Part 2 – Component, Provision Method & Constructor Injection

Watch the next part

In part 2 of the Dagger 2 tutorial, we will add the google/dagger dependencies to a new Android Studio project (for now we ignore the dagger.android classes) and then start learning how to use the Dagger 2 API.
We create a new Java interface that we annotate with @Component and which has 1 method declaration that returns our requested fully instantiated object. Dagger will then at compile time generate the implementation for this so-called provision method so we can use it in our activity. In order for this to work, we have to tell Dagger how to create all the necessary dependencies. One way of doing this is with the help of constructor injection, for which we have to annotate all the constructors Dagger has to know about with @Inject. This javax annotation is standardized under JSR 330.
Dagger will then internally generate a directed acyclic graph (DAG) that contains all the dependencies in the correct order. For this, it doesn’t use any Java reflection like earlier dependency injection frameworks (like Spring, Guice or Dagger 1).

Show less

Links & DependenciesCar.javaEngine.javaWheels.javaMainActivity.javaCarComponent.java

Dagger 2 dependencies:

github.com/google/dagger

package com.codinginflow.dagger2example;

import android.util.Log;

import javax.inject.Inject;

public class Car {
    private static final String TAG = "Car";

    private Engine engine;
    private Wheels wheels;

    @Inject
    public Car(Engine engine, Wheels wheels) {
        this.engine = engine;
        this.wheels = wheels;
    }

    public void drive() {
        Log.d(TAG, "driving...");
    }
}

package com.codinginflow.dagger2example;

import javax.inject.Inject;

public class Engine {

    @Inject
    public Engine() {

    }
}

package com.codinginflow.dagger2example;

import javax.inject.Inject;

public class Wheels {

    @Inject
    public Wheels() {

    }
}

package com.codinginflow.dagger2example;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {
    private Car car;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        CarComponent component = DaggerCarComponent.create();

        car = component.getCar();
        car.drive();
    }
}

package com.codinginflow.dagger2example;

import dagger.Component;

@Component
public interface CarComponent {

    Car getCar();
}