Jackson Generic Type + Java Type Erasure

The problem
We are save configuration elements to Dynamodb: the key is the configuration name, the value is the JSON string.
Recently I am trying to refactor the code to make the service code much shorter and easier to read. But I made one mistake - related with Java Type erasure.

The following code tries to read configuration: its value is a Map. I want to create one generic method that can read different Map.

  @Nonnull
  private <T> Map<String, T> getMapConfig(final String configName) {
      final ConfigElement c = configDao.getConfigByName(configName);

      Map<String, T> resultMap = null;
      if (c != null) {
          String json = c.getValue();
          resultMap = objectMapper.readValue(json, new TypeReference<Map<String, T>>() {});
      }
      if (resultMap == null) {
          resultMap = new HashMap<>();
      }
      return resultMap;
  }

  private <T> T convertMapValue(final String json) {
      return objectMapper.readValue(json, new TypeReference<T>(){});
  }

But this doesn't work: because of type erasure, T is just a type variable, at runtime, Java can't get the real value of the type.

The Solution
We have to specify class type like below:

  @Nonnull
  private <T> Map<String, T> getMapConfig(final String configName, final Class<T> classType) {
      final ConfigElement c = configDao.getConfigByName(configName);

      Map<String, T> resultMap = null;
      if (c != null) {
          String json = c.getValue();
          final MapType mapType = TypeFactory.defaultInstance().constructMapType(Map.class, String.class, classType);            
          resultMap = objectMapper.readValue(json, mapType);
      }
      if (resultMap == null) {
          resultMap = new HashMap<>();
      }
      return resultMap;
  }
  private <T> T convertMapValue(final String json,final Class<T> valueType) {
      return objectMapper.readValue(json, valueType);
  }

One related stuff I found is that in eclipse you can change Eclipse's Warning settings so that if Eclipse will show warnings if some parameter are not used: Change -> Window/Preferences /Java/Compiler/ErrorsWarnings -> Unnecessary Code > Value of parameter is not used to warning.

Read More
Jackson Essentials - the JSON Libaray
Using Jackson JSON View to Protect Mass Assignment Vulnerabilities
Merge JSON Objects: Jackson + BeanUtils.copyProperties
Jackson Generic Type + Java Type Erasure
Jackson Date Serialize + Deserialize
Apache Commons TypeLiteral
Google Guice TypeLiteral
Effects of Type Erasure and Bridge Methods