How to show Dependency Injection on a UML class diagram?

You display class dependencies by using a dashed line with a pointed arrow that points from the client model element to the supplier model element.

I think that how you provide the dependencies to the client is irrelevent at the UML level.

First of all, UML and DI are very much orthogonal concepts:

• UML concerns itself with describing interactions, relationships and dependencies between concrete entities.
• DI describes how dependencies are to be resolved.

Thus, UML diagrams don't really have a mechanism to describe DI. However, I'm sure if DI is involved, you would see certain patterns evolve, including (but not limited to):

• at least two entities per dependency, that is (1) the interface and (2) at least one concrete implementation
• functions/methods that take in dependency interfaces, but (usually) no functions/methods that take in implementations of dependency implementations
• one or multiple entities to represent the injection mechanism itself

Martin Fowler shows it like this: