单例写法，双重检查

  public static ConversionService getSharedInstance() {
    ApplicationConversionService sharedInstance = ApplicationConversionService.sharedInstance;
    if (sharedInstance == null) {
      synchronized (ApplicationConversionService.class) {
        sharedInstance = ApplicationConversionService.sharedInstance;
        if (sharedInstance == null) {
          sharedInstance = new ApplicationConversionService();
          ApplicationConversionService.sharedInstance = sharedInstance;
        }
      }
    }
    return sharedInstance;
  }