code: private String getSign(String body) {
  
          JSONReader reader = new JSONValidatingReader(new ExceptionErrorListener());
          Object rootObj = reader.read(body);
          Map<?, ?> rootJson = (Map<?, ?>) rootObj;
  
          return (String) rootJson.get(AlipayConstants.SIGN);
      }
docs: 获取签名
  
  @param body
  @return