# Doing Dependency Inversion right

Dependency Injection is a well-known pattern and de-facto standard for implementing a Dependency Inversion Principle. Most modern frameworks have some level of support for Dependency Injection - from weaving the application via public setters at runtime using XML as a spec (e.g. Java Spring), to compile-time constructor injection (e.g. macwire). However, while doing most of the heavy lifting, these tools and frameworks leave capturing the more sophisticated and valuable promises of DIP to the developers. Sadly, most of the time the result is…. suboptimal - that is to say it is not completely wrong, but could have been better. This shortcoming is subtle, but “getting it right” often solves or even removes a lot of other questions/concerns - including some that spawn “both implementations are fine, let’s discuss which one to choose till the thermodynamic death of the Universe” discussions.

# Dependency Inversion Principle

To recap, the precise definition of DIP is as follows (see page 6):

A. High-level modules should not depend on low-level modules. Both should depend on abstractions (e.g. interfaces).

B. Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.

To make it more concrete, let’s consider the classical “layered” architecture1.

This is how it usually looks like before DIP is applied (arrows show the direction of dependencies):

Then someone remembers about DIP and realizes that “infrastructure” layer is technically the lowest level - so “domain” and “application” should not depend on it - and draws a new diagram:

… or maybe the team has started with DIP in mind right away, so that’s their actual architecture diagram from the get-go. In any case, the next step is to implement it in code - and this is where that suboptimality I’m talking about creeps in.

# How to do DIP in code

Ok, both sentences in DIP definition can be boiled down to “interfaces should depend on interfaces, implementations should depend on interfaces (and not other implementations)”. That’s easy - we just convert

public class HighLevelComponent {
private final LowLevelComponent dependency;
}

public class LowLevelComponent { ... }


to

public class HighLevelComponent {
private final ILowLevelComponent dependency;
}

public interface ILowLevelComponent { ... }
public class LowLevelComponentImplementation implements ILowLevelComponent { ... }


And already we start reaping numerous benefits of DIP - we can now replace implementations (including plugging test doubles), code high-level component to completeness not even having a low-level component started (against a test double or “simplified” implementation) and our code is already somewhat better documented.

However, let me put some more context into it - what if our high-level component and low-level component belonged to a different layers - e.g. domain and infrastructure?

package foo.bar.domain
public class ClientService {
private final IClientRepository dependency;
}
public class Client { ... }

package foo.bar.infrastructure
public interface IClientRepository { void save(Client entity); }
public class InMemoryClientRepository implements IClientRepository { void save(Client entity); }


Can you spot the problem? There is one2.

Just in case it’s not apparent - domain.ClientService depends on infrastructure.IClientRepository, which makes the app look more like the first diagram, whereas we wanted the second one. What’s worse - the dependency not only goes in the undesired direction but actually is two-way:

• domain.ClientService depends on infrastructure.IClientRepository
• infrastructure.IClientRepository -> domain.Client

… and we’re one step closer to “everything depends on everything else” aka Big Ball of Mud.

# Solution

Unsurprisingly, the solution is already invented. It was right there, in the plain sight, in the paper where the Dependency Inversion Principle was coined (page 7, figure 4). It just didn’t make it to the “short” description.

So, I give you The Missing Part of The Dependency Inversion Principle:

C. Implementations should depend on abstractions in the same layer as they are. Implementations can implement abstractions from the higher layers, or the same layer.

The best thing about it is that it’s not too hard to implement in code. Continuing on our Client example:

package foo.bar.domain
public class ClientService {
private final IClientRepository dependency;
}
public interface IClientRepository { void save(Client entity); }
public class Client { ... }

package foo.bar.infrastructure
public class InMemoryClientRepository implements IClientRepository { void save(Client entity); }


The only thing changed in the code is the move of the IClientRepository interface into the domain layer. However, conceptually (or semantically) it changes a lot:

1. It becomes apparently clear that repositories in the domain layer should speak “domain” language3 - i.e. receive and return domain objects. If infrastructure needs a dedicated persistence model, it becomes the responsibility of the infrastructure to map the domain model to the persistence model and back.
2. It removes “circular” dependency between domain and infrastructure
3. It helps keep the domain clean of infra - if this approach is applied persistently, the domain layer becomes a leaf in the dependency tree, as all the implementations only depend on “same layer” interfaces.
1. … which gives an ability to develop domain logic before the gory details of infrastructure are known
2. … and swap infra in the middle of the project, or three years down the line
4. It puts an additional barrier on “leaking” persistence/messaging/other infrastructure concerns and concepts into the domain model (which is a constant battle in DDD-based projects)

… and probably some other less obvious benefits.

## Bonus track: code smells

There are two code smells that indicate “part C” is not observed and are quite easy to spot.

Analyze imports: Ctrl+Shift+F (or equivalent hotkey for “Search in Folder” command) in the “domain” package. Any hits for “infrastructure” or other lower-level layers are good indicators that something is not right. Obviously works not only for “domain” and “infra”, but any pair of packages you’d want to check.

impl packages or *Impl class name suffixes:4 this means that the implementation implements an abstraction from the same package - which almost always means ‘the same architecture layer’. Not an offence on it’s own, but chances are high that the interface in question belongs to a higher layer.

# Conclusion

Dependency Inversion Principle in the real world is quite often reduced only to the “interface abstraction” - replacing concrete dependencies with abstract ones and leveraging Dependency Injection to keep the app running. However, it prevents from reaping all the benefits of DIP - such as clearer isolation of application layers - that in turn bring even higher productivity and code quality benefits.

Although refactoring to the “proper DIP” is relatively straightforward, the bigger benefit is the change in the ways developers think about the system at hand - so while it is easy (if not even “mechanical”) to change the code, it’s not as easy to change the principles. So realizing full DIP potential calls for mindset change (and lots of explaining ).

1. The same logic is valid for the more advanced architectures too, (e.g. Hexagon), “layered” is chosen for simplicity and because “everyone knows it”. ↩︎

2. pun intended. ↩︎

3. This removes the question of “should DB repositories use domain model or persistence model?” - the one that often spawns “both implementations are equivalent…” discussions (at least in my current team), and needs careful monitoring in the code reviews afterward. ↩︎

4. Yes, I’m attacking a Java best practice. Yes, I know what I’m talking about. Yes, I’m sure  ↩︎