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Disambiguating Between Instances With Google Guice

Biju Kunjummen user avatar by
Biju Kunjummen
·
Feb. 11, 15 · Interview
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Google guice provides a neat way to select a target implementation if there are multiple implementations of an interface. My samples are based on anexcellent article by Josh Long(@starbuxman) on a similar mechanism that Spring provides.

So, consider an interface called MarketPlace having two implementations, an AndroidMarketPlace and AppleMarketPlace:

interface MarketPlace {
}

class AppleMarketPlace implements MarketPlace {

    @Override
    public String toString() {
        return "apple";
    }
}

class GoogleMarketPlace implements MarketPlace {

    @Override
    public String toString() {
        return "android";
    }
}

and consider a user of these implementations:

class MarketPlaceUser {
    private final MarketPlace marketPlace;
    
    public MarketPlaceUser(MarketPlace marketPlace) {
        System.out.println("MarketPlaceUser constructor called..");
        this.marketPlace = marketPlace;
    }

    public String showMarketPlace() {
        return this.marketPlace.toString();
    }

}

A good way for MarketPlaceUser to disambiguate between these implementations is to use a guice feature called Binding Annotations. To make use of this feature, start by defining annotations for each of these implementations this way:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@BindingAnnotation
@interface Android {}

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@BindingAnnotation
@interface Ios {}

and inform the Guice binder about these annotations and the appropriate implementation corresponding to the annotation:

class MultipleInstancesModule extends AbstractModule {

    @Override
    protected void configure() {
        bind(MarketPlace.class).annotatedWith(Ios.class).to(AppleMarketPlace.class).in(Scopes.SINGLETON);
        bind(MarketPlace.class).annotatedWith(Android.class).to(GoogleMarketPlace.class).in(Scopes.SINGLETON);
        bind(MarketPlaceUser.class).in(Scopes.SINGLETON);
    }
}

Now, if MarketPlaceUser needs to use one or the other implementation, this is how the dependency can be injected in:

import com.google.inject.*;

class MarketPlaceUser {
    private final MarketPlace marketPlace;

    @Inject
    public MarketPlaceUser(@Ios MarketPlace marketPlace) {
        this.marketPlace = marketPlace;
    }

}

This is very intuitive. If you have concerns about defining so many annotations, another approach could be to use @Named built-in Google Guice annotation, this way:

class MultipleInstancesModule extends AbstractModule {

    @Override
    protected void configure() {
        bind(MarketPlace.class).annotatedWith(Names.named("ios")).to(AppleMarketPlace.class).in(Scopes.SINGLETON);
        bind(MarketPlace.class).annotatedWith(Names.named("android")).to(GoogleMarketPlace.class).in(Scopes.SINGLETON);
        bind(MarketPlaceUser.class).in(Scopes.SINGLETON);
    }
}

and use it this way, where the dependency is required:

import com.google.inject.*;

class MarketPlaceUser {
    private final MarketPlace marketPlace;

    @Inject
    public MarketPlaceUser(@Named("ios") MarketPlace marketPlace) {
        this.marketPlace = marketPlace;
    }

}
If you are interested in exploring this further, here is the Google guice sample and an equivalent sample using Spring framework
Google Guice Google (verb)

Published at DZone with permission of Biju Kunjummen, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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