Android's recommended app architecture encourages dividing your code into classes to benefit from separation of concerns, a principle where each class of the hierarchy has a single defined responsibility. This leads to more, smaller classes that need to be connected together to fulfill each other's dependencies.
![Android apps are usually made up of many classes, and some of them
depend on each other.](https://developer.android.google.cn/static/topic/libraries/architecture/images/final-architecture.png?authuser=9)
The dependencies between classes can be represented as a graph, in which each
class is connected to the classes it depends on. The representation of all your
classes and their dependencies makes up the application graph.
In figure 1, you can see an abstraction of the application graph.
When class A (ViewModel
) depends on class B (Repository
), there's
a line that points from A to B representing that dependency.
Dependency injection helps make these connections and enables you to swap out
implementations for testing. For example, when testing a ViewModel
that depends on a repository, you can pass different implementations of
Repository
with either fakes or mocks to test the different cases.
Basics of manual dependency injection
This section covers how to apply manual dependency injection in a real Android app scenario. It walks through an iterated approach of how you might start using dependency injection in your app. The approach improves until it reaches a point that is very similar to what Dagger would automatically generate for you. For more information about Dagger, read Dagger basics.
Consider a flow to be a group of screens in your app that correspond to a feature. Login, registration, and checkout are all examples of flows.
When covering a login flow for a typical Android app, the LoginActivity
depends on LoginViewModel
, which in turn depends on UserRepository
.
Then UserRepository
depends on a UserLocalDataSource
and a
UserRemoteDataSource
, which in turn depends on a Retrofit
service.
![](https://developer.android.google.cn/static/images/training/dependency-injection/2-application-graph.png?authuser=9)
LoginActivity
is the entry point to the login flow and the user
interacts with the activity. Thus, LoginActivity
needs to create the
LoginViewModel
with all its dependencies.
The Repository
and DataSource
classes of the flow look like this:
Kotlin
class UserRepository( private val localDataSource: UserLocalDataSource, private val remoteDataSource: UserRemoteDataSource ) { ... } class UserLocalDataSource { ... } class UserRemoteDataSource( private val loginService: LoginRetrofitService ) { ... }
Java
class UserLocalDataSource { public UserLocalDataSource() { } ... } class UserRemoteDataSource { private final Retrofit retrofit; public UserRemoteDataSource(Retrofit retrofit) { this.retrofit = retrofit; } ... } class UserRepository { private final UserLocalDataSource userLocalDataSource; private final UserRemoteDataSource userRemoteDataSource; public UserRepository(UserLocalDataSource userLocalDataSource, UserRemoteDataSource userRemoteDataSource) { this.userLocalDataSource = userLocalDataSource; this.userRemoteDataSource = userRemoteDataSource; } ... }
Here's what LoginActivity
looks like:
Kotlin
class LoginActivity: Activity() { private lateinit var loginViewModel: LoginViewModel override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) // In order to satisfy the dependencies of LoginViewModel, you have to also // satisfy the dependencies of all of its dependencies recursively. // First, create retrofit which is the dependency of UserRemoteDataSource val retrofit = Retrofit.Builder() .baseUrl("https://example.com") .build() .create(LoginService::class.java) // Then, satisfy the dependencies of UserRepository val remoteDataSource = UserRemoteDataSource(retrofit) val localDataSource = UserLocalDataSource() // Now you can create an instance of UserRepository that LoginViewModel needs val userRepository = UserRepository(localDataSource, remoteDataSource) // Lastly, create an instance of LoginViewModel with userRepository loginViewModel = LoginViewModel(userRepository) } }
Java
public class MainActivity extends Activity { private LoginViewModel loginViewModel; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // In order to satisfy the dependencies of LoginViewModel, you have to also // satisfy the dependencies of all of its dependencies recursively. // First, create retrofit which is the dependency of UserRemoteDataSource Retrofit retrofit = new Retrofit.Builder() .baseUrl("https://example.com") .build() .create(LoginService.class); // Then, satisfy the dependencies of UserRepository UserRemoteDataSource remoteDataSource = new UserRemoteDataSource(retrofit); UserLocalDataSource localDataSource = new UserLocalDataSource(); // Now you can create an instance of UserRepository that LoginViewModel needs UserRepository userRepository = new UserRepository(localDataSource, remoteDataSource); // Lastly, create an instance of LoginViewModel with userRepository loginViewModel = new LoginViewModel(userRepository); } }
There are issues with this approach:
There's a lot of boilerplate code. If you wanted to create another instance of
LoginViewModel
in another part of the code, you'd have code duplication.Dependencies have to be declared in order. You have to instantiate
UserRepository
beforeLoginViewModel
in order to create it.It's difficult to reuse objects. If you wanted to reuse
UserRepository
across multiple features, you'd have to make it follow the singleton pattern. The singleton pattern makes testing more difficult because all tests share the same singleton instance.
Managing dependencies with a container
To solve the issue of reusing objects, you can create your own
dependencies container class that you use to get dependencies. All instances
provided by this container can be public. In the example, because you only need
an instance of UserRepository
, you can make its dependencies private with the
option of making them public in the future if they need to be provided:
Kotlin
// Container of objects shared across the whole app class AppContainer { // Since you want to expose userRepository out of the container, you need to satisfy // its dependencies as you did before private val retrofit = Retrofit.Builder() .baseUrl("https://example.com") .build() .create(LoginService::class.java) private val remoteDataSource = UserRemoteDataSource(retrofit) private val localDataSource = UserLocalDataSource() // userRepository is not private; it'll be exposed val userRepository = UserRepository(localDataSource, remoteDataSource) }
Java
// Container of objects shared across the whole app public class AppContainer { // Since you want to expose userRepository out of the container, you need to satisfy // its dependencies as you did before private Retrofit retrofit = new Retrofit.Builder() .baseUrl("https://example.com") .build() .create(LoginService.class); private UserRemoteDataSource remoteDataSource = new UserRemoteDataSource(retrofit); private UserLocalDataSource localDataSource = new UserLocalDataSource(); // userRepository is not private; it'll be exposed public UserRepository userRepository = new UserRepository(localDataSource, remoteDataSource); }
Because these dependencies are used across the whole application, they need
to be placed in a common place all activities can use: the
Application
class. Create a custom
Application
class that contains an AppContainer
instance.
Kotlin
// Custom Application class that needs to be specified // in the AndroidManifest.xml file class MyApplication : Application() { // Instance of AppContainer that will be used by all the Activities of the app val appContainer = AppContainer() }
Java
// Custom Application class that needs to be specified // in the AndroidManifest.xml file public class MyApplication extends Application { // Instance of AppContainer that will be used by all the Activities of the app public AppContainer appContainer = new AppContainer(); }
Now you can get the instance of the AppContainer
from the application and
obtain the shared of UserRepository
instance:
Kotlin
class LoginActivity: Activity() { private lateinit var loginViewModel: LoginViewModel override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) // Gets userRepository from the instance of AppContainer in Application val appContainer = (application as MyApplication).appContainer loginViewModel = LoginViewModel(appContainer.userRepository) } }
Java
public class MainActivity extends Activity { private LoginViewModel loginViewModel; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // Gets userRepository from the instance of AppContainer in Application AppContainer appContainer = ((MyApplication) getApplication()).appContainer; loginViewModel = new LoginViewModel(appContainer.userRepository); } }
In this way, you don't have a singleton UserRepository
. Instead, you have an
AppContainer
shared across all activities that contains objects from the graph
and creates instances of those objects that other classes can consume.
If LoginViewModel
is needed in more places in the application, having a
centralized place where you create instances of LoginViewModel
makes
sense. You can move the creation of LoginViewModel
to the container and provide
new objects of that type with a factory. The code for a LoginViewModelFactory
looks like this:
Kotlin
// Definition of a Factory interface with a function to create objects of a type interface Factory<T> { fun create(): T } // Factory for LoginViewModel. // Since LoginViewModel depends on UserRepository, in order to create instances of // LoginViewModel, you need an instance of UserRepository that you pass as a parameter. class LoginViewModelFactory(private val userRepository: UserRepository) : Factory{ override fun create(): LoginViewModel { return LoginViewModel(userRepository) } }
Java
// Definition of a Factory interface with a function to create objects of a type public interface Factory<T> { T create(); } // Factory for LoginViewModel. // Since LoginViewModel depends on UserRepository, in order to create instances of // LoginViewModel, you need an instance of UserRepository that you pass as a parameter. class LoginViewModelFactory implements Factory{ private final UserRepository userRepository; public LoginViewModelFactory(UserRepository userRepository) { this.userRepository = userRepository; } @Override public LoginViewModel create() { return new LoginViewModel(userRepository); } }
You can include the LoginViewModelFactory
in the AppContainer
and make the
LoginActivity
consume it:
Kotlin
// AppContainer can now provide instances of LoginViewModel with LoginViewModelFactory class AppContainer { ... val userRepository = UserRepository(localDataSource, remoteDataSource) val loginViewModelFactory = LoginViewModelFactory(userRepository) } class LoginActivity: Activity() { private lateinit var loginViewModel: LoginViewModel override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) // Gets LoginViewModelFactory from the application instance of AppContainer // to create a new LoginViewModel instance val appContainer = (application as MyApplication).appContainer loginViewModel = appContainer.loginViewModelFactory.create() } }
Java
// AppContainer can now provide instances of LoginViewModel with LoginViewModelFactory public class AppContainer { ... public UserRepository userRepository = new UserRepository(localDataSource, remoteDataSource); public LoginViewModelFactory loginViewModelFactory = new LoginViewModelFactory(userRepository); } public class MainActivity extends Activity { private LoginViewModel loginViewModel; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // Gets LoginViewModelFactory from the application instance of AppContainer // to create a new LoginViewModel instance AppContainer appContainer = ((MyApplication) getApplication()).appContainer; loginViewModel = appContainer.loginViewModelFactory.create(); } }
This approach is better than the previous one, but there are still some challenges to consider:
You have to manage
AppContainer
yourself, creating instances for all dependencies by hand.There is still a lot of boilerplate code. You need to create factories or parameters by hand depending on whether you want to reuse an object or not.
Managing dependencies in application flows
AppContainer
gets complicated when you want to include more functionality in
the project. When your app becomes larger and you start introducing different
feature flows, there are even more problems that arise:
When you have different flows, you might want objects to just live in the scope of that flow. For example, when creating
LoginUserData
(that might consist of the username and password used only in the login flow) you don't want to persist data from an old login flow from a different user. You want a new instance for every new flow. You can achieve that by creatingFlowContainer
objects inside theAppContainer
as demonstrated in the next code example.Optimizing the application graph and flow containers can also be difficult. You need to remember to delete instances that you don't need, depending on the flow you're in.
Imagine you have a login flow that consists of one activity (LoginActivity
)
and multiple fragments (LoginUsernameFragment
and LoginPasswordFragment
).
These views want to:
Access the same
LoginUserData
instance that needs to be shared until the login flow finishes.Create a new instance of
LoginUserData
when the flow starts again.
You can achieve that with a login flow container. This container needs to be created when the login flow starts and removed from memory when the flow ends.
Let's add a LoginContainer
to the example code. You want to be able to create
multiple instances of LoginContainer
in the app, so instead of making it a
singleton, make it a class with the dependencies the login flow needs from
the AppContainer
.
Kotlin
class LoginContainer(val userRepository: UserRepository) { val loginData = LoginUserData() val loginViewModelFactory = LoginViewModelFactory(userRepository) } // AppContainer contains LoginContainer now class AppContainer { ... val userRepository = UserRepository(localDataSource, remoteDataSource) // LoginContainer will be null when the user is NOT in the login flow var loginContainer: LoginContainer? = null }
Java
// Container with Login-specific dependencies class LoginContainer { private final UserRepository userRepository; public LoginContainer(UserRepository userRepository) { this.userRepository = userRepository; loginViewModelFactory = new LoginViewModelFactory(userRepository); } public LoginUserData loginData = new LoginUserData(); public LoginViewModelFactory loginViewModelFactory; } // AppContainer contains LoginContainer now public class AppContainer { ... public UserRepository userRepository = new UserRepository(localDataSource, remoteDataSource); // LoginContainer will be null when the user is NOT in the login flow public LoginContainer loginContainer; }
Once you have a container specific to a flow, you have to decide when to create
and delete the container instance. Because your login flow is self-contained in
an activity (LoginActivity
), the activity is the one managing the lifecycle
of that container. LoginActivity
can create the instance in onCreate()
and
delete it in onDestroy()
.
Kotlin
class LoginActivity: Activity() { private lateinit var loginViewModel: LoginViewModel private lateinit var loginData: LoginUserData private lateinit var appContainer: AppContainer override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) appContainer = (application as MyApplication).appContainer // Login flow has started. Populate loginContainer in AppContainer appContainer.loginContainer = LoginContainer(appContainer.userRepository) loginViewModel = appContainer.loginContainer.loginViewModelFactory.create() loginData = appContainer.loginContainer.loginData } override fun onDestroy() { // Login flow is finishing // Removing the instance of loginContainer in the AppContainer appContainer.loginContainer = null super.onDestroy() } }
Java
public class LoginActivity extends Activity { private LoginViewModel loginViewModel; private LoginData loginData; private AppContainer appContainer; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); appContainer = ((MyApplication) getApplication()).appContainer; // Login flow has started. Populate loginContainer in AppContainer appContainer.loginContainer = new LoginContainer(appContainer.userRepository); loginViewModel = appContainer.loginContainer.loginViewModelFactory.create(); loginData = appContainer.loginContainer.loginData; } @Override protected void onDestroy() { // Login flow is finishing // Removing the instance of loginContainer in the AppContainer appContainer.loginContainer = null; super.onDestroy(); } }
Like LoginActivity
, login fragments can access the LoginContainer
from
AppContainer
and use the shared LoginUserData
instance.
Because in this case you're dealing with view lifecycle logic, using lifecycle observation makes sense.
Conclusion
Dependency injection is a good technique for creating scalable and testable Android apps. Use containers as a way to share instances of classes in different parts of your app and as a centralized place to create instances of classes using factories.
When your application gets larger, you will start seeing that you write a lot of boilerplate code (such as factories), which can be error-prone. You also have to manage the scope and lifecycle of the containers yourself, optimizing and discarding containers that are no longer needed in order to free up memory. Doing this incorrectly can lead to subtle bugs and memory leaks in your app.
In the Dagger section, you'll learn how you can use Dagger to automate this process and generate the same code you would have written by hand otherwise.