<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="session-mgmt"
  xmlns:xlink="http://www.w3.org/1999/xlink">
  <info>
    <title>Session Management</title>
  </info>
  <para>HTTP session related functonality is handled by a combination of the
      <classname>SessionManagementFilter</classname> and the
      <interfacename>SessionAuthenticationStrategy</interfacename> interface, which the filter
    delegates to. Typical usage includes session-fixation protection attack prevention, detection of
    session timeouts and restrictions on how many sessions an authenticated user may have open
    concurrently.</para>
  <section>
    <title>SessionManagementFilter</title>
    <para>The <classname>SessionManagementFilter</classname> checks the contents of the
        <interfacename>SecurityContextRepository</interfacename> against the current contents of the
        <classname>SecurityContextHolder</classname> to deterine whether a user has been
      authenticated during the current request, typically by a non-interactive authentication
      mechanism, such as pre-authentication or remember-me <footnote><para>Authentication by
          mechanisms which perform a redirect after authenticating (such as form-login) will not be
          detected by <classname>SessionManagementFilter</classname>, as the filter will not be
          invoked during the authenticating request. Session-management functionality has to be
          handled separately in these cases. </para></footnote>. If the repository contains a
      security context, the filter does nothing. If it doesn't, and the thread-local
        <interfacename>SecurityContext</interfacename> contains a (non-anonymous)
        <interfacename>Authentication</interfacename> object, the filter assumes they have been
      authenticated by a previous filter in the stack. It will then invoke the configured
        <interfacename>SessionAuthenticationStrategy</interfacename>.</para>
    <para>If the user is not currently authenticated, the filter will check whether an invalid
      session ID has been requested (because of a timeout, for example) and will redirect to the
      configured <literal>invalidSessionUrl</literal> if set. The easiest way to configure this is
      through the namespace, <link xlink:href="#ns-session-mgmt">as described earlier</link>.</para>
  </section>
  <section>
    <title><interfacename>SessionAuthenticationStrategy</interfacename></title>
    <para>
      <interfacename>SessionAuthenticationStrategy</interfacename> is used by both
        <classname>SessionManagementFilter</classname> and
        <classname>AbstractAuthenticationProcessingFilter</classname>, so if you are using a
      customized form-login class, for example, you will need to inject it into both of these. In
      this case, a typical configuration, combining the namespace and custom beans might look like this:<programlisting><![CDATA[
  <http>  
    <custom-filter position="FORM_LOGIN_FILTER" ref="myAuthFilter" />     
    <session-management session-authentication-strategy-ref="sas"/>      
  </http>
  
  <beans:bean id="myAuthFilter" 
      class="org.springframework.security.web.authentcation.UsernamePasswordAuthenticationFilter">
    <beans:property name="sessionAuthenticationStrategy" ref="sas" />
    ...
  </beans:bean>
  
  <beans:bean id="sas" 
      class="org.springframework.security.web.session.DefaultAuthenticatedSessionStrategy">
    <beans:property name="sessionRegistry" ref="sessionRegistry" />
    <beans:property name="maximumSessions" value="1" />
  </beans:bean>
]]>       
      </programlisting></para>
  </section>
  <section xml:id="concurrent-sessions">
    <title>Concurrency Control</title>
    <para>Spring Security is able to prevent a principal from concurrently authenticating to the
      same application more than a specified number of times. Many ISVs take advantage of this to
      enforce licensing, whilst network administrators like this feature because it helps prevent
      people from sharing login names. You can, for example, stop user "Batman" from logging onto
      the web application from two different sessions.</para>
    <para>This feature is supported by the namespace, so please check the earlier namespace chapter
      for the simplest configuration. Sometimes you need to customize things though. </para>
    <para>The implementation uses a specialized version of
        <interfacename>SessionAuthenticationStrategy</interfacename>, called
        <classname>ConcurrentSessionControlStrategy</classname>. <note><para>Previously the
          concurrent authentication check was made by the <classname>ProviderManager</classname>,
          which could be injected with a <literal>ConcurrentSessionController</literal>. The latter
          would check if the user was attempting to exceed the number of permitted sessions.
          However, this approach required that an HTTP session be created in advance, which is
          undesirable. In Spring Security 3, the user is first authenticated by the
            <interfacename>AuthenticationManager</interfacename> and once they are successfully
          authenticated, a session is created and the check is made whether they are allowed to have
          another session open.</para></note></para>
    <para>To use concurrent session support, you'll need to add the following to
        <literal>web.xml</literal>: <programlisting><![CDATA[
  <listener>
    <listener-class>
      org.springframework.security.web.session.HttpSessionEventPublisher
    </listener-class>
  </listener> ]]>       
        </programlisting></para>
    <para>In addition, you will need to add the <literal>ConcurrentSessionFilter</literal> to your
        <classname>FilterChainProxy</classname>. The <classname>ConcurrentSessionFilter</classname>
      requires two properties, <literal>sessionRegistry</literal>, which generally points to an
      instance of <literal>SessionRegistryImpl</literal>, and <literal>expiredUrl</literal>, which
      points to the page to display when a session has expired. A configuration using the namespace
      to create the <classname>FilterChainProxy</classname> and other default beans might look like
      this: <programlisting><![CDATA[
  <http>
    <custom-filter position="CONCURRENT_SESSION_FILTER" ref="concurrencyFilter" />  
    <custom-filter position="AUTHENTICATION_PROCESSING_FILTER" ref="myAuthFilter" />     
        
    <session-management session-authentication-strategy-ref="sas"/>      
  </http>
  
  <beans:bean id="concurrencyFilter" 
      class="org.springframework.security.web.session.ConcurrentSessionFilter">
    <beans:property name="sessionRegistry" ref="sessionRegistry" />
    <beans:property name="expiredUrl" value="/session-expired.htm" />
  </beans:bean>
  
  <beans:bean id="myAuthFilter" 
      class="org.springframework.security.web.authentcation.UsernamePasswordAuthenticationFilter">
    <beans:property name="sessionAuthenticationStrategy" ref="sas" />
    ...
  </beans:bean>
  
  <beans:bean id="sas" 
      class="org.springframework.security.web.session.ConcurrentSessionControlStrategy">
    <beans:property name="sessionRegistry" ref="sessionRegistry" />
    <beans:property name="maximumSessions" value="1" />
  </beans:bean>
  
  <beans:bean id="sessionRegistry" class="org.springframework.security.core.session.SessionRegistryImpl" />
]]>       
    </programlisting></para>
    <para>Adding the listener to <filename>web.xml</filename> causes an
        <literal>ApplicationEvent</literal> to be published to the Spring
        <literal>ApplicationContext</literal> every time a <literal>HttpSession</literal> commences
      or terminates. This is critical, as it allows the <classname>SessionRegistryImpl</classname>
      to be notified when a session ends. Without it, a user will never be able to log back in again
      once they have exceeded their session allowance, even if they log out of another session or it
      times out.</para>
  </section>
</chapter>
