Spring DI : Aggregation or Composition

Question

Aggregation: If two objects have there own life cycle and not tightly coupled with each other(can exists independently).

Class A {
    B b;
} 

The default value for b will be null and A's object can exists if there is no instance injected in A.

Composition: if two objects are tightly coupled and one can not exists without second.

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

Spring DI: In spring if we are using @Autowired annotation over a member then this member is required before instantiation of object.

Class A{
  @Autowired  */ now this member is required before creating of object of A.
  B b;
}

Query: So from the above facts we can think Spring DI as a composition here .I am asking this question because in a interview I had a discussion over it and as per the interviewer Spring is using Aggregation here. Could any one please clarify if I am missing or not getting some thing here.

Answer 1

I think it's depend on B scope. If B is singletone then B manage itslef life cycle, so the relation between A and B is Aggregation. If B is not singletone (prototype) then every time A is creating a new instance of B created and injected into A and A manage B life cycle, so the relation between A and B in this scenario ll be Composition

Answer 2

I think, in Spring Autowire means more Aggregation when classes service each other.

For example in MVC design pattern in Service layer . you may have two Service class that service each other.like Teacher that teach student and Student learn from teacher.for implement sample like this you should have two classes like below:

Class Teacher(){
     public void teach(Lesson lesson,Student student){
      //...
     }
}

Class Student(){
     @Autowire
     private ITeacher iTeacher;

     public void learn(Lesson lesson){
       iTeacher.teach(lesson,this);
       //...
     }
}

As you see Student depends to interface of Teacher . and Spring choose which implement of Interface choose for injection.From this point of view, beans not coupled together and you can remove and replace implementation by another.