Dependency injection with Hilt in Android Kotlin

Introduction

Dependency injection helps your app get the things it needs without creating them itself. Hilt makes this easy and organized in Android apps.

When you want to share the same object across different parts of your app without creating it multiple times.

When you want to make your code easier to test by providing fake or mock objects.

When your app has many classes that depend on each other and you want to manage them cleanly.

When you want to reduce boilerplate code for creating and passing dependencies manually.

Syntax

Android Kotlin

1. Add @HiltAndroidApp to your Application class.
2. Use @AndroidEntryPoint on Activities or Fragments.
3. Create modules with @Module and @InstallIn annotations.
4. Provide dependencies with @Provides or @Inject constructors.
5. Use @Inject to get dependencies in your classes.

@HiltAndroidApp triggers code generation for dependency injection setup.

@AndroidEntryPoint tells Hilt where to inject dependencies in Android components.

Examples

This marks your Application class to start Hilt.

Android Kotlin

@HiltAndroidApp
class MyApp : Application()

This injects UserRepository into MainActivity automatically.

Android Kotlin

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
  @Inject lateinit var repository: UserRepository
}

This module tells Hilt how to create a Retrofit instance.

Android Kotlin

@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {
  @Provides
  fun provideRetrofit(): Retrofit {
    return Retrofit.Builder().baseUrl("https://api.example.com").build()
  }
}

This shows constructor injection where Hilt provides Retrofit automatically.

Android Kotlin

class UserRepository @Inject constructor(private val retrofit: Retrofit) {
  // Use retrofit here
}

Sample App

This example shows a simple Android app using Hilt. The GreetingService is provided by a module and injected into MainActivity. When the app runs, it prints a greeting message.

Android Kotlin

import android.app.Application
import dagger.hilt.android.HiltAndroidApp

@HiltAndroidApp
class MyApp : Application()

import androidx.activity.ComponentActivity
import android.os.Bundle
import dagger.hilt.android.AndroidEntryPoint
import javax.inject.Inject

@AndroidEntryPoint
class MainActivity : ComponentActivity() {
    @Inject lateinit var greetingService: GreetingService

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        println(greetingService.greet())
    }
}

import dagger.Module
import dagger.Provides
import dagger.hilt.InstallIn
import dagger.hilt.components.SingletonComponent
import javax.inject.Singleton

@Module
@InstallIn(SingletonComponent::class)
object GreetingModule {
    @Provides
    @Singleton
    fun provideGreetingService(): GreetingService = GreetingService()
}

class GreetingService {
    fun greet() = "Hello from Hilt!"
}

Important Notes

Always add @HiltAndroidApp to your Application class once per app.

Use @AndroidEntryPoint on every Activity or Fragment where you want injection.

Modules define how to create objects that Hilt cannot create by itself.

Summary

Hilt helps manage dependencies automatically in Android apps.

Use annotations like @HiltAndroidApp, @AndroidEntryPoint, @Module, and @Inject.

This makes your code cleaner, easier to test, and reduces manual setup.

Practice

1. What is the primary purpose of using @HiltAndroidApp annotation in an Android application?

easy

A. To inject dependencies into a ViewModel

B. To mark an Activity as injectable by Hilt

C. To trigger Hilt's code generation and setup the application-level dependency container

D. To define a module that provides dependencies

Solution

1. Step 1: Understand the role of @HiltAndroidApp

   This annotation is placed on the Application class to start Hilt's code generation and create the application-level dependency container.

2. Step 2: Differentiate from other annotations

   @AndroidEntryPoint marks Activities or Fragments, @Module defines providers, and @Inject is for injection points, so they do not setup the application container.

3. Final Answer:

   To trigger Hilt's code generation and setup the application-level dependency container -> Option C

4. Quick Check:

   @HiltAndroidApp = Setup app container [OK]

Hint: Remember: @HiltAndroidApp is only on Application class [OK]

Common Mistakes:

• Confusing @HiltAndroidApp with @AndroidEntryPoint
• Thinking @HiltAndroidApp is for Activities
• Using @HiltAndroidApp on modules

2. Which of the following is the correct way to inject a dependency into an Android Activity using Hilt?

easy

A. Annotate the Activity with @AndroidEntryPoint and use @Inject lateinit var for the dependency

B. Use @Module annotation on the Activity and call inject() manually

C. Add @HiltAndroidApp on the Activity and initialize dependencies in onCreate()

D. Declare dependencies as constructor parameters without annotations

Solution

1. Step 1: Identify how to enable injection in Activity

   Using @AndroidEntryPoint on the Activity allows Hilt to generate necessary code for injection.

