Injecting a repository into a Service in Android using Hilt

I have an Android project with Hilt dependency injection. I have defined MyApplication and MyModule as follows.

@HiltAndroidApp
class MyApplication : Application()

@Module
@InstallIn(ApplicationComponent::class)
abstract class MyModule {
    @Binds
    @Singleton
    abstract fun bindMyRepository(
        myRepositoryImpl: MyRepositoryImpl
    ): MyRepository
}

MyRepositoryImpl implements the MyRepository interface:

interface MyRepository {
    fun doSomething(): String
}

class MyRepositoryImpl
@Inject
constructor(

) : MyRepository {
    override fun doSomething() = ""
}

I can now inject this implementation of MyRepository into a ViewModel:

class MyActivityViewModel
@ViewModelInject
constructor(
    private val myRepository: MyRepository,
) : ViewModel() { }

This works as expected. However, if I try to inject the repository into a service, I get an error java.lang.Class<MyService> has no zero argument constructor:

class MyService
@Inject
constructor(
    private val myRepository: MyRepository,
): Service() { }

The same error occurs with an activity, too:

class MyActivity
@Inject
constructor(
    private val myRepository: MyRepository,
) : AppCompatActivity(R.layout.my_layout) { }

What am I doing wrong with the injection?

From the documentation on how we Inject dependencies into Android classes, we can learn the following:

Hilt can provide dependencies to other Android classes that have the @AndroidEntryPoint annotation.

Hilt currently supports the following Android classes:

• Application (by using @HiltAndroidApp)
• ViewModel (by using @HiltViewModel)
• Activity
• Fragment
• View
• Service
• BroadcastReceiver

So when you subclass any of these Android classes, you don't ask Hilt to inject dependencies through the constructors. Instead, you annotate it with @AndroidEntryPoint, and ask Hilt to inject its dependencies by annotating the property with @Inject:

@AndroidEntryPoint
class ExampleActivity : AppCompatActivity() { 

    @Inject
    lateinit var mAdapter: SomeAdapter 

    ...

}

So, in your case you should inject MyRepository in MyActivity and MyService like this:

@AndroidEntryPoint
class MyService: Service() {

    @Inject
    lateinit var myRepository: MyRepository
   
    ...

}

@AndroidEntryPoint
class MyActivity: AppCompatActivity(R.layout.my_layout) { 

    @Inject
    lateinit var myRepository: MyRepository

    ...

}

And remember:

Fields injected by Hilt cannot be private

That's it for Android classes that is supported by Hilt.

If you wonder what about classes not supported by Hilt (ex: ContentProvider)?! I recommend learning how from this tutorial @EntryPoint annotation on codelab (also don't forget to check the documentation for how to Inject dependencies in classes not supported by Hilt).