Java J2E无法在我的servlet中使用sendRedirect（）

Question

我有一个错误，我自己似乎无法纠正

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    HttpSession session = request.getSession();
    if (session.getAttribute("username") != null) {
        if ((boolean)session.getAttribute("usertype") == true) {
            int userID = Integer.parseInt(request.getParameter("id"));
            UserDAO dao = new UserDAO();
            User user = dao.ReturnUserID(userID);
            dao.DeleteUser(user);
            dao.Close();

            response.sendRedirect("/SupTracking/admin");
        }
        response.sendRedirect("/SupTracking/index");
    }
    response.sendRedirect("/SupTracking/index");
}

我在响应中得到一个错误。sendRedirect（“/SupTracking/admin”），表示无法使用sendRedirect（），因为响应已发送

这个错误应该很简单，只是我不明白我在这段代码中发送了什么东西

下面是确切的堆栈跟踪：

java.lang.IllegalStateException: Impossible d''appeler "sendRedirect()" après que la réponse ait été envoyée
org.apache.catalina.connector.ResponseFacade.sendRedirect(ResponseFacade.java:494)
com.SupTracking.servlets.admindeluser.doGet(admindeluser.java:39)
javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

我做错了什么

Answer 1

因为当条件变为真时，响应对象会多次调用sendRedirect（）

试试这个：

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    HttpSession session = request.getSession();
    if (session.getAttribute("username") != null) {
        if ((boolean)session.getAttribute("usertype") == true) {
            int userID = Integer.parseInt(request.getParameter("id"));
            UserDAO dao = new UserDAO();
            User user = dao.ReturnUserID(userID);
            dao.DeleteUser(user);
            dao.Close();

         }
    }
    response.sendRedirect("/SupTracking/index");
}

Answer 2

对于响应对象，只能调用sendRedirect一次。您应该更改逻辑，以便知道它只会被调用一次，如下所示：

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    HttpSession session = request.getSession();
    if (session.getAttribute("username") != null) {
        if ((boolean)session.getAttribute("usertype") == true) {
            int userID = Integer.parseInt(request.getParameter("id"));
            UserDAO dao = new UserDAO();
            User user = dao.ReturnUserID(userID);
            dao.DeleteUser(user);
            dao.Close();

            response.sendRedirect("/SupTracking/admin");
        }
        else
        {
            response.sendRedirect("/SupTracking/index");
        }
    }
    else
    {
        response.sendRedirect("/SupTracking/index");
    }
}

你的问题是：

 if ((boolean)session.getAttribute("usertype") == true) {
            int userID = Integer.parseInt(request.getParameter("id"));
            UserDAO dao = new UserDAO();
            User user = dao.ReturnUserID(userID);
            dao.DeleteUser(user);
            dao.Close();

            response.sendRedirect("/SupTracking/admin");******Called Here**********
        }
        response.sendRedirect("/SupTracking/index");*******Also Called Here**********

您不能多次重定向，响应只能重定向到一个页面

Answer 3

在每次重定向后添加return语句。否则，如果一个或两个条件都为真，则会多次重定向

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    HttpSession session = request.getSession();
    if (session.getAttribute("username") != null) {
        if ((boolean)session.getAttribute("usertype") == true) {
            int userID = Integer.parseInt(request.getParameter("id"));
            UserDAO dao = new UserDAO();
            User user = dao.ReturnUserID(userID);
            dao.DeleteUser(user);
            dao.Close();

            response.sendRedirect("/SupTracking/admin");
            return;
        }
        response.sendRedirect("/SupTracking/index");
        return;
    }
    response.sendRedirect("/SupTracking/index");
}