code: final Mappings mappingsOf(final Class<?> clazz) {
          if (!mappings.containsKey(clazz)) {
              mappings.putIfAbsent(clazz, new Mappings(clazz));
          }
          return mappings.get(clazz);
      }
docs: Gets the mapping definition for a given class.
  @param clazz The class.
  @return The mapping definition.