apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/environment/Env.java/Env/valueOf
class Env:  public static Env valueOf(String name) {
    name = getWellFormName(name);
    if (exists(name)) {
      return STRING_ENV_MAP.get(name);
    } else {
      throw new IllegalArgumentException(name + " not exist");
    }
  }