过滤器、监听器常见应用

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1. 用户登录之后，向Session中放入用户的数据
2. 进入主页的时候要判断用户是否已经登陆，要求：在过滤器中实现.

public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        if (request.getSession().getAttribute("USER_SESSION") == null ){
            response.sendRedirect(request.getContextPath()+"/login.jsp");
        }

        filterChain.doFilter(request, response);
    }