How to use Dagger in Java library module in Android Studio?

Question

I'm using Dagger in a Java library module in an Android Studio project and here's what my build.gradle for the module looks like:

apply plugin: 'java-library'

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])

    implementation 'com.google.dagger:dagger:2.24'
    annotationProcessor 'com.google.dagger:dagger-compiler:2.24'
}

sourceCompatibility = "7"
targetCompatibility = "7"

I can see that the Dagger is properly generating implementations and they are present in build/generated/sources/annotationProcessor but for some reason I cannot access them in the code. Also, the generated files shows an error at the package statement that states:

Package name "com.example.javamodule" does not correspond to the file path "java.main.com.example.javamodule"

I have two questions here. First, how can I access the Dagger generated classes in my java module code and second, how to remove the above-mentioned error from the generated classes?

Carlos Leonardo Camilo Vargas · Accepted Answer · 2020-10-28 06:22:15Z

In your java library's gradle file:

plugins {
    id 'java-library'
    id 'kotlin'
    id 'kotlin-kapt'
}

java {
    sourceCompatibility = JavaVersion.VERSION_1_7
    targetCompatibility = JavaVersion.VERSION_1_7
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"

    //Dependency injection
    implementation 'com.google.dagger:dagger:2.27'
    kapt 'com.google.dagger:dagger-compiler:2.24'
}

Then create a class and its dependencies:

class First
@Inject
constructor(
    private val second: Second,
    private val third: Third
) {
    fun firstFunction() {
        print(second.secondMessage())
        print(third.name)
    }
}

class Second(
    private val name: String
) {
    fun secondMessage(): String {
        return name
    }
}

class Third(
    val name: String
)

Then create your dagger module:

@Module
class ModuleUtil {

    @Provides
    fun providesSecond(): Second {
        return Second("second")
    }

    @Provides
    fun providesThird(): Third {
        return Third("third")
    }

}

Then create your dagger component:

@Singleton
@Component(modules = [
    ModuleUtil::class
])
interface MainComponent {

    fun maker(): First

}

An object to handle the generated component:

object DaggerWrapper {

    lateinit var mainComponent: MainComponent

    fun initComponent() {
        mainComponent = DaggerMainComponent
            .builder()
            .build()
    }

}

And finally in your app android module(eg. inside an Activity):

DaggerWrapper.initComponent()
            val mainComponent = DaggerWrapper.mainComponent
            val first = mainComponent.maker()
            first.firstFunction()

you might also need ` implementation 'com.google.dagger:hilt-core:$VERSION`

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