Guice and interface that has multiple implementations

java guice
newbie picture newbie · Nov 8, 2011 · Viewed 26.4k times · Source

If I have interface Validator and multiple implementations for this interface. How can I inject any of the multiple implementations with Guice? Now I know that I can use following code to inject one, but it allows only one implementation:

public class MyModule extends AbstractModule {
  @Override 
  protected void configure() {
    bind(Validator.class).to(OneOfMyValidators.class);
  }
}

What I would like to do is:

Validator v1 = injector.getInstance(Validator1.class);
Validator v2 = injector.getInstance(Validator2.class);

Is it possible at all?

Answer

Andrew McNamee picture Andrew McNamee · Nov 8, 2011

Short answer: binding annotations. They're basically a way of letting the depender give a hint that points towards a particular instance or implementation without requiring a dependency on the full concrete implementation class.

See: https://github.com/google/guice/wiki/BindingAnnotations

For example, in the module, you might do:

bind(Validator.class).annotatedWith(ValidatorOne.class).to(OneOfMyValidators.class);
bind(Validator.class).annotatedWith(ValidatorTwo.class).to(SomeOtherValidator.class);

And in your constructor, you'd do:

@Inject
MyClass(@ValidatorOne Validator someValidator,
    @ValidatorTwo Validator otherValidator) {
  ...
}

To get an annotated value straight from an Injector, you'll have to use the Guice Key class, like:

Validator v1 = injector.getInstance(Key.get(Validator.class, ValidatorOne.class));

On a side note, binding annotations are very useful for injecting runtime constants. See the comments for bindConstant in:

https://google.github.io/guice/api-docs/latest/javadoc/index.html?com/google/inject/Binder.html

Related questions

Overriding Binding in Guice

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 …

Read More
How to instantiate spring managed beans at runtime?

I stuck with a simple refactoring from plain java to spring. Application has a "Container" object which instantiates its parts at runtime. Let me explain with the code: public class Container { private List<RuntimeBean> runtimeBeans = new ArrayList<…

Read More
Why java classes do not inherit annotations from implemented interfaces?

I've been using Guice's AOP to intercept some method calls. My class implements an interface and I would like to annotate the interface methods so Guice could select the right methods. Even if the annotation type is annotated with Inherited …

Read More