About Dagger 2

Dagger 2 is dependency injection framework. It is based on the Java Specification Request (JSR) 330. It uses code generation and is based on annotations. The generated code is very relatively easy to read and debug.

To learn more about Dagger 2, here are some excellent resources.

  1. Excellent video on Dependency Injection
  2. Excellent video on Dagger 2 by Jake Wharton
  3. Excellent tutorial on Dagger 2 and Android tutorial
  4. SO - @Named
  5. SO - @Inject constructor params

Annotations

Dagger 2 uses the following annotations:

  • @Module and @Provides: define classes and methods which provide dependencies.

  • @Inject: request dependencies. Can be used on a constructor, a field, or a method.

  • @Component: enable selected modules and used for performing dependency injection.

Dagger 2 uses generated code to access the fields and not reflection. Therefore it is not allowed to use private fields for field injection.

Dependency providers

The term dependency injection context is typically used to describe the set of objects which can be injected.

  • In Dagger 2, classes annotated with @Module are responsible for providing objects which can be injected.
  • Such classes can define methods annotated with @Provides. The returned objects from these methods are available for dependency injection.
  • Methods annotated with @Provides can also express dependencies via method parameters. These dependencies are fulfilled by Dagger 2, if possible.

Dependency consumers

You use the @Inject annotation to define a dependency.

Note that if you annotate a constructor with @Inject, Dagger 2 can also use an instance of this object to fulfill dependencies. This was done to avoid the definition of lots of @Provides methods for these objects.

Connecting consumers and providers

The @Component is used on an interface. Such an interface is used by Dagger 2 to generate code. This interface defines the connection between the provider of objects (modules) and the objects which express a dependency (consumers).

  • The name of the the generated class is Dagger + the name of the interface.
  • This generated class has a create() method which allows configuring the objects based on the given configuration. Or you can use the builder() to get more sophisticated about providing modules.
  • The methods defined on the interface are available to access the generated objects.

Special treatment of fields

Dagger 2 does not inject fields automatically. It can also not inject private fields. If you want to use field injection you have to define a method in your @Component interface which takes the instance into which you would like Dagger 2 to inject an object into this field.

Scope annotations

You can use the @Singleton annotation to indicate that there should be only one instance of the object.

Simplest Java Example

This is the most basic example of using Dagger 2 without component dependencies, subcomponents, complex modules, or custom scopes. The main Dagger 2 features used are:

  • @Component.
  • @Inject.
  • @Module and @Provider.

There’s a Log interface that is that allows a message to be logged. And an implementation of this interface ConsoleLogger that just dumps it to the console. The goal is to leave creation of objects to Dagger 2 (along w/ providing a concrete implementation of the Log interface).

Dagger 2 will be providing this application w/ 2 objects:

  1. Main object.
  2. Log object.

Here are the basic classes.

interface Log {
    void log(String msg);
}
class ConsoleLogger implements Log {
    @Override
    public void log(String msg) {
        System.out.println(msg);
    }
}

Here’s a link to the repo.

Object that does not require @Module

Let’s look at the simplest case first. We don’t want to create a new Main object using new Main() in the main() method of the Main class. We want Dagger 2 to generate this, since the whole point of dependency injection is not having to use new.

There’s only one Main class, there’s no interface that has to be implemented by this class. So it is pretty straightforward to tell Dagger 2 to create this object and expose it as a dependency.

@Inject
Main() {}

The way in which to do this is to annotate the no arg constructor of Main with @Inject. This tells Dagger 2 that it should create an instance of this class when it’s needed by any of the objects in the object graph that it generates.

This requires the creation of a @Component since that’s the entry point for Dagger 2 to start creating all the objects that are required by the application. This is what the interface looks like.

@Component
@Singleton
interface LogComponent {
    Main providesMain();
}

Please note that it’s called in the main() method of the Main class. So we don’t actually create an instance of Main. It’s done for us by Dagger 2.

public static void main(String[] args) {
    DaggerLogComponent.builder().build().providesMain().run();
}

How did this work? Dagger 2 knows how it can create a Main class, since the no-arg constructor annotated w/ @Inject in Main is a way to tell Dagger 2 that when it needs to, it can use this constructor to create an instance. The @Singleton annotation on the component also ensures that it keeps this instance around for the lifetime of this component (once the instance has been created).

By using @Inject you can bypass the need to create a @Module and have a @Provide annotated method, which is what is usually required. Dagger 2 is smart enough to figure out that for this simple class (that it doesn’t have to worry about mapping to some interface) it can just create a singleton if needed and pass it around the object graph of dependencies.

The situation will get more complex when we have to deal with generating a Log implementation object and make it available to the object graph.

Object that requires @Module

The key difference between providing a Main object and a Log object is that Log is an interface, and many different implementations may be available for this interface, and Dagger 2 doesn’t know (without being told) how to provide the desired implementation for this interface.

This is where @Module and @Provides come into play.

So we add some more complexity to our component.

@Component(modules = {LogModule.class})
@Singleton
interface LogComponent {
    Main providesMain();
}

@Module
class LogModule {
    @Provides
    @Singleton
    Log providesLogger(ConsoleLogger logger) {
        return logger;
    }
}

Note that the providesLogger() method is declared, but an object is NOT created here! We could simply do the following in this method and it would work.

Log providesLogger() {
    return new ConsoleLogger();
}

However, by adding a parameter to this providesLog() method, we are telling Dagger 2 to figure out how to get an instance of ConsoleLogger on its own, but then return it as a Log interface. That’s pretty slick!

In order to make this happen, we have to make the ConsoleLogger class more complex though.

@Singleton
class ConsoleLogger implements Log {

    @Inject
    ConsoleLogger(){}

    @Override
    public void log(String msg) {
        System.out.println(msg);
    }
}

By simply adding a no-arg constructor and marking it w/ @Inject we are able to tell Dagger 2 how to provide an instance of ConsoleLogger that it can provide! How awesome is that!

Also note that the @Singleton annotation is applied to the class and not the method, since we are using a @Inject. If we were using a provider and module then the @Singleton annotation would not be at the class level; this is only for @Inject.

Finally we have to change the Main class so that it knows how to work w/ the LogModule that we just added.

class Main {

    public static void main(String[] args) {
        DaggerLogComponent.builder().build().providesMain().run();
    }

    @Inject
    Main() {}

    @Inject
    Log logger;

    public void run() {
        logger.log("Hello world!!! Simple Dagger");
    }

}

@Bind shortcut

Instead of this:

@Provides @Singleton 
Log providesLogger(ConsoleLogger logger) { return logger; }

We could write:

@Binds @Singleton 
abstract Log providesLogger(ConsoleLogger logger);

It is possible to have arguments in the constructor that are annotated w/ @Inject as long as the parameters in the constructor are objects that Dagger 2 knows how to create. In order to use this:

  1. Simply make the Module class abstract
  2. Make the non-@Binds methods, ie the @Provides methods static.

Summary

There you have it! This is the simplest way in which you can use Dagger 2.

  1. It was super easy to make Main objects w/ a no arg constructor that’s annotated w/ @Inject. But it was harder when we have an interface that may be implemented by many different classes, and so a provider and module was needed to tell Dagger 2 how to bind an implementation to the interface.

  2. However it was not necessary to tell Dagger 2 how to explicitly create an instance of the class implementing the interface (this was achieved w/ @Inject annotation again, on the no arg constructor of the implementation class).

  3. The Module and Provider method simply was a way to Dagger 2 how to make that association or binding between the implementation of our choice and the interface.

References

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