apollo-demo/src/main/java/com/ctrip/framework/apollo/demo/api/ApolloConfigDemo.java/ApolloConfigDemo/getConfig
class ApolloConfigDemo:  private String getConfig(String key) {
    String result = config.getProperty(key, DEFAULT_VALUE);
    if (DEFAULT_VALUE.equals(result)) {
      result = publicConfig.getProperty(key, DEFAULT_VALUE);
    }
    if (DEFAULT_VALUE.equals(result)) {
      result = yamlConfig.getProperty(key, DEFAULT_VALUE);
    }
    logger.info(String.format("Loading key : %s with value: %s", key, result));
    return result;
  }