2. Step 2: Inject dependencies inside Activity

   Use @Inject lateinit var to declare dependencies that Hilt will provide automatically.

3. Final Answer:

   Annotate the Activity with @AndroidEntryPoint and use @Inject lateinit var for the dependency -> Option A

4. Quick Check:

   @AndroidEntryPoint + @Inject lateinit var = Activity injection [OK]

Hint: Use @AndroidEntryPoint on Activity and @Inject lateinit var inside [OK]

Common Mistakes:

• Trying to use @Module on Activity
• Forgetting @AndroidEntryPoint annotation
• Injecting dependencies via constructor in Activity

3. Given the following Hilt module code, what will be the output when repository.getData() is called?

@Module
@InstallIn(SingletonComponent::class)
object DataModule {
    @Provides
    fun provideData(): String = "Hello Hilt"
}

class Repository @Inject constructor(private val data: String) {
    fun getData() = data
}

medium

A. "DataModule"

B. Compilation error due to missing @Inject on Repository

C. NullPointerException at runtime

D. "Hello Hilt"

Solution

1. Step 1: Understand the provided dependency

   The module provides a String "Hello Hilt" via the provideData() function.

2. Step 2: Check Repository injection

   Repository constructor is annotated with @Inject and receives the String dependency correctly.

3. Final Answer:

   "Hello Hilt" -> Option D

4. Quick Check:

   Provided String = "Hello Hilt" [OK]

Hint: Look for @Provides return value matching injected type [OK]

Common Mistakes:

• Assuming @Inject missing on Repository causes error (it is present)
• Confusing module name with provided value
• Expecting runtime null error without reason

4. Identify the error in the following Hilt setup:

@Module
@InstallIn(ActivityComponent::class)
object NetworkModule {
    @Provides
    fun provideRetrofit(): Retrofit {
        return Retrofit.Builder().baseUrl("https://api.example.com").build()
    }
}

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
    @Inject lateinit var retrofit: Retrofit

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }
}

medium

A. Retrofit cannot be provided in ActivityComponent scope because it should be Singleton

B. Missing @Inject constructor in Retrofit class

C. MainActivity must be annotated with @HiltAndroidApp

D. Retrofit instance must be created manually, not with @Provides

Solution

1. Step 1: Check the component scope for Retrofit

   Retrofit is usually a singleton because it is expensive to create and shared across app.

2. Step 2: Analyze the module's @InstallIn scope

   The module is installed in ActivityComponent, which creates a new instance per Activity, not singleton.

3. Step 3: Identify the mismatch

   Providing Retrofit in ActivityComponent scope causes multiple instances, which is incorrect for Retrofit.

4. Final Answer:

   Retrofit cannot be provided in ActivityComponent scope because it should be Singleton -> Option A

5. Quick Check:

   Retrofit = Singleton scope needed [OK]

Hint: Provide expensive objects in SingletonComponent scope [OK]

Common Mistakes:

• Thinking Retrofit needs @Inject constructor
• Confusing @HiltAndroidApp with @AndroidEntryPoint
• Believing manual creation is required with @Provides

5. You want to inject two different implementations of an interface Logger using Hilt in your Android app. Which approach correctly distinguishes these implementations for injection?

hard

A. Provide both implementations in the same module without qualifiers and inject List<Logger>

B. Use @Qualifier annotations with different names on each provider and injection site

C. Use @Singleton on both implementations and inject Logger directly

D. Create separate modules for each implementation without any annotations

Solution

1. Step 1: Understand multiple implementations injection

   When multiple implementations of the same interface exist, Hilt needs a way to distinguish them.

2. Step 2: Use @Qualifier annotations

   Custom @Qualifier annotations with different names allow Hilt to know which implementation to inject where.

3. Step 3: Reject other options

   Injecting List<Logger> is possible but does not distinguish single injection points; @Singleton does not differentiate implementations; separate modules without qualifiers cause ambiguity.

4. Final Answer:

   Use @Qualifier annotations with different names on each provider and injection site -> Option B

5. Quick Check:

   Multiple implementations = use @Qualifier [OK]

Hint: Use @Qualifier to name different implementations [OK]

Common Mistakes:

• Injecting interface without qualifiers causes errors
• Assuming @Singleton differentiates implementations
• Not using qualifiers when multiple providers exist