Java Programming

Dear All,
I am using j_security_check to redirect my JSP page to a login, so I can redirect with out any problem, and the login page ask me for user name and password.
I don't know where I should specify the user name and password.
I have searched alot but I could not find the point which I need, I have found some links regarding this issue but when I implement them, it was not working.
this is the j_security_check which I am using:

<form method="POST" action="j_security_check">
  <table border="0" cellspacing="5">
    <tr>
      <th align="right">Username:</th>
      <td align="left"><input type="text" name="j_username"></td>
    </tr>
    <tr>
      <th align="right">Password:</th>
      <td align="left"><input type="password" name="j_password"></td>
    </tr>
    <tr>
      <td align="right"><input type="submit" value="Log In"></td>
      <td align="left"><input type="reset"></td>
    </tr>
  </table>
</form>

web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>guestbook</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
    <description></description>
    <display-name>Main</display-name>
    <servlet-name>Main</servlet-name>
    <servlet-class>xtokhi.Main</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>Main</servlet-name>
    <url-pattern>/Main</url-pattern>
  </servlet-mapping>
  <servlet>
    <description></description>
    <display-name>DeletedNode</display-name>
    <servlet-name>DeletedNode</servlet-name>
    <servlet-class>xtokhi.DeletedNode</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>DeletedNode</servlet-name>
    <url-pattern>/DeletedNode</url-pattern>
  </servlet-mapping>
  <!--
    Define a security constraint and tell it which abstract roles are
    permitted to access this resource
  -->
  <security-constraint>
    <display-name>Security Constraint</display-name>
    <web-resource-collection>
      <web-resource-name>Protected Area</web-resource-name>
      <url-pattern>/Admin.jsp</url-pattern>
      <http-method>GET</http-method>
      <http-method>POST</http-method>
    </web-resource-collection>
    <auth-constraint>
      <role-name>guest</role-name>
    </auth-constraint>
  </security-constraint>

  <!-- Default a login configuration that uses form-based authentication -->
  <login-config>
    <auth-method>FORM</auth-method>
    <realm-name>Form-Based Authentication Area</realm-name>
    <form-login-config>
      <form-login-page>/login.jsp</form-login-page>
      <form-error-page>/error.jsp</form-error-page>
    </form-login-config>
  </login-config>

  <security-role>
    <description>A Funky User</description>
    <role-name>guest</role-name>
  </security-role>
</web-app>

I want the user name and password to be stored in an xml file. how can I do this?
Where the user name and password should be stored? and how?