code: public static Class<?> getGenericFieldType(Class<?> clazz, Field field) {
          Type fieldType = field.getGenericType();
          return getRealGenericClass(clazz, fieldType);
      }
docs: 泛型类clazz,field字段的真实class对象.
  @param clazz 泛型class
  @param field 字段
  @return