apollo-client-config-data/src/main/java/com/ctrip/framework/apollo/config/data/internals/PureApolloConfig.java/PureApolloConfig/getProperty
class PureApolloConfig:  @Override
  public String getProperty(String key, String defaultValue) {
    // step 1: check local cached properties file
    String value = this.getPropertyFromRepository(key);

    // step 2: check properties file from classpath
    if (value == null) {
      value = this.getPropertyFromAdditional(key);
    }

    this.tryWarnLog(value);

    return value == null ? defaultValue : value;
  }