Generic @Inject'd fields in an abstract superc

问题:

Consider a MVP-ish set of types. An abstract Presenter exists, with a View interface:

public interface View {
    //...
}

public abstract class AbstractPresenter<V extends View> {
    @Inject V view;
    //...
}

Then, lets have a specific concrete presenter subclass, with its view interface and implementation:

public interface LoginView extends View {
    //...
}
public LoginPresenter extends AbstractPresenter<LoginView> {
    //...
}

public class LoginViewImpl implements LoginView {
    //...
}

In a Dagger module, of course we would define a @Provides method:

@Provides
LoginView provideLoginView() {
    return new LoginViewImpl();
}

In Guice you could write this the same way, or just bind(LoginView.class).to(LoginViewImpl.class).

However, in Dagger (both v1 and the 2.0-SNAPSHOT from Google), this produces an error, since it can't figure out what V is when creating the binding wiring for AbstractPresenter<V>. On the other hand, Guice figures out that that because it is actually creating a LoginPresenter, so it needs an implementation of LoginView.

Dagger 1.2.2:

foo.bar.AbstractPresenter$$InjectAdapter.java:[21,31] cannot find symbol
  symbol:   class V
  location: class foo.bar.AbstractPresenter$$InjectAdapter

Dagger 2.0-SNAPSHOT:

Caused by: java.lang.IllegalArgumentException: V
    at dagger.internal.codegen.writer.TypeNames$2.defaultAction(TypeNames.java:39)
    at dagger.internal.codegen.writer.TypeNames$2.defaultAction(TypeNames.java:36)
    at javax.lang.model.util.SimpleTypeVisitor6.visitTypeVariable(SimpleTypeVisitor6.java:179)
    at com.sun.tools.javac.code.Type$TypeVar.accept(Type.java:1052)
    at dagger.internal.codegen.writer.TypeNames.forTypeMirror(TypeNames.java:36)
    at dagger.internal.codegen.MembersInjectorGenerator.write(MembersInjectorGenerator.java:142)
    at dagger.internal.codegen.MembersInjectorGenerator.write(MembersInjectorGenerator.java:61)
    at dagger.internal.codegen.SourceFileGenerator.generate(SourceFileGenerator.java:53)
    at dagger.internal.codegen.InjectBindingRegistry.generateSourcesForRequiredBindings(InjectBindingRegistry.java:101)
    at dagger.internal.codegen.ComponentProcessor.process(ComponentProcessor.java:149)

My question: Is this a bug? Is this a missing feature? Or is this a performance issue that Dagger is protecting us from (a la SerializableTypeOracleBuilder in GWT RPC)?

Note that this same issue occurs when V is referred to as Provider<V>, Lazy<V>, etc.

回答1:

That looks like a bug as it shouldn't throw an exception, but it should log a warning explaining that type parameters need to be bound to a specific type.

The rest is for Dagger2, and I'm using 2.1-SNAPSHOT. You haven't provided an example @Component that will do the injection and without it Dagger2 2.1-SNAPSHOT doesn't actually report a problem. It's possible that it has already fixed your problem and I'm seeing a slightly different version but if not then I presume your component looks something like this:

@Component
public interface PresenterComponent {
  <V extends View> void inject(AbstractPresenter<V> presenter);
}

When Dagger2 is processing this it cannot determine a concrete type for V, and so it doesn't know what type to insert. It can't just insert say LoginView because that would break if it was passed a AbstractPresenter<LogoutView>.

However, if you use say the following then Dagger2 can determine that it needs to inject a LoginView into AbstractPresenter<LoginView> and will do so safely.

@Module
public class LoginModule {
  @Provides LoginView provideLoginView() {
    return new LoginViewImpl();
  }
}

@Component(modules = LoginModule.class)
public interface LoginComponent {
    void inject(LoginPresenter presenter);
}

Unless you have no control over when an object is created, e.g. if some framework creates it for you and then passes in for you to initialize, it is much better to use @Inject on the constructor if you can, e.g. like this:

public LoginPresenter extends AbstractPresenter<LoginView> {
    //...
    @Inject LoginPresenter(LoginView view) {
        super(view);
    //...
    }
}

回答2:

This is because of the Type arguments. Injects does not work when u have a type arguments. U need to do something like this,

bind(new LoginPresenter<LoginViewImpl>(){});