Request.getattribute 在一个 Servlet 中工作，同时在另一个中抛出空指针 [重复]答案

【问题标题】：Request.getattribute Working in One Servlet while throwing Null Pointer in Other [duplicate]Request.getattribute 在一个 Servlet 中工作，同时在另一个中抛出空指针 [重复]
【发布时间】：2020-05-16 12:12:52
【问题描述】：

在我的 JSP Servlet 应用程序中，其中一个 JSP 页面能够向 Servlet 发送表单参数，而其他 JSP 页面在执行相同操作时会抛出空指针异常。为什么这样？非常感谢您对此的帮助，请在下面找到代码。

Name.jsp文件

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<h1>What's Your Name</h1>

<form action="Name" method="POST">
 First Name: <input type="text" name="firstname"></br>
  Last Name: <input type="text" name="lastname"></br>

  <input type="submit" value="Submit">

</form>
</body>
</html>

NameServlet文件

package com.servelets;

import java.io.IOException;
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.http.HttpSession;

import com.objects.Name;

/**
 * Servlet implementation class NameServlet
 */
@WebServlet("/Name")
public class NameServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

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

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Get Name");
        response.getWriter().append("Served at: ").append(request.getContextPath());
        request.getRequestDispatcher("WEB-INF/views/Name.jsp").forward(request, response);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Post Name");
        Name name = new Name();
        HttpSession session = request.getSession();
        name.setFirstname(request.getParameter("firstname"));
        name.setLastname(request.getParameter("lastname"));


        session.setAttribute("name", name);

        //request.getRequestDispatcher("/WEB-INF/views/Bio.jsp").forward(request, response);
        response.sendRedirect("Bio");
        //request.getRequestDispatcher("Bio").include(request, response);
    }

}

Bio.jsp页面

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<h1>Give us your Age, Weight, Height</h1>
<h1>Welcome ${name.firstname}  ${name.lastname} </h1>
<form action="Bio" method="POST">
 Age: <input type="text" name="age"></br>
  Weight: <input type="number" name="weight">
  <select id="weightunit" name="weightunit">
    <option value="kg">Kg</option>
    <option value="Lb">Lb</option>

  </select></br>
  Height: <input type="number" name="height"> Unit: <input type="text" name="heightunit"></br>

  <input type="submit" value="Submit">

</form>
</body>
</html>

BioServlet文件

package com.servelets;

import java.io.IOException;
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.http.HttpSession;

import com.objects.Bio;

/**
 * Servlet implementation class BioServlet
 */
@WebServlet(description = "Servlet for Bio Page", urlPatterns = { "/Bio" })
public class BioServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

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

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        System.out.println("Get Bio");
        response.getWriter().append("Served at: ").append(request.getContextPath());
        request.getRequestDispