code: private String getSign(String body) {
  
          String signNodeName = "<" + AlipayConstants.SIGN + ">";
          String signEndNodeName = "</" + AlipayConstants.SIGN + ">";
  
          int indexOfSignNode = body.indexOf(signNodeName);
          int indexOfSignEndNode = body.indexOf(signEndNodeName);
  
          if (indexOfSignNode < 0 || indexOfSignEndNode < 0) {
              return null;
          }
  
          //  签名
          return body.substring(indexOfSignNode + signNodeName.length(), indexOfSignEndNode);
      }
docs: 获取签名
  
  @param body
  @return