java.lang.IllegalStateException：在servletfilter中提交响应后无法转发

Question

I am using a filter to determine whether or not a requested page has a valid session or not.

This is my code. web.xml :

    <filter>
    <filter-name>SessionFilter</filter-name>
    <filter-class>
        com.imagemanagementutility.filter.SessionFilter
    </filter-class>
    <init-param>
        <param-name>avoid-urls</param-name>
        <param-value>index.jsp</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>SessionFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Filter class:

    public class SessionFilter implements Filter {

    private ArrayList<String> urlList;

    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse res,
            FilterChain chain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        String url = request.getServletPath();
        boolean allowedRequest = false;
        System.out.println(url.replace("/",""));
        if (urlList.contains(url)) {
            allowedRequest = true;
        }

        if (!allowedRequest) {
            HttpSession session = request.getSession(false);
            if (null == session) {
                System.out.println("redirect in servlet session filter");
                RequestDispatcher dispatcher = request
                        .getRequestDispatcher("//index.jsp");
                dispatcher.forward(request, response);
            }
        }

            chain.doFilter(req, res);

        System.out.println("end");
    }

    public void init(FilterConfig config) throws ServletException {
        String urls = config.getInitParameter("avoid-urls");
        StringTokenizer token = new StringTokenizer(urls, ",");

        urlList = new ArrayList<String>();

        while (token.hasMoreTokens()) {
            urlList.add(token.nextToken());

        }
    }
}

I have a login page which is used to check whether or not the user is valid. If the user is valid then control is transfer to this servlet.

    public class MainService extends HttpServlet {
    private static final long serialVersionUID = 1L;
    static Logger log = Logger.getLogger(MainService.class.getName());

    /**
     * @see HttpServlet#HttpServlet()
     */
    public MainService() {
        super();
    }
/*
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
     *      response)
     */
    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
     *      response)
     */
    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession(true);
        response.setContentType("text/html;charset=UTF-8");
        LogUtil.logInfo("i m here");
        List<Images> imageList = null;
        response.setContentType("text/html");
        try {
            UserDataService userDataService = new UserDataImpl();



            String userId = null;
            String username = null;
            String password = null;
            username = request.getParameter("user");
            password = request.getParameter("password");

            userId = userDataService.checkUser(username, password);

            if (userId!=null) {

            System.out.println(userId);
            session.setAttribute("userId", userId);

                imageList = userDataService.getImages(userId);
                session.setAttribute("imageList", imageList);


            /*response.sendRedirect("//showUserImages.jsp");*/

                RequestDispatcher dispatcher = request
                        .getRequestDispatcher("//showUserImages.jsp");
                dispatcher.forward(request, response);


            } else {

                /*response.sendRedirect("//index.jsp");*/

                RequestDispatcher dispatcher = request
                        .getRequestDispatcher("//index.jsp");
                dispatcher.forward(request, response);

            }
        } catch (ImageException e) {

            e.getMessage();
            e.printStackTrace();
        }
    }

}

This works without the filter, but with the filter shows an error when I forward control to "//showuserImages.jsp".

Answer 1

Put the invocation of doFilter in an else block:

if (!allowedRequest) {
    HttpSession session = request.getSession(false);
    if (null == session) {
        System.out.println("redirect in servlet session filter");
        RequestDispatcher dispatcher = request
                .getRequestDispatcher("//index.jsp");
        dispatcher.forward(request, response);
    }
}else{
    chain.doFilter(req, res);
}