Overriding Binding in Guice

Question

I've just started playing with Guice, and a use-case I can think of is that in a test I just want to override a single binding. I think I'd like to use the rest of the production level bindings to ensure everything is setup correctly and to avoid duplication.

So imagine I have the following Module

public class ProductionModule implements Module {
    public void configure(Binder binder) {
        binder.bind(InterfaceA.class).to(ConcreteA.class);
        binder.bind(InterfaceB.class).to(ConcreteB.class);
        binder.bind(InterfaceC.class).to(ConcreteC.class);
    }
}

And in my test I only want to override InterfaceC, while keeping InterfaceA and InterfaceB in tact, so I'd want something like:

Module testModule = new Module() {
    public void configure(Binder binder) {
        binder.bind(InterfaceC.class).to(MockC.class);
    }
};
Guice.createInjector(new ProductionModule(), testModule);

I've also tried the following, with no luck:

Module testModule = new ProductionModule() {
    public void configure(Binder binder) {
        super.configure(binder);
        binder.bind(InterfaceC.class).to(MockC.class);
    }
};
Guice.createInjector(testModule);

Does anyone know if it's possible to do what I want or am I completely barking up the wrong tree??

--- Follow up: It would seem I can achieve what I want if I make use of the @ImplementedBy tag on the interface and then just provide a binding in the test case, which works nicely when there is a 1-1 mapping between the interface and implementation.

Also, after discussing this with a colleague it would seem we'd head down the road of overriding an entire module and ensuring we have our modules defined correctly. This seems like it might cause a problem though where a binding is misplaced in a module and needs to be moved, thus possibly breaking a load of tests as bindings may no longer be available to be overriden.

Answer 1

This might not be the answer you're looking for, but if you're writing unit tests, you probably shouldn't be using an injector and rather be injecting mock or fake objects by hand.

On the other hand, if you really want to replace a single binding, you could use Modules.override(..):

public class ProductionModule implements Module {
    public void configure(Binder binder) {
        binder.bind(InterfaceA.class).to(ConcreteA.class);
        binder.bind(InterfaceB.class).to(ConcreteB.class);
        binder.bind(InterfaceC.class).to(ConcreteC.class);
    }
}
public class TestModule implements Module {
    public void configure(Binder binder) {
        binder.bind(InterfaceC.class).to(MockC.class);
    }
}
Guice.createInjector(Modules.override(new ProductionModule()).with(new TestModule()));

See details here.

But as the javadoc for Modules.overrides(..) recommends, you should design your modules in such a way that you don't need to override bindings. In the example you gave, you could accomplish that by moving the binding of InterfaceC to a separate module.