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/