How to check if the type is instanceof an interface in java?

Question

I have a class (CreateAccountRequest) that implement an interface(Iloggable), the interface has no method, Just for marking purpose.

public interface Iloggable {}    

public class CreateAccountRequest implements Iloggable{
 //some private fields, getters and setters
}

In my custom RequestBodyAdviceAdapter class, trying to check if the request is an instance of Iloggable, to proceed or ignore the request (for instance do the logging or not )

I know we can use instanceOf operator to check if an object implements an interface or not and the unit test as below approve it:

@Test
void CreateAccountRequest_Object_Instance_Of_Iloggable_Test() {
    CreateAccountRequest request = new CreateAccountRequest();
    assertTrue(request instanceof Iloggable);
}

But in RequestBodyAdviceAdapter supports method the result is false or true all the time, I tried different ways to distinguish if the parameter implement the interface or not

@ControllerAdvice
public class CustomRequestBodyAdviceAdapter extends RequestBodyAdviceAdapter {
@Override
public boolean supports(MethodParameter methodParameter, Type type, Class<? extends HttpMessageConverter<?>> aClass) {
    //if(methodParameter.getParameterType().isInstance(Iloggable.class)) //return false all the time
    //if(methodParameter.getParameter().getType().isInstance(Iloggable.class))// return false all the time
    if(methodParameter.getParameter().getClass().isInstance(Iloggable.class))// return false all the time
    // if(type instanceof Iloggable)// return false all the time
    //if(type.getClass().isInstance(Iloggable.class)) //return true all the time
    //if(type != null && Iloggable.class.isAssignableFrom(type.getClass()))//return false all the time
      return true;
    return false;
}
    //other override methods
}

For removing any doubt, I put a snapshot of the debugging in the support method:

I don't think Type will be an instance of any object. Can you share a snippet of how this method is called? — Seeker, Nov 08 '21 at 15:42
Type is the class, not an instance. You can't use instanceof on the type, you need to check if the type implements the interface. See https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html. — ewramner, Nov 08 '21 at 15:43

xerx593 · Accepted Answer · 2021-11-08T17:04:47.880

3

In this case type is a java.lang.reflect.Type (instance) and not an CreateAccountRequest (instance;)

You could get more lucky with:

if (type instanceof Class) { // type != null
  Class<?> aClazz = (Class<?>) type;
  return Iloggable.class.isAssignableFrom(aClazz);
}

Class.isAssignableFrom(...)-javadoc-17

Type -> Class

In depth:

if(type instanceof Iloggable) //always false, because type is the
// (java.lang.reflect.)Type(->class) (occasionally) of the Iloggable and not/ever an instance of it.

if(type.getClass().isInstance(Iloggable.class)) //equivalent to 
// java.lang.reflect.Type.class.isInstance(Iloggable.class)
// always false, except when Iloggable *implements* Type (pervert, but possible!;)

Iloggable.class.isAssignableFrom(type.getClass()) //incorporates 
// my "wrong assumptions" on type... type.getClass() would 
// evaluate to java.lang.reflect.Type.class, which is similar to bullet 2

But I assume (when correctly used -> experiment) some MethodParameter methods should also lead to the desired. Especially:

edited Nov 08 '21 at 17:04

answered Nov 08 '21 at 15:59

xerx593

12,237
5
33
64

Thanks , I tryied: type != null && Iloggable.class.isAssignableFrom(type.getClass()), it return false all the time. isAssignableFrom(type) doesn't compile – Mostafa Nov 08 '21 at 16:04
sry...my bad: `type` is *not a class*! ..but it holds obviously its name! ..i update my answer! – xerx593 Nov 08 '21 at 16:07
Perhaps you forgot to update the answer, I don't see any changes ;) – Mostafa Nov 08 '21 at 16:14
check now, plz! ;) – xerx593 Nov 08 '21 at 16:32
That worked, Could you please add more explanation to your answer, especially the reason why my other solutions ( updated question) didn't work, so I could accept your brilliant answer? – Mostafa Nov 08 '21 at 16:40
..for the `methodParameter.*`, i would also like to debug ;)...but the `type*` approaches are easy! – xerx593 Nov 08 '21 at 16:47
1

see, update, thx4accept!+1;) – xerx593 Nov 08 '21 at 17:05

score 2 · Answer 2 · answered Nov 08 '21 at 16:32

You want to make sure that type represents a class that implements Illoggable.

@Override
public boolean supports(MethodParameter methodParameter, Type type, Class<? extends HttpMessageConverter<?>> aClass) {
    if ( type instanceof Class ) {
        cls = ( Class ) type;
        result = Iloggable.class.isAssignableFrom( cls );
    } else {
        result = false;
    }
    return result;
}

score 0 · Answer 3 · answered Nov 08 '21 at 17:58

0

Try following comparison, target.getTypeName()==Iloggable.class.getTypeName()

answered Nov 08 '21 at 17:58

Ruchira Sandaruwan

36
2

How to check if the type is instanceof an interface in java?

3 Answers3