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java - 在 servlet 和 JSP 之间的 MVC 中存储会话变量（错误状态 500）

问题描述

当我运行我的动态网页时，在看到我获得状态 500 页面后，我的控制台中出现错误“java.lang.NumberFormatException：null”。

该代码应该要求用户猜测 0-1000 之间的数字。如果用户猜错了，它允许用户使用相同的会话变量再次猜测（存储目标数和猜测数）。这一直持续到用户正确猜出目标数字是多少。我也避免使用隐藏文本选项来存储变量。

我已经查看了错误消息的含义，但由于状态错误非常模糊，我无法弄清楚它适用于何处。

任何帮助表示赞赏...

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（第一页）： 

<%@ 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);
    }

}

猜测.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!");
        }

标签： javaeclipsemodel-view-controllersession-variables

解决方案

当我使用提到的 jsp 和 servlet 部署您的应用程序时，它会正常工作，但我注意到您的 GameServlet 的 tomcat 控制台中有一些错误，因为您将请求转发两次，这已经完成

即 java.lang.IllegalStateException: 响应提交后无法转发

因此，在您的 GameServlet java 中，在 doPost 下

评论最后一行（如下所述）

  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只有在文本框中传递字符串消息时才会发生。 可以在运行时对其应用附加检查。

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