@WebParam on a parameter of interface method - how to get name?

I try to get the name "best_user" of the annotation @WebParam on a method parameter User user in an interface method. Because it is an interface, the following code does not give me access to the annotation:

String methodName = "doSomething";

Class<?> clazz = Class.forName(packageName + "." + "UserService");

Method serviceMethod = null;

for (Method method: clazz.getMethods())
{
    if (method.getName().equals(methodName))
    {
        serviceMethod = method;
        break;
    }
}

Class<?>[] parameterTypes = serviceMethod.getParameterTypes();
WebParam webParam = parameterTypes[0].getAnnotation(WebParam.class); //webParam is null
String parameterName = webParam.name();

However the web service framework is able to get this data. How can I do it as well?

Interface:

@WebService(name = "UserService", targetNamespace = "my.targetNamespace")
public interface UserService {
    @WebMethod
    public String doSomething
        (
            @WebParam(name = "best_user")
            User user
        )
}