CODEWITHSUNDEEP
javaguice
I.G.
I.G.

Reputation: 370

Guice. How to inject different objects in a field of parent class?

I have a problem with Guice field injection. Consider following hierarchy of classes.

abstract class Base{
    @Inject
    protected MyService myService;
}

class A extends Base{
    @Inject
    private AnotherService anotherService;
 }

class B extends Base{
   ...
}

I want to have two different instances of MyService at runtime - one to inject in all objects of class A and the other for objects of class B. I know how to achieve that behaviour with constructor injecton:

  bind(MyService.class).annotatedWith(Names.named("forA")).to(MyServiceImpl.class).in(Singleton.class);
  bind(MyService.class).annotatedWith(Names.named("forB")).to(MyServiceImpl.class).in(Singleton.class);

  class A extends Base{
    @Inject @Named(value = "forA")
    public A(MyService service1, AnotherService service2) {
      this.myService = service1;
      this.anotherService = service2;
    }

The problem is that migration to constructor injection will be pretty complicated, so I would like to stick with the field injection. Is it possible to tune field injection like I want?

Upvotes: 0

Views: 653

Answers (1)

Sneh
Sneh

Reputation: 3737

I would use Custom Injections from Guice to achieve this.

Firstly I will create an annotation to annotate the MyService which I want to be injected based on super class type.

@BindingAnnotation
@interface MyServiceInject {
}

Then I would annotate my MyService field with the annotation.

static abstract class Base {

    @MyServiceInject
    protected MyService myService;
}

Now we need a custom TypeListener which will be called everytime we encounter an injection point.

This is how we will create it.

static class ClassBasedMyServiceInjectionListener implements TypeListener {

    @Override
    public <I> void hear(TypeLiteral<I> typeLiteral, TypeEncounter<I> encounter) {
        Class<?> clazz = typeLiteral.getRawType();

        while (clazz != null) {
            for (Field field : clazz.getDeclaredFields()) {
                if (field.getType() == MyService.class && field.isAnnotationPresent(MyServiceInject.class)) { //if type of field is MyService and it has MyServiceInject annotation
                    encounter.register(new ClassBasedMyServiceInjector<>(field,
                                    typeLiteral.getType(),
                                    encounter.getProvider(MyServiceA.class),
                                    encounter.getProvider(MyServiceB.class)
                            )
                    ); //Now register a MemberInjector for this encounter.
                }
            }
            clazz = clazz.getSuperclass();
        }
    }
}

Now we need the MemberInjector which will look like this.

static class ClassBasedMyServiceInjector<T> implements MembersInjector<T> {

    private final Field field;
    private final Type superClassType;
    private final Provider<MyServiceA> myServiceAProvider;
    private final Provider<MyServiceB> myServiceBProvider;

    ClassBasedMyServiceInjector(Field field, Type superClassType, Provider<MyServiceA> myServiceAProvider, Provider<MyServiceB> myServiceBProvider) {
        this.field = field;
        this.superClassType = superClassType;
        this.myServiceAProvider = myServiceAProvider;
        this.myServiceBProvider = myServiceBProvider;

        field.setAccessible(true);
    }

    public void injectMembers(T t) { //this will be called when guice wants to inject members
        try {
            if (superClassType == A.class) {//if super class is of type A
                field.set(t, myServiceAProvider.get()); //inject MyServiceA
            } else if (superClassType == B.class) { //if super class is of type B
                field.set(t, myServiceBProvider.get()); //inject MyServiceB
            }
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }
}

Finally I would bind our custom TypeListener in our module's configure method like this.

    @Override
    protected void configure() {
        bindListener(Matchers.any(), new ClassBasedMyServiceInjectionListener());
    }

Hope this helps.

Upvotes: 1

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