Dependency injection: Scoping by region (Guice, Spring, Whatever)

Question

Here's a simplified version of my needs.

I have a program where every B object has its own C and D object, injected through Guice. In addition an A object is injected into every C and D objects.

What I want: that for each B object, its C and D objects will be injected with the same A object.

[Edit-Start]

(1) Guice supports "singleton" and "prototype" modes. However, what I need is something in between: I need A to be singleton WRT to a given B object (so that the C and D injected into a B object will share an A object). For another B object, I want another A. So it's a singleton but for a limited scope of the program (actually, a limited scope of a data structure).

(2) I don't mind a solution that uses method (setter)- or field- injection.

(3) I tried, several times, to achieve this and it always felt that I only need to implement some custom thingy of the DI container to make this work - but it never worked. Thus, I am looking for a detailed solution (not just "hand waving")

[Edit-End]

Specifically, I want the output of the program (below) to be:

Created C0 with [A0]
Created D0 with [A0]
Created B0 with [C0, D0]
Created C1 with [A1]
Created D1 with [A1]
Created B1 with [C1, D1]

Where it currently produces the following output:

Created C0 with [A0]
Created D0 with [A1]  <-- Should be A0
Created B0 with [C0, D0]
Created C1 with [A2]  <-- Should be A1
Created D1 with [A3]  <-- Should be A1
Created B1 with [C1, D1]

I am expecting DI containers to allow this kind of customization but so far I had no luck in finding a solution. Below is my Guice-based code, but a Spring-based (or other DI containers-based) solution is welcome.

  import java.util.Arrays;
  import com.google.inject.*;

  public class Main {

    public static class Super {
      private static Map<Class<?>,Integer> map = new HashMap<Class<?>,Integer>();

      private Integer value;

      public Super(Object... args) {
        value = map.get(getClass());
        value = value == null ? 0 : ++value;
        map.put(getClass(), value);

        if(args.length > 0)
          System.out.println("Created " + this + " with " + Arrays.toString(args));
      }

      @Override
      public final String toString() {
        return "" + getClass().getSimpleName().charAt(0) + value;
      }
    }

    public interface A { }  
    public static class AImpl extends Super implements A  { } 

    public interface B { }  
    public static class BImpl extends Super implements B {
      @Inject public BImpl(C c, D d) { super(c,d); }
    }

    public interface C { }  
    public static class CImpl extends Super implements C  {
      @Inject public CImpl(A a) { super(a); }
    }

    public interface D { }  
    public static class DImpl extends Super implements D {
      @Inject public DImpl(A a) { super(a); }
    }


    public static class MyModule extends AbstractModule {
      @Override
      protected void configure() {
        bind(A.class).to(AImpl.class);
        bind(B.class).to(BImpl.class);      
        bind(C.class).to(CImpl.class);      
        bind(D.class).to(DImpl.class);      
      }    
    }

    public static void main(String[] args) {
      Injector inj = Guice.createInjector(new MyModule());
      inj.getInstance(B.class);    
      inj.getInstance(B.class);    
    }  
  }

Answer 1

Here's one solution based on your original code - there are three changes:

1. Move the bindings for A, C, and D into a separate sub-module
2. Mark A as a singleton in the sub-module
3. Use a @Provides method in the main-module to provide instances of BImpl with
   a new child injector for each request - this is where the sub-module comes in

This works because the singleton binding for A is now limited to each child injector.

[ Note: you could always cache the sub-module instance in a field of the
main-module if you don't want to keep creating it for each request of B ]

  import java.util.*;
  import com.google.inject.*;

  public class Main {

    public static class Super {
      private static Map<Class<?>,Integer> map = new HashMap<Class<?>,Integer>();

      private Integer value;

      public Super(Object... args) {
        value = map.get(getClass());
        value = value == null ? 0 : ++value;
        map.put(getClass(), value);

        if(args.length > 0)
          System.out.println("Created " + this + " with " + Arrays.toString(args));
      }

      @Override
      public final String toString() {
        return "" + getClass().getSimpleName().charAt(0) + value;
      }
    }

    public interface A { }  
    public static class AImpl extends Super implements A  { } 

    public interface B { }  
    public static class BImpl extends Super implements B {
      @Inject public BImpl(C c, D d) { super(c,d); }
    }

    public interface C { }  
    public static class CImpl extends Super implements C  {
      @Inject public CImpl(A a) { super(a); }
    }

    public interface D { }  
    public static class DImpl extends Super implements D {
      @Inject public DImpl(A a) { super(a); }
    }

    public static class MyModule extends AbstractModule {
      @Override
      protected void configure() {} 

  // >>>>>>>>
      @Provides
      B builder( Injector injector ) {
        return injector.createChildInjector( new SubModule() ).getInstance( BImpl.class );
      }
  // <<<<<<<<
    }

  // >>>>>>>>
    public static class SubModule extends AbstractModule {
      @Override
      protected void configure() {
        bind(A.class).to(AImpl.class).in( Scopes.SINGLETON );
        bind(C.class).to(CImpl.class);      
        bind(D.class).to(DImpl.class);      
      }    
    }
  // <<<<<<<<

    public static void main(String[] args) {
      Injector inj = Guice.createInjector(new MyModule());
      inj.getInstance(B.class);    
      inj.getInstance(B.class);    
    }  
  }

Answer 2

Now, I don't know if you absolutely need to have BImpl, CImpl and DImpl created by Guice (to allow for AOP, for example), but if not this is simple:

public static class MyModule extends AbstractModule {
  @Override
  protected void configure() {
    bind(A.class).to(AImpl.class);
  }

  @Provides
  protected B provideB(A a) {
    C c = new CImpl(a);
    D d = new DImpl(a);
    return new BImpl(c, d);
  }
}

Alternatively (and I know you didn't specify this in your question), if you can bind each instance of B that you use with a different binding annotation, you can use a private module like this, which you'd add once per binding annotation when creating your injector:

public static class MyOtherModule extends PrivateModule {
  private final Annotation annotation;

  public MyOtherModule(Annotation annotation) {
    this.annotation = annotation;
  }

  @Override
  protected void configure() {
    bind(A.class).to(AImpl.class).in(Scopes.SINGLETON);
    bind(C.class).to(CImpl.class);
    bind(D.class).to(DImpl.class);
    bind(B.class).annotatedWith(annotation).to(BImpl.class);
    expose(B.class).annotatedWith(annotation);
  }
}

main for this looks like:

public static void main(String[] args) {
  Injector inj = Guice.createInjector(new MyOtherModule(Names.named("first")),
      new MyOtherModule(Names.named("second")));
  inj.getInstance(Key.get(B.class, Names.named("first")));
  inj.getInstance(Key.get(B.class, Names.named("second")));
}

I imagine there are other possibilities as well.