OptionalBinding using Guice to avoid binding by users

Question

Following the documentation of OptionalBinder

An API to bind optional values, optionally with a default value. OptionalBinder fulfills two roles:

1. It allows a framework to define an injection point that may or may not be bound by users.
2. It allows a framework to supply a default value that can be changed by users.

I am trying to follow up on the first point above for which I have the following setup:

interface Reporting<R> {} // service to be bind optionally

class InternalServiceImpl implements InternalService {
    @Inject
    Reporting reporting;
    ... // use this in a method
}

public class FrameworkModule extends AbstractModule {
   protected void configure() {
     OptionalBinder.newOptionalBinder(binder(), Reporting.class);
   }
}

in the user modules(class UserWorkingModule) if I do not provide a binding such as

bind(new TypeLiteral<Reporting<ReportingEvent>>(){}).to(ReportingImpl.class).in(Singleton.class);

the application fails to start with the following logs:

1) No implementation for Reporting was bound.   while locating Reporting
    for field at InternalServiceImpl.reporting(InternalServiceImpl.java:21) at
FrameworkModule.configure(FrameworkModule.java:55) (via modules: UserWorkingModule -> FrameworkModule)

Is it still a must to provide a binding for Reporting in the UserWorkingModule?

Answer 1

bind(new TypeLiteral<Reporting<ReportingEvent>>(){}).to(ReportingImpl.class).in(Singleton.class);

is a binding for the generic Reporting<ReportingEvent>, while

OptionalBinder.newOptionalBinder(binder(), Reporting.class);

is actually specifying the raw type, Reporting. Instead, you want to use the overload for newOptionalBinder which specifies a TypeLiteral arg, so that you are talking about the same thing in both the optional request and the binding:

OptionalBinder.newOptionalBinder(binder(), new TypeLiteral<Reporting<ReportingEvent>>(){});

Without this, you are basically saying "any binding to Reporting will satisfy this requirement", even something like Reporting<Object> - and what happens if you bind more than one?

---

On the other hand, if you actually want to allow any binding of any Reporting type (which is what your error suggests, then instead the opposite thing is wrong: instead of binding to raw Reporting, you specified the generic impl instead. Change that bind() call to say "this actually only works for raw requests":

bind(Reporting.class).to(ReportingImpl.class).in(Singleton.class);

Answer 2

You're not actually specifying a binding, when using the OptionalBinder. You still need to specify a default binding when using the OptionalBinder try:

     OptionalBinder.newOptionalBinder(binder(), Reporting.class)
        .setDefault()
        .to(ReportingImpl.class);