Mocking java.lang.Class getDeclaredMethod(String name, ...Class args) with Mockito not working

Question

I am trying to mock the getDeclaredMethod on a java .class object of a certain defined java type:

AccessRulesMethods accessRulesMock = mock(AccessRulesMethods.class);
Method mockMethod = mock(Method.class);
when(accessRulesMock.getClass().getDeclaredMethod(param.getValue(), String.class, BigInteger.class)).thenReturn(mockMethod); 

but I get the following exception:

java.lang.NoSuchMethodException: myOwnPackage.AccessRulesMethods.methodName(java.lang.String, java.math.BigInteger)
at java.lang.Class.getDeclaredMethod(Class.java:2130)

Lokking into java.lang.Class at line 2130 it seems that the method is not mocked at all. I found in another discussion here that this is the correct way but with no examples... Anyone knows how can I manage to get what I need?

Thanks a lot, Saverio

Business logic:

try {
                    Method method = AccessRulesMethods.class.getDeclaredMethod(name, String.class, BigInteger.class);
                    String parameterName = Arrays.asList(method.getParameters()).get(0).getName();
                    Field inputField = input.getClass().getDeclaredField(parameterName);
                    inputField.setAccessible(true);
                    String parameterValueFromInput = (String) inputField.get(input);
                    AccessRulesMethods accessRulesInstance = beanFactory.getBean(AccessRulesMethods.class);
                    methodOutput = (MethodOutput) method.invoke(accessRulesInstance, parameterValueFromInput, input.getIdBp());
                }catch (InvocationTargetException ite){
                    throw ite.getTargetException();
                }catch (NoSuchMethodException | IllegalAccessException | NoSuchFieldException e) {
                    throw new CoreApplicationException("Errore tecnico: invocazione metodo fallita", ErrorConstant.getLocalErrorKey(ErrorConstant.LOCAL_ERROR_CODE_ERRVISREFL001), HttpStatus.INTERNAL_SERVER_ERROR);
                }

I have a bean with some methods: AccessRulesMethod. I get method names from a table in a db and the I call this methods with reflection in a given order. In order to call these method, first I get the name of the required parameter (the second one is fixed) and then I pass this parameter from the input to the API, also with reflection