Using Guice with circular dependencies

Question

Consider this simple example.

Class A {
   B b;
   A() {
       this.b = new B(this);
   }
}

In this example instance A knows about instance B, and instance B knows about instance A.

My question is: how to instantiate instance A with Guice, i.e. how to make Guice take care of this complex circle dependencies?

Answer 1

Your example is not an issue at all, since you're constructing B directly. But if you want to both A and B to be created by Guice, one or both should be an interface. You can do:

public interface A { /* skipping methods */ }
public interface B { /* skipping methods */ }

public class AImpl implements A {
   private final B b;

   @Inject
   public AImpl(B b) {
      this.b = b;
   }
   // ...
}

public class BImpl implements B {
   private final A a;

   @Inject
   public BImpl(A a) {
      this.a = a;
   }
   // ...
}

Even if AImpl and BImpl are scoped as singletons, Guice can handle this injection (by way of a proxy). This works in a simple case like this at any rate... I imagine there could be more complex circular dependencies that it couldn't handle. Anyway, eliminating circular dependencies would be preferable, of course.

Answer 2

The answer is that you should not use a dependency injection framework while you have circular dependences in your code.

So, you have to refactor you code beforehand. As far as I know, there are two solutions for tightly coupled classes: either merge two classes into one or introduce new class and move common logic into it (for detail look here)