Question

我试图制作一个非常简单的登录屏幕。它必须检查密码和用户名。还有一个默认用户，在init()中设置。奇怪的是，它仍在NullPointerException上给我一个rd.forward(); 有人能解释一下吗我很抱歉我的英语不好，如果我需要详细说明，请告诉我。

package servlets;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Scanner;
import javax.servlet.FilterConfig;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import classes.user;

public class loginServlet extends HttpServlet {
ArrayList<user> alleUsers = new ArrayList<>();
private String defaultUser="", defaultPassword="";
private user userding;

public void init(ServletConfig config) throws ServletException{
    super.init(config);
    defaultUser = config.getInitParameter("defaultUser");
    defaultPassword = config.getInitParameter("defaultPassword");
    user userding = new user(defaultUser, defaultPassword);
}

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    String a = req.getParameter("back");
    String b = req.getParameter("username");
    String c = req.getParameter("password");
    Object o = getServletContext().getAttribute("arraylist");
    RequestDispatcher rd = null;

    if (a.equals("Register")) {
        rd = req.getRequestDispatcher("registerScherm.jsp");
    }

    if (a.equals("Login")) {
        System.out.println(b + " "+ c);
        if(!b.equals(null) && !c.equals(null)){
            if (o instanceof ArrayList) {
                    alleUsers = (ArrayList) o;
                    alleUsers.add(userding);
                    if (alleUsers != null) {
                        for (user u : alleUsers) {
                            if (b.equals(u.getUsername()) && c.equals(u.getPassword())) {
                                getServletContext().setAttribute("naam", b);
                                rd = req.getRequestDispatcher("ingelogdScherm.jsp");
                                break;
                            } else {
                                rd = req.getRequestDispatcher("loginScherm.jsp");
                            }
                        }
                    }
                }
            }else{
                rd = req.getRequestDispatcher("loginScherm.jsp");
            }
        }
    rd.forward(req, resp);
    //It gives an error on this forward
    }
}

这是我的loginscreen jsp页面：

<body>
    <%
        String userName = "";
        for (Cookie c : request.getCookies()) {
            if (c.getName().equals("username")) {
            userName = c.getValue();
            break;
        }
    }

%>
<div id="account">
<div id="inputbox">
    <form action="loginServlet.do" method="post">
        <input class="ltf" type="text" name="username" value="<%= userName %>" /><br/> 
        <input class="ltf" type="password" name="password" /><br/> 
        <input type="submit" name="back" value="Login" /> 
        <input type="submit" name="back" value="Register" />
    </form>
</div>

好吧我想我修好了。我删除了一些帮助的守卫。我仍然困惑为什么它没有通过它们，但它现在有效。谢谢！

Answer 1

在执行此语句时，在整个条件语句中，rd可能尚未初始化：

rd.forward(req, resp);

这可能是您获得NullPointerException的原因，rd尚未初始化且尚未在内存中分配任何空间。这一切真的是一个指向null的指针。

如果这是您的应用程序中可以或允许发生的情况，您可以通过添加以下条件来避免异常

 if(rd != null) 
      rd.forward(req, resp);

您可能希望在前端通知客户端无法初始化rd。

Answer 2

你的缩进被打破了，这使得阅读变得困难;我为你搞砸了..

package servlets;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Scanner;
import javax.servlet.FilterConfig;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import classes.user;

public class loginServlet extends HttpServlet {
ArrayList<user> alleUsers = new ArrayList<>();
private String defaultUser="", defaultPassword="";
private user userding;

public void init(ServletConfig config) throws ServletException{
    super.init(config);
    defaultUser = config.getInitParameter("defaultUser");
    defaultPassword = config.getInitParameter("defaultPassword");
    user userding = new user(defaultUser, defaultPassword);
}

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    String a = req.getParameter("back");
    String b = req.getParameter("username");
    String c = req.getParameter("password");
    Object o = getServletContext().getAttribute("arraylist");
    RequestDispatcher rd = null;

    if (a.equals("Register")) {
        rd = req.getRequestDispatcher("registerScherm.jsp");
    }

    if (a.equals("Login")) {
        System.out.println(b + " "+ c);
        if(!b.equals(null) && !c.equals(null)){
            if (o instanceof ArrayList) {
                    alleUsers = (ArrayList) o;
                    alleUsers.add(userding);
                    if (alleUsers != null) {
                        for (user u : alleUsers) {
                            if (b.equals(u.getUsername()) && c.equals(u.getPassword())) {
                                getServletContext().setAttribute("naam", b);
                                rd = req.getRequestDispatcher("ingelogdScherm.jsp");
                                break;
                            } else {
                                rd = req.getRequestDispatcher("loginScherm.jsp");
                            }
                        }
                    }
            }
        }else{
            rd = req.getRequestDispatcher("loginScherm.jsp");
        }
    }
    rd.forward(req, resp); //It gives an error on this forward
}

现在已经完成了.. 请注意，如果 a 不等于“login”或“Register”，则rd自初始化后未更改：

    String a = req.getParameter("back");
    RequestDispatcher rd = null;

所以看看“a”是什么会很有趣..

根据数据，您可以在这种情况下结束的其他地方，包括以下行：

if (o instanceof ArrayList)

Answer 3

如果a的值不等于“登录”或“注册”，则rd仍为null，因此rd.forward(req, resp);将NullPointerException。
您可以通过打印a的值来调试它，也可以在rd.forward(req, resp); req.getRequestDispatcher("name.jsp");的任何地方添加package servlets; import java.io.BufferedReader; import java.io.File; import java.io.FileReader; import java.io.IOException; import java.io.PrintWriter; import java.sql.Connection; import java.sql.DriverManager; import java.sql.SQLException; import java.util.ArrayList; import java.util.Scanner; import javax.servlet.FilterConfig; import javax.servlet.RequestDispatcher; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import classes.user; public class loginServlet extends HttpServlet { ArrayList<user> alleUsers = new ArrayList<>(); private String defaultUser="", defaultPassword=""; private user userding; public void init(ServletConfig config) throws ServletException{ super.init(config); defaultUser = config.getInitParameter("defaultUser"); defaultPassword = config.getInitParameter("defaultPassword"); user userding = new user(defaultUser, defaultPassword); } protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String a = req.getParameter("back"); String b = req.getParameter("username"); String c = req.getParameter("password"); Object o = getServletContext().getAttribute("arraylist"); RequestDispatcher rd = null; if (a.equals("Register")) { rd = req.getRequestDispatcher("registerScherm.jsp"); rd.forward(req, resp); } if (a.equals("Login")) { System.out.println(b + " "+ c); if(!b.equals(null) && !c.equals(null)){ if (o instanceof ArrayList) { alleUsers = (ArrayList) o; alleUsers.add(userding); if (alleUsers != null) { for (user u : alleUsers) { if (b.equals(u.getUsername()) && c.equals(u.getPassword())) { getServletContext().setAttribute("naam", b); rd = req.getRequestDispatcher("ingelogdScherm.jsp"); rd.forward(req, resp); break; } else { rd = req.getRequestDispatcher("loginScherm.jsp"); rd.forward(req, resp); } } } } }else{ rd = req.getRequestDispatcher("loginScherm.jsp"); rd.forward(req, resp); } } } }，如下所示：

var count = (from p in _db.Personen
             join pc in _db.Postcodes on p.Postcode equals pc.postcode
             join r in _db.Regios on pc.RegioId equals r.RegioId
             where p.Leeftijd >= leeftijdgetal[0] && leeftijd[1] <= p.Leeftijd && 
                   r.RegioNaam == regio && 
                   p.Geslacht == geslacht &&
                   p.Showit == 1
            )
            .Distinct()
            .Count();

Javaservlet nullpointerexception

3 个答案: