code: public String resolveConfigEnvPath(String envPath) { // almost same as Lasta Di's logic
          if (envPath == null) {
              throw new IllegalArgumentException("The argument 'envPath' should not be null.");
          }
          final String configEnv = getConfigEnv();
          final String envMark = "_env.";
          if (configEnv != null && envPath.contains(envMark)) {
              // e.g. maihama_env.properties to maihama_env_production.properties
              final int markIndex = envPath.indexOf(envMark);
              final String front = envPath.substring(0, markIndex);
              final String rear = envPath.substring(markIndex + envMark.length());
              return front + "_env_" + configEnv + "." + rear;
          } else {
              return envPath;
          }
      }
docs: ==============================