It’s the reference implementation of Jakarta EE’s CDI specification. I will introduce an implementation of that role by using the Weld framework. The only thing that’s missing is the injector. The CoffeeMachine interface fulfills the interface role.The BasicCoffeeMachine class acts as the services.The CoffeeApp implements the client role.It also provides three of the four roles required by the dependency inversion technique: ![]() The class BasicCoffeeMachine implements the CoffeeMachine interface.Īs you can see in the diagram, this application already follows the dependency inversion principle. This small application enables you to control different coffee machines, and you can use it to brew a fresh cup of filter coffee. It consists of a CoffeeApp class that calls the brewFilterCoffee method on the CoffeeMachine interface to brew a fresh cup of coffee. Let’s change it so that it uses the dependency injection technique. I used the CoffeeApp example in my article about the dependency inversion principle. Using Dependency Injection to make the CoffeeApp more flexible You already implement most of the required roles, and the dependency injection technique enables you to remove the last dependency to the service implementation. All frameworks that I listed at the beginning of this article provide ready-to-use implementations of it.Īs you can see, dependency injection is a great fit for applications that follow the dependency inversion principle. But that’s not an issue because you don’t need to implement it. The injector is the only role that isn’t required by the dependency inversion principle. But most often, it’s better to introduce an interface to remove the dependency between the client and the service implementation. But by doing that, you break with the dependency inversion principle and your client has an explicit dependency on the service class. You can skip the interface role and inject the service object directly into the client. The service and the client are the two classes between which the dependency inversion principle intends to remove the dependency by introducing an interface. You already implement three of these four roles by following the dependency inversion principle. The injector which creates a service instance and injects it into the client.An interface that’s used by the client and implemented by the service.If you want to use this technique, you need classes that fulfill four basic roles. These are: This reduces the amount of required boilerplate code and improves flexibility.īut before we take a look at an example, I want to tell you more about the dependency injection technique. The goal of the dependency injection technique is to remove this dependency by separating the usage from the creation of the object. That prevents you from replacing the implementation of the interface with a different one. The interface only decouples the usage of the lower level class but not its instantiation. At some place in your code, you need to instantiate the implementation of the interface. But even if you implement it perfectly, you still keep a dependency on the lower level class. That principle improves the reusability of your code and limits the ripple effect if you need to change lower level classes. I explained this approach in great details in my article about the dependency inversion principle. If you do that, both classes depend on the interface and no longer on each other. ![]() You can introduce interfaces to break the dependencies between higher and lower level classes.
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