Dependency Injection #

In previous chapters we would have seen plenty mention of the concept of dependency injection. The Diego Framework is designed around dependency injection principles, therefore it’s a good idea to get a good understanding of the concept so that you can work more efficiently and effectively with the framework.

As Wikipedia puts it:

Dependency injection is a technique whereby one object supplies the dependencies of another object.

If class A requires instances of classes B and C, B and C are considered dependencies of A. When instantiating class A, you can either supply these dependencies yourself or rely on something called a dependency container to do this for you. A dependency container holds all the dependencies of your classes and injects them into your classes when they are required.

Dependency injection in this framework is implemented using Google Guice.

💡 You can read more about this design pattern on the Guice Wiki.

The dependency container #

In Diego, it’s always better to use the dependency container, where possible, to create instances of your classes. You can access the dependency container in one of two ways:

Using the @Inject annotation
Using the load() method from the UsesDependencyService trait

Using the `@Inject` annotation #

The main way of getting objects from the dependency container is by using the @com.google.Inject annotation in your classes. A good example of where this is useful is when you want to inject a Service or Database into your application:

public class MyApplication {
    private final Database db;
    
    @Inject
    public MyApplication(Database db) {
        this.db = db;
    }
}

You can place the @Inject annotation on constructors (ideal), methods or (least ideal) fields. All injection sites must be marked as public.

Using the load() method #

Another, more direct way, of creating instances of classes with dependencies is by accessing the dependency container directly. To get access to the dependency container, have your class implement the UsesDependencyService interface, which will give you access to the load(Class<?>) method (as seen in the Database chapter).

public class MyScheduledJob implements UsesDependecyService {
    private Database db = load(Database);
}

Some classes such as Serviceuse this interface out of the box, so you don’t need to explicitly declare it.

Adding modules to the dependency container #

You can add your custom modules to the dependency container. First, you create a Module (tutorial here) and tell the dependency container to load it when creating your application using the @diego.init.Uses annotation:

@Uses(MyModule.class)
public class MyApp implements WebMvcConfiguration {}

Providers in your module will have access to:

The loaded application settings via the Settings singleton and the underlying Lightbend Config data;
The configured ObjectMapper singleton; and
The configured Freemarker Configuration singleton.

Dependency Injection #

The dependency container #

Using the @Inject annotation #

Using the load() method #

Adding modules to the dependency container #

Further reading #

Using the `@Inject` annotation #