java - 使用 HttpSession 时出现空指针异常

Question

我目前正在学习Servlet。使用下面的代码时，我收到 NullPointerException 。每当在 SqServlet 中调用 getAttribute() 函数以获取 k 中的值时，它都会返回 null 值。谁能告诉我哪里出错了？谢谢。

public class AddServelet extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse res) {
        try {
            int i = Integer.parseInt(req.getParameter("num1"));
            int j = Integer.parseInt(req.getParameter("num2"));
            int k = i + j;

            PrintWriter out = res.getWriter();
            HttpSession session = req.getSession();

            req.setAttribute("k", k);
            res.sendRedirect("sq");
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}

public class SqServlet extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse res) {
        try {
            HttpSession session = req.getSession(false);
            int k = (int)session.getAttribute("k");

            k = k * k;
            PrintWriter out = res.getWriter();
            out.println("Square of sum: " + k);
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}

Answer 1

您在请求中设置“k”，然后从 session 中获取它

在AddServelet中，你应该这样做:session.setAttribute("k", k);

而不是req.setAttribute("k", k);