CODEWITHSUNDEEP
javaguice
vinoths
vinoths

Reputation: 1121

Guice - bind different instances based on enclosing class

Is it possible to bind Named instance based on the enclosing class? For ex: in the below sample, classes A and B will get DataStore instance injected. However, I need to have primary store as StoreA and secondary store as StoreB in the scope/context of class A but primary store as StoreC and secondary store as StoreD in the scope/context of class B. How can this be achieved?

class A {
   @Inject
   public A(DataStore dataStore) {
      ... 
   }
}

class B {
   @Inject
   public B(DataStore dataStore) {
      ... 
   }
}

class DataStore {
   @Inject
   public A(@Named("primary") Store primaryStore, @Named("secondary") Store store) {
      ... 
   }
}

bind(Store.class).annotatedWith(Names.named("primary")).to(StoreA.class);//for class A
bind(Store.class).annotatedWith(Names.named("secondary")).to(StoreB.class);//for class A
bind(Store.class).annotatedWith(Names.named("primary")).to(StoreC.class);//for class B
bind(Store.class).annotatedWith(Names.named("secondary")).to(StoreD.class);//for class B

Upvotes: 0

Views: 596

Answers (1)

Jeff Bowman
Jeff Bowman

Reputation: 95634

This is sometimes known as the "robot legs" problem, as if building a robot with identical legs but differing left and right feet. Each Leg to bind a Foot, but the Foot requested depends on the Leg requested. In your case, each DataStore binds to a primary and secondary Store, but which Stores depends on the DataStore requested.

A Guice-injected instance can't be bound directly based on its target (a similar feature was rejected), but as mentioned in the FAQ you can use PrivateModule to create a similar effect.

install(new PrivateModule() {
  @Override public void configure() {
    bind(Store.class).annotatedWith(Names.named("primary")).to(StoreA.class);
    bind(Store.class).annotatedWith(Names.named("secondary")).to(StoreB.class);
    expose(A.class);
  }
});
install(new PrivateModule() {
  @Override public void configure() {
    bind(Store.class).annotatedWith(Names.named("primary")).to(StoreC.class);
    bind(Store.class).annotatedWith(Names.named("secondary")).to(StoreD.class);
    expose(B.class);
  }
});

Consequently, @Named("primary") Store will not be accessible outside of A or B (or its dependencies), but that makes sense; you're never defining a general Store, but A and B each have the private bindings they need.

(Disclaimer: I didn't get a chance to test this, so it may need refinement.)

Upvotes: 1

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