Citing Robert C Martin's original source

One might question why I use the word “inversion”. Frankly, it is because more traditional software development methods, such as Structured Analysis and Design, tend to create software structures in which high level modules depend upon low level modules, and in which abstractions depend upon details. Indeed one of the goals of these methods is to define the subprogram hierarchy that describes how the high level modules make calls to the low level modules. ... Thus, the dependency structure of a well designed object oriented program is “inverted” with respect to the dependency structure that normally results from traditional procedural methods.

One point to note when reading Uncle Bob's paper on the DIP is that C++ didn't (and at time of writing, still doesn't) have interfaces, so achieving this abstraction in C++ is typically implemented through an abstract / pure virtual base class, whereas in Java or C# the abstraction to loosen the coupling would usually be through decoupling by abstracting an interface from the dependency, and coupling the higher level module(s) to the interface.

Edit Just to clarify:

"In some place I also see it called dependency inversion"

Note that Dependency Injection (DI) is ONE of the possible implementations to achieve the Dependency Inversion Principle (DIP) - the "D" in SOLID design principles, so DI and DIP are not entirely interchangeable.

Other DIP implementations include the Service locator pattern (which is nowadays often regarded as an anti-pattern); and Plugin.