XML and JSON for Ajax - Setting Up a Simple XML Document

Before we delve into the code, we need to make some decisions. We're going to return data using XML, but how should that XML be structured? What should our XML response look like? We don't want anything complex, so well aim to create an XML document that looks like this:

  <converted-values>
      <decimal>97</decimal>
      <hexadecimal>0x61</hexadecimal>
      <octal>0141</octal>
      <hyper>&0x61;</hyper>
      <binary>1100001B</binary>
  </converted-values>

With this format, the browser can use its document object model (DOM) parser to index and retrieve the data.

There are many ways to create this XML document. For the sake of simplicity, well first use aStringBuffer to wrap the data with XML tags. Later, we'll look at other ways to create the XML document.

When I talk about XML formatting, Im referring to the server wrapping the data in XML. The client receives the XML-formatted string in theHTTPResponseand parses it for the individual data fields. The client passes data through the request using eitherHTTPPost()orHTTPGet(). There is no reason for the client to send XML data to the server, because the data is already wrapped in the request as name/value pairs.

Using a Servlet to Build an XML Document

Let's start by looking at the servlet code that wraps the data in XML. This servlet is shown in Example4-1.

Example 4-1. The AjaxResponseServlet

/*
 * Converts a character to hex, decimal, binary, octal, and HTML, then
 * wraps each of the fields with XML and sends them back through the response.
 */
package com.AJAXbook.servlet;

import java.io.IOException;

import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;

public class AjaxResponseServlet extends HttpServlet {

    public void doGet(HttpServletRequest req, HttpServletResponse res)
            throws ServletException, IOException{
        // key is the parameter passed in from the JavaScript
        // variable named url (see index.html)
        String key = req.getParameter("key");
        StringBuffer returnXML = null;
        if (key != null) {
            // extract the first character from key
            // as an int, then convert that int to a String
            int keyInt = key.charAt(0);
            returnXML = new StringBuffer("\r\n<converted-values>");
            returnXML.append("\r\n<decimal>"+
                            Integer.toString(keyInt)+"</decimal>");
            returnXML.append("\r\n<hexadecimal>0x"+
                            Integer.toString(keyInt,16)+"</hexadecimal>");
            returnXML.append("\r\n<octal>0"+
                            Integer.toString(keyInt,8)+"</octal>");
            returnXML.append("\r\n<hyper>&amp;0x"+
                            Integer.toString(keyInt,16)+";</hyper>");
            returnXML.append("\r\n<binary>"+
                            Integer.toString(keyInt,2)+"B</binary>");
            returnXML.append("\r\n</converted-values>");

            // set up the response
            res.setContentType("text/xml");
            res.setHeader("Cache-Control", "no-cache");
            // write out the XML string
            res.getWriter().write(returnXML.toString());
        }
        else {
            // if key comes back as a null, return a question mark
            res.setContentType("text/xml");
            res.setHeader("Cache-Control", "no-cache");
            res.getWriter().write("?");
        }
    }
}

This code is similar to the code from Chapter 3. The only thing that has been added is the code to wrap the data with XML tags:

  returnXML = new StringBuffer("\r\n<converted-values>");
  returnXML.append("\r\n<decimal>"+ 
                   Integer.toString(keyInt)+"</decimal>");
  returnXML.append("\r\n<hexadecimal>0x"+
                   Integer.toString(keyInt,16)+"</hexadecimal>");
  returnXML.append("\r\n<octal>0"+
                   Integer.toString(keyInt,8)+"</octal>");
  returnXML.append("\r\n<hyper>&amp;0x"+
                   Integer.toString(keyInt,16)+";</hyper>");
  returnXML.append("\r\n<binary>"+
                   Integer.toString(keyInt,2)+"B</binary>");
  returnXML.append("\r\n</converted-values>");

This code simply sets up aStringBuffercalledreturnXML. We then convert the incoming value to decimal, hex, etc.; wrap it with an appropriate XML tag; and append it to the buffer. When we've finished all five conversions and added the closing tag (</converted-values>), we send the response back to the Ajax client usingres.getWriter().write(). We return a question mark (without any XML wrapping) if the key we received wasnull.

Next: Other Ways to Build the XML Document >>
 

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