con = DriverManager.getConnection(url+db, user, pass);
try{
Statement st = con.createStatement();
ResultSet res = st.executeQuery(“SELECT user_name FROM users”);
while (res.next()) {
String un = res.getString(“user_name”);
if(username.equals(un)){
con.close();
isExist = true;
break;
}
}
con.close();
}
catch (SQLException s){
System.out.println(“SQL code does not execute.”);
}
}
catch (Exception e){
e.printStackTrace();
}
return isExist;
}
}

This servlet finds the username parameter and checks if this username is available in the database. If it exists then sends “User Name exists. Please select other name.” otherwise “Congratulations....User Name is available.” to the user. After getting the response from the server JavaScript function “callback()” is called to handle the response data.

if( xmlHttp.readyState==4 ){
if( xmlHttp.status==200 ) {
document.getElementById(‘text’).innerHTML =
“<br/><b>”+xmlHttp.responseText+”</b>”;
}
}

The function first checks if everything is fine. To get the text response, responseText property of the xmlHttp object is used. Now the html div component of id “text” is set to the response text.

4. Show User Details:

This Ajax example shows how to show user details like first and last name of the user without refreshing the current page. When the user changes user selection, its first and last names are displayed on the page. JavaScript is used to extract the XML data of the response.