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java - 在 MVC 中 servlet 和 JSP 之间存储 session 变量(错误状态 500)

转载 作者：行者123 更新时间：2023-11-30 02:00:47 26 4
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当我运行动态网页时，在看到状态 500 页面后，我在控制台中收到错误“java.lang.NumberFormatException: null”。

该代码应该要求用户猜测 0-1000 之间的数字。如果用户猜测错误，则允许用户使用相同的 session 变量再次猜测(存储目标数量和猜测次数)。这一直持续到用户正确猜测出目标数字是多少。我还避免使用隐藏文本选项来存储变量。

我已经查过错误消息的含义，但我无法弄清楚它适用于哪里，因为状态错误非常模糊。

感谢任何帮助...

来自控制台的错误消息: 

Oct 20, 2018 11:59:12 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [controllers.GameServlet] in context with path [/GuessingGame_MVC_Version] threw exception
java.lang.NumberFormatException: null
    at java.lang.Integer.parseInt(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at controllers.GameServlet.doPost(GameServlet.java:47)
    at controllers.GameServlet.doGet(GameServlet.java:38)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:668)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:770)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1415)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

index.jsp(首页): 

<%@ page language="java" contentType="text/html; charset=US-ASCII"
    pageEncoding="US-ASCII"%>

<%@ page import="model.GameNumber" %>


<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Guessing Game - MVC Version</title>
</head>
<body>
  <h1>Guessing Game - MVC Version</h1>

  <p>
     Welcome to our guessing game!
  </p>

  <p>
     Please guess a number between 0 and 1000.
  </p>

  <form name="guessForm" action="doGuess" method="get">
     <label>
        Guess 1: 
     </label>
     <input type="text" name="guess" /><br />
     <input type="submit" name="submit" value="Make Guess">

  </form>


</body>
</html>

GameServlet.java(servlet):

package controllers;

import java.io.IOException;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.util.*;

import model.GameNumber;

/**
 * Servlet implementation class GameServlet
 */
@WebServlet(
        description = "A servlet to control our simple guessing game", 
        urlPatterns = { 
                "/GameServlet", 
                "/doGuess"
        }, 
        initParams = { 
                @WebInitParam(name = "Guesses", value = "1"), 
                @WebInitParam(name = "Minimum", value = "0"),
                @WebInitParam(name = "Maximum", value = "1000")
        })
public class GameServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    Random rand = new Random();
    int target = rand.nextInt(1000);

    int guesses=1;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public GameServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {


        // get input - guess
        GameNumber guess = new GameNumber(Integer.parseInt(request.getParameter("guess")));

        // initialize output
        String msg = "";
        String url = "/guess.jsp";

        // compare the guess with the target
       if( guess.getValue() == target ){
           // winner
           msg = "Correct! You got it in " + guesses + " guesses.";
           url = "/correct.jsp";
       } else {
           // next guess
           guesses++;
           if ( guess.getValue() < target ) {
               //low
               msg = "Incorrect guess! Guess higher next time.";
           } else {
               // high
               msg = "Incorrect guess! Guess lower next time.";
           }
       }
       //store guesses and target as session attributes
       HttpSession session = request.getSession();
        session.setAttribute("target", target);
        session.setAttribute("guesses", guesses);

       // add values to request object to pass to the destination
       request.setAttribute("msg", msg);


       // send control to the next component
       RequestDispatcher dispatcher = request.getRequestDispatcher(url);
       dispatcher.forward(request, response);

       RequestDispatcher dispatcherr = getServletContext().getRequestDispatcher(url);
       dispatcherr.forward(request, response);
    }

}

guess.jsp:

<%@ page language="java" contentType="text/html; charset=US-ASCII"
    pageEncoding="US-ASCII"%>

<%@ page import="model.GameNumber" %>

<%
  // get the inputs from the request and session attributes

  String msg = (String) request.getAttribute("msg");

  Integer targett = (Integer) session.getAttribute("target");
  int target = targett.intValue();

  Integer guessess = (Integer) session.getAttribute("guesses");
  int guesses = guessess.intValue();
  // output the form to the client
%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Guessing Game - MVC Version</title>
</head>
<body>
  <h1>Guessing Game - MVC Version</h1>

  <p>
     <%= msg %>
  </p>

  <form name="guessForm" action="doGuess" method="get">
     <label>
        Guess <%= guesses %>: 
     </label>
     <input type="text" name="guess" /><br />
     <input type="submit" name="submit" value="Make Guess">
  </form>
</body>
</html>

正确的.jsp:

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>

<%
  String msg = (String) request.getAttribute("msg");
%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Guessing Game - MVC Version</title>
</head>
<body>
<H1>Guessing Game - MVC Version</h1>

<%= msg %>

<p>
<a href=index.jsp>
  Play Again
</a>
</p>
</body>
</html>

游戏编号(型号):

package model;

import java.util.Random;

public class GameNumber {

    private int value;

    /**
     * 
     */
    public GameNumber() {
        value = 0;
    }

    /**
     * @param value
     */
    public GameNumber(int value) {
        if (value < 0) {
            this.value = 0;
        } else {
           this.value = value;
        }
    }

    /**
     * @return the value
     */
    public int getValue() {
        return value;
    }

    /**
     * @param value the value to set
     */
    public void setValue(int value) {
        if (value < 0) {
            this.value = 0;
        } else {
           this.value = value;
        }
    }

    /**
     * 
     * A simple method that sets value to a random integer between minimum and maximum
     * @param minimum the minimum value in the range
     * @param maximum the maximum value in the range
     */
    public void setRandom(int minimum, int maximum){
        Random random = new Random();
        this.value = random.nextInt(maximum - minimum) + minimum;
    }

    /**
     * a simple method to adjust the value up 1 unit
     */
    public void increment(){
        this.value++;
    }


}

尝试 catch 猜测(给出相同的错误):

GameNumber guess = null;
        try {
        guess = new GameNumber(Integer.parseInt(request.getParameter("guess")));
        } catch (NumberFormatException e) {
            System.out.println("Number format Excdption!");
        }

最佳答案

当我使用提到的 jsp 和 servlet 部署您的应用程序时它会工作得很好，但我注意到你的 GameServlet 的 tomcat 控制台中有一些错误因为您转发请求两次，并且已经完成

即java.lang.IllegalStateException:提交响应后无法转发

因此在您的 GameServlet java 中的 doPost 下

注释最后一行(下面提到)

  RequestDispatcher dispatcherr = getServletContext().getRequestDispatcher(url);
   dispatcherr.forward(request, response);

并添加

return; // not a must but helps in certain situations

关于您提到的错误消息仅当您在文本框中传递字符串消息时才会发生这种情况。可以在运行时应用额外的检查。

关于java - 在 MVC 中 servlet 和 JSP 之间存储 session 变量(错误状态 500)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/52912127/

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