<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="form">
    <info><title>Form Authentication Mechanism</title></info>
    
    <section xml:id="form-overview">
        <info><title>Overview</title></info>
        
        <para>HTTP Form Authentication involves using the
            <literal>UsernamePasswordAuthenticationFilter</literal> to process a login
            form. This is the most common way for an application to authenticate end
            users. Form-based authentication is entirely compatible with the DAO, LDAP
            and JAAS authentication providers.</para>
        <para>This is also the mechanism used by the &lt;form-login&gt; element from the namespace
            and it's recommended that you use that unless you have specific customization requirements.
        </para>
    </section>
    
    <section xml:id="form-config">
        <info><title>Configuration</title></info>
        
        <para>The login form simply contains <literal>j_username</literal> and
            <literal>j_password</literal> input fields, and posts to a URL that is
            monitored by the filter (by default
            <literal>/j_spring_security_check</literal>). You should add an 
            <literal>UsernamePasswordAuthenticationFilter</literal> to your application context:
            <programlisting><![CDATA[    
<bean id="authenticationProcessingFilter" class=
"org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter">
  <property name="authenticationManager" ref="authenticationManager"/>
  <property name="filterProcessesUrl" value="/j_spring_security_check"/>
</bean> ]]>        
            </programlisting></para>
        <para>
            The configured <interfacename>AuthenticationManager</interfacename>
            processes each authentication request. The destination following a successful authentication
            or an authentication failure is controlled by the <interfacename>AuthenticationSuccessHandler</interfacename>
            and <interfacename>AuthenticationFailureHandler</interfacename> interfaces, respectively.
            The filter has properties which allow you to set these 
            <footnote><para>In versions prior to 3.0, the application flow at this point had evolved to a stage
                was controlled by a mix of properties on this class and strategy plugins. The
            decision was made for 3.0 to refactor the code to make these two strategies entirely responsible.
            </para></footnote>.
            Some standard implementations are supplied for these such as
            <classname>SimpleUrlAuthenticationSuccessHandler</classname>,
            <classname>SavedRequestAwareAuthenticationSuccessHandler</classname>, 
            <classname>SimpleUrlAuthenticationFailureHandler</classname> and 
            <classname>ExceptionMappingAuthenticationFailureHandler</classname>. Have a look at the Javadoc
            for these classes to see how they work. 
            </para>
        
        <para>If authentication is successful, the resulting
            <interfacename>Authentication</interfacename> object will be placed into the
            <classname>SecurityContextHolder</classname>.
            The configured AuthenticationSuccessHandler will then be called to either redirect or forward
            the user to the approprate destination. By default a <classname>SavedRequestAwareAuthenticationSuccessHandler</classname>
            is used, which means that the user will be redirected to the original destination they requested before they were asked to
            login.
            <note>
                <para>
                    The <classname>ExceptionTranslationFilter</classname> caches the original request a user makes.
                    When the user authenticates, the request handler makes use of this cached request to obtain the original
                    URL and redirect to it. The original request is then rebuilt and used as an alternative.  
                </para>
            </note>
            If authentication fails, the configured <interfacename>AuthenticationFailureHandler</interfacename> will be invoked.
        </para>
    </section>
</chapter>