Dagger 2 constructor injection in kotlin with Named arguments

问题

I have this dependency:

@Singleton
class SpiceMix @Inject constructor(@field:[Named("oregano")] private val oregano: Spice,
                                   @field:[Named("sage")] private val sage: Spice,
                                   @field:[Named("rosemary")] private val rosemary: Spice) 

And a module to fulfill its dependencies:

@Module
class SpiceModule {

    @Provides
    @Named("oregano")
    @Singleton
    fun provideOregano(): Spice = Oregano()

    @Provides
    @Named("sage")
    @Singleton
    fun provideSage(): Spice = Sage()

    @Provides
    @Named("rosemary")
    @Singleton
    fun provideRosemary(): Spice = Rosemary()

The SpiceMix is then injected in various locations of my app.

However, this does not compile and I get an error:

Spice cannot be provided without an @Provides-annotated method

I think the @Named annotations do not quite work in my constructor signature. I am not quite sure how I can make it work.

Note: this compiles fine if I ditch the Named annotations and change the types of the constructor parameters to their concrete forms. However, Spice is an interface, and I need it for mocking purposes in my tests.

What can I do?

回答1:

You want to annotate the constructor parameters if you're doing constructor injection, and not the fields - use the @param: annotation target:

@Singleton
class SpiceMix @Inject constructor(@param:Named("oregano") private val oregano: Spice,
                                   @param:Named("sage") private val sage: Spice,
                                   @param:Named("rosemary") private val rosemary: Spice)

Edit: actually, since the resolution order for annotation targets is

• param;
• property;
• field.

according to the docs, having no annotation target should also annotate the parameter of the constructor. So you can just drop the target altogether:

@Singleton
class SpiceMix @Inject constructor(@Named("oregano") private val oregano: Spice,
                                   @Named("sage") private val sage: Spice,
                                   @Named("rosemary") private val rosemary: Spice)

来源：https://stackoverflow.com/questions/48442623/dagger-2-constructor-injection-in-kotlin-with-named-arguments