<property name="sessionAuthenticationStrategy" ref="sas" />

<bean id="sas" class="org.springframework.security.web.authentication.session.CompositeSessionAuthenticationStrategy">
    <constructor-arg>
      <list>
        <bean class="org.springframework.security.web.authentication.session.ConcurrentSessionControlAuthenticationStrategy">
          <constructor-arg ref="sessionRegistry"/>
          <!-- 這里是配置session數(shù)量，此處為1，表示同一個用戶同時只會有一個session在線 --> 
          <property name="maximumSessions" value="1" />
          <property name="exceptionIfMaximumExceeded" value="false" />
        </bean>
        <bean class="org.springframework.security.web.authentication.session.SessionFixationProtectionStrategy">
        </bean>
        <bean class="org.springframework.security.web.authentication.session.RegisterSessionAuthenticationStrategy">
          <constructor-arg ref="sessionRegistry"/>
        </bean>
      </list>
    </constructor-arg>
  </bean>

  <bean id="sessionRegistry"
        class="org.springframework.security.core.session.SessionRegistryImpl" />

<bean id="concurrencyFilter"
        class="org.springframework.security.web.session.ConcurrentSessionFilter">
    <constructor-arg name="sessionRegistry" ref="sessionRegistry" />
    <constructor-arg name="sessionInformationExpiredStrategy" ref="redirectSessionInformationExpiredStrategy" />
  </bean>


  <bean id="redirectSessionInformationExpiredStrategy"
        class="org.springframework.security.web.session.SimpleRedirectSessionInformationExpiredStrategy">
    <constructor-arg name="invalidSessionUrl" value="/login.html" />
  </bean>

/**
   * In addition to the steps from the superclass, the sessionRegistry will be updated
   * with the new session information.
   */
  public void onAuthentication(Authentication authentication,
      HttpServletRequest request, HttpServletResponse response) {

    //根據(jù)所登錄的用戶信息，查詢相對應(yīng)的現(xiàn)存session列表
    final List<SessionInformation> sessions = sessionRegistry.getAllSessions(
        authentication.getPrincipal(), false);

    int sessionCount = sessions.size();
    //獲取session并發(fā)數(shù)量，對于XML中的maximumSessions
    int allowedSessions = getMaximumSessionsForThisUser(authentication);

    //判斷現(xiàn)有session列表數(shù)量和并發(fā)控制數(shù)間的關(guān)系
    //如果是首次登錄，根據(jù)xml配置，這里應(yīng)該是0<1，程序?qū)^續(xù)向下執(zhí)行，
    //最終執(zhí)行到SessionRegistryImpl的registerNewSession進(jìn)行新session的保存
    if (sessionCount < allowedSessions) {
      // They haven't got too many login sessions running at present
      return;
    }

    if (allowedSessions == -1) {
      // We permit unlimited logins
      return;
    }

    if (sessionCount == allowedSessions) {
      //獲取本次http請求的session
      HttpSession session = request.getSession(false);

      if (session != null) {
        // Only permit it though if this request is associated with one of the
        // already registered sessions
        for (SessionInformation si : sessions) {
          //循環(huán)已保存的session列表，判斷本次http請求session是否已經(jīng)保存
          if (si.getSessionId().equals(session.getId())) {
            //本次http請求是有效請求，返回執(zhí)行下一個filter
            return;
          }
        }
      }
      // If the session is null, a new one will be created by the parent class,
      // exceeding the allowed number
    }

    //本次http請求為新請求，進(jìn)入具體判斷
    allowableSessionsExceeded(sessions, allowedSessions, sessionRegistry);
  }

/**
   * Allows subclasses to customise behaviour when too many sessions are detected.
   *
   * @param sessions either <code>null</code> or all unexpired sessions associated with
   * the principal
   * @param allowableSessions the number of concurrent sessions the user is allowed to
   * have
   * @param registry an instance of the <code>SessionRegistry</code> for subclass use
   *
   */
  protected void allowableSessionsExceeded(List<SessionInformation> sessions,
      int allowableSessions, SessionRegistry registry)
      throws SessionAuthenticationException {
    //根據(jù)exceptionIfMaximumExceeded判斷是否要將新http請求拒絕
    //exceptionIfMaximumExceeded也可以在XML中配置
    if (exceptionIfMaximumExceeded || (sessions == null)) {
      throw new SessionAuthenticationException(messages.getMessage(
          "ConcurrentSessionControlAuthenticationStrategy.exceededAllowed",
          new Object[] { Integer.valueOf(allowableSessions) },
          "Maximum sessions of {0} for this principal exceeded"));
    }

    // Determine least recently used session, and mark it for invalidation
    SessionInformation leastRecentlyUsed = null;

    //若不拒絕新請求，遍歷現(xiàn)存seesion列表
    for (SessionInformation session : sessions) {
      //獲取上一次/已存的session信息
      if ((leastRecentlyUsed == null)
          || session.getLastRequest()
              .before(leastRecentlyUsed.getLastRequest())) {
        leastRecentlyUsed = session;
      }
    }

    //將上次session信息寫為無效（欺騙）
    leastRecentlyUsed.expireNow();
  }

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
      throws IOException, ServletException {
    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;

  //獲取本次http請求的session
    HttpSession session = request.getSession(false);
  
    if (session != null) {
      //從本地session關(guān)系表中取出本次http訪問的具體session信息
      SessionInformation info = sessionRegistry.getSessionInformation(session
          .getId());
      //如果存在信息，則繼續(xù)執(zhí)行
      if (info != null) {
        //判斷session是否已經(jīng)失效（這一步在本文4.2中被執(zhí)行）
        if (info.isExpired()) {
          // Expired - abort processing
          if (logger.isDebugEnabled()) {
            logger.debug("Requested session ID "
                + request.getRequestedSessionId() + " has expired.");
          }
          //執(zhí)行登出操作
          doLogout(request, response);

          //從XML配置中的redirectSessionInformationExpiredStrategy獲取URL重定向信息，頁面跳轉(zhuǎn)到登錄頁面
          this.sessionInformationExpiredStrategy.onExpiredSessionDetected(new SessionInformationExpiredEvent(info, request, response));
          return;
        }
        else {
          // Non-expired - update last request date/time
          sessionRegistry.refreshLastRequest(info.getSessionId());
        }
      }
    }

    chain.doFilter(request, response);
  }