Question

If I have two classes like:

Class A {
     public String importantValue = "stringvalue";

     @Autowire
     public B b;
}

@Component
@Scope("prototype");
Class B {
     // This should be set automatically
     // from IOC Container upon injection.
     public String importantValueFromA;
}

Is this even possible? As soon as B class has been injected to A it should automatically set the value in B.

Answer 1

Do you want class A to do some setup on injected class B? That’s simple:

@Service
class A {
    private String importantValue = "stringvalue";

    @Autowire
    private B b;

    @PostConstruct
    public void initB() {
        b.importantValueFromA = this.importantValue;
    }
}

Obviously you cannot access b.importantValueFromA in A.A constructor because injection didn’t yet happen. But @PostConstruct callback is guaranteed to be called after injection.

Another approach is to use setter injection, but it feels kind of hacky:

private B b;

@Autowire
public void setB(B b) {
    this.b = b;
    b.importantValueFromA = this.importantValue;
}

Two suggestions:

• keep your fields private and use setters/methods to access them.
• injecting prototype scoped bean to singleton bean might have some unexpected results. Enough to say only one instance of B will be created.