Question

我有以下JSP文件和Servlet文件

<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>ABC Corporation</title>
    <h1>Terminal Login</h1>
</head>
<body>
    <form name="login" action="/WebAccount/LoginServlet?" method="post" />
    Username: <input type="text" name="username" value=""/>
    Password: <input type="text" name="password" value=""/>
    <input type="submit" value="LOGIN"/>

    Not User? Register Here: <input type="submit" action="/WebAccount/register.jsp" value="REGISTER">

</body>
</html>

Servlet代码：

class LoginServlet extends HttpServlet {

    /** 
     * Processes requests for both HTTP <code>GET</code> and <code>POST</code> methods.
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException, SQLException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        String username = request.getParameter("Username").toString();
        String password = request.getParameter("Password").toString();
        try {

            Class.forName("com.mysql.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/account";
            Connection conn = DriverManager.getConnection(url, "root", "school");

            Statement statement = (Statement) conn.createStatement();

            ResultSet rs = statement.executeQuery("SELECT * from Users where username='" + username + "' and password='" + password + "';");
            String user;
            String pass;

            while (rs.next()) {
                user = rs.getString(username).toString();
                pass = rs.getString(password).toString();

                if (username.equals(user) && password.equals(pass)) {
                    response.sendRedirect("http://www.google.com");
                    conn.close();
                }
            }
            if (!rs.next()) {
                out.println("Login failed");
                conn.close();
            }
        } catch (SQLException ex) {
             throw new ServletException("Cannot Connect", ex);
        } catch (ClassNotFoundException ex) {
            throw new ServletException("Login failed", ex);

        } finally {
            out.close();
        }
    }

    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /** 
     * Handles the HTTP <code>GET</code> method.
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        try {
            processRequest(request, response);
        } catch (SQLException ex) {
            Logger.getLogger(LoginServlet.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    /** 
     * Handles the HTTP <code>POST</code> method.
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        try {
            processRequest(request, response);
        } catch (SQLException ex) {
            Logger.getLogger(LoginServlet.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    /** 
     * Returns a short description of the servlet.
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}

我在处理servlet时收到此错误：

显示java.lang.NullPointerException

LoginServlet.processRequest(LoginServlet.java:38)
LoginServlet.doPost(LoginServlet.java:104)
javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

有任何错误吗？

Answer 1

究竟在＆＃34; LoginServlet.java，第38行＆＃34;？它看起来像＆＃34; out＆＃34;可能是null？

如果问题确实存在于回应中，那么＃34;作家＆＃34;对象，这里有一个可能的解释：

http://www.coderanch.com/t/484052/Tomcat/Tomcat-Returning-NullPointer-upon-closing

谢谢问题是response.getWriter（）对象被声明为   在servlet中是全局的，因此它在所有线程中都很常见。如果一个   线程关闭它，它用于在其他线程中给出NullPtrException。

Answer 2

显示java.lang.NullPointerException
在LoginServlet.processRequest（LoginServlet.java:38）

只需阅读跟踪的第1行即可。在LoginServlet方法内，processRequest()类的第38行的某些内容为null，而您的代码却没有预料到它。

好吧，我会试试我的魔法球。您声明了2个输入字段如下：

Username: <input type="text" name="username" value=""/>
Password: <input type="text" name="password" value=""/>

因此，参数名称分别为username和password。

但是，您尝试按如下方式访问它：

String username = request.getParameter("Username").toString();
String password = request.getParameter("Password").toString();

请注意大写的第1个字符！这不匹配。 getParameter()会在此处返回null，而toString()上的（不必要的btw）null调用将导致NullPointerEXception。

相应修复：

String username = request.getParameter("username");
String password = request.getParameter("password");

有关使用servlet的更多提示，您可能会发现我们的servlets wiki page也很有用。

Answer 3

错误消息告诉您究竟在哪一行引发异常。阅读错误消息。它包含有用的信息。

我可以通过检查此行将导致异常的代码告诉您，因为输入字段名为username（小写u），并且您读取（因此为空）参数Username（大写U）：

String username = request.getParameter("Username").toString();

顺便说一句，您为什么要在toString()上致电String？

处理servlet时出错

3 个答案: