With the use of Ajax, transferring of large amount of data to the server can be minimized by sending only user name to the server where user name existence can be checked.

In the above code, there is a button component with label “Check User Name Availability”. It has “onClick” attribute which is set to the JavaScript function “checkUserExist()”. This method is called every time user clicks the button component. div area of id “text” is used to show the result of the response from the server using Ajax.

“checkUserExist()” first tries to get HTTP request object maintaining the browser compatibility. In this example, this object is stored in the variable named “xmlHttp”. Read how to get HTTP request object on page 12

Next step is to determine which method should be invoked after getting the response from the server. In this example, the method “callback()” is assigned for this purpose. Now the request for the servlet “AJAXCheckUserServlet” is forwarded to the server along with user name as parameter.

AJAXCheckUserServlet.java

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
import java.sql.*;
public class AJAXCheckUserServlet extends HttpServlet {
public void doGet( HttpServletRequest request, HttpServletResponse response ) throws
ServletException, IOException {
String username = request.getParameter(“username”);
boolean isExist = check(username);
response.setContentType(“text/html”);
response.setHeader(“Cache-Control”, “no-cache”);
if(isExist){
response.getWriter().write(“User Name exists. Please select other name.”);
}
else{
response.getWriter().write(“Congratulations....User Name is available.”);
}
}
public boolean check(String username) {
Connection con = null;
String url = “jdbc:mysql://192.168.10.59:3306/”;
String db = “javajazzup”;
String driver = “com.mysql.jdbc.Driver”;
String user = “root”;
String pass = “root”;
boolean isExist= false;
try{
Class.forName(driver).newInstance();