apollo-client/src/main/java/com/ctrip/framework/apollo/spring/property/PlaceholderHelper.java/PlaceholderHelper/normalizeToPlaceholder
class PlaceholderHelper:  private String normalizeToPlaceholder(String strVal) {
    int startIndex = strVal.indexOf(PLACEHOLDER_PREFIX);
    if (startIndex == -1) {
      return null;
    }
    int endIndex = strVal.lastIndexOf(PLACEHOLDER_SUFFIX);
    if (endIndex == -1) {
      return null;
    }

    return strVal.substring(startIndex, endIndex + PLACEHOLDER_SUFFIX.length());
  }