use @autowired in abstract base class

Question

As I know, field injection is not recommended. Should use constructor instead.

What I'm trying to do here is using @Autowired in the constructor of the base class, and make it accessible for all the subclasses. In some subclasses, I also need some specific beans to be @Autowired from their constructors. Demo code is as following:

Base class:

public abstract class Base {

    protected final MyDemoService myDemoService;

    @Autowired
    public Base(MyDemoService myDemoService) {
        this.myDemoService = myDemoService;
    }
}

Inherited(sub) class:

public class Sub extends Base {

    private final MySubService mySubService;

    @Autowired
    public Sub(MySubService mySubService) {
        this.mySubService = mySubService;
    }
} 

This will give me a 'no default constructor' error.
It's similar to the question: similar question and answer, but it doesn't work here.

---

I have dived into this for a while, I found this article about dependency injection: further read

I'm thinking is the Setter Injection a right way for my question?

Setter injection:

public abstract class Base {

    protected MyDemoService myDemoService;

    @Autowired
    public void setMyDemoService(MyDemoService myDemoService) {
        this.myDemoService = myDemoService;
    }
}

I'm new to Java Spring Boot, and want to get some expertise advise from you guys. Any discussion will be highly appreciated!

Answer 1

The code that you provide won't compile. As long as in your base class you don't have the default constructor, you should call super(MyDemoService) in child.

Inherited(sub) class:

public class Sub extends Base {

    private final MySubService mySubService;

    @Autowired
    public Sub(MySubService mySubService, MyDemoService myDemoService){
        super(myDemoService);
        this.mySubService = mySubService;
    }
} 

Or, if MySubService is an implementation of MyDemoService

@Autowired
public Sub(MySubService mySubService){
    super(mySubService);
}

As long as your field MyDemoService myDemoService in abstract class is protected you can use it in subclasses.

If you have multiple implementation of MyDemoService, than you have to use @Qualifier as described in the answer that you have mentioned.

public Sub(@Qualifier("MySubService") MyDemoService mySubService){
    super(mySubService);
}