How to call method from a reference send to annotation in java

Question

I have a Interface I and a Abstract Class A , I have My custom annotation MyAnnotation which should take parameter as subclass S of A, now while processing annotation I want to call method of concrete class S

public interface I{
   void m1();
}

public abstract class A implements I {
    public abstract void m1();
}

public @interface MyAnnotation {
    public Class< ? extends A> ref();
    public Class< ? super A> ref2();
}

public S extends A{
    public void m1() {}
}

I am annotating method like

@MyAnnotation(ref= new XX() )  or @MyAnnotation(ref= XX.class )
@MyAnnotation(ref= new yy() ) or @MyAnnotation(ref= yy.class )

whichever works

//In spring aspect before processing I am getting method annotation and trying to call m1()  
annotation.ref().m1() //Error
annotation.ref2().m1() //Error

`abstarct` and `public S extends A` aren't correct – Andrew Tobilko Oct 07 '18 at 17:17 — Andrew Tobilko, Oct 07 '18 at 17:17

Michael · Accepted Answer · 2018-10-07T18:23:56.067

3

You can't use new XX() in an annotation. Annotations parameters can use a very specific set of types:

primitive

String

Class

an Enum

another Annotation

an array of any of the above

See this answer.

So to accomplish what you're trying to accomplish, you'd have to use a class.

You would then have to use reflection to create an instance and invoke the method.

Class<?> clazz = annotation.ref();
I instance = (I) cls.getConstructor().newInstance();
instance.m1();

See this answer.

Your classes must all have no-argument constructors, else you'll only be able to instantiate some this way but not others (leading you to have to conditionally branch based on the class).

edited Oct 07 '18 at 18:23

answered Oct 07 '18 at 17:33

Michael

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I think `((I) cls.newInstance()).m1()` is a better option than `...getMethod(...).invoke(...)` – ernest_k Oct 07 '18 at 18:06
1

@ernest_k `newInstance` is deprecated since 9; it's better to call `Constructor::newInstance` – Eugene Oct 07 '18 at 18:09
@Eugene That's right. My comment is just about avoiding the `getMethod(...).invoke(...)` part, if you notice... – ernest_k Oct 07 '18 at 18:11
@ernest_k + Eugene, You're both right. It was a bit of a lazy answer, mostly just copied and pasted from the sources I listed. I've improved it now – Michael Oct 07 '18 at 18:24

score 0 · Answer 2 · answered Oct 07 '18 at 17:20

You can't do that simply like that. You need an instance of the class first. If your A class is a Spring's bean, you can inject ApplicationContext and get the bean from there. Then you can call a method.

@Autowired
private ApplicationContext context;

void test(MyAnnotation annotation) {
    A bean = context.getBean(annotation.ref());
    bean.m1();
}

How to call method from a reference send to annotation in java

2 Answers2