The role of dependency injection

The package structure described previously goes a long way toward achieving a clean architecture, but an essential requirement of such an architecture is that the application layer does not have dependencies on the incoming and outgoing adapters, as we have learned in Chapter 3, Inverting Dependencies.

For incoming adapters, such as our web adapter, this is easy since the control flow points in the same direction as the dependency between the adapter and the domain code. The adapter simply calls the service within the application layer. In order to clearly bring out the entry points to our application, we’ll want to hide the actual services behind port interfaces.

For outgoing adapters, such as our persistence adapter, we have to make use of the Dependency Inversion Principle to turn the dependency against the direction of the control flow.

We have already seen how that works. We create an interface within the application layer, which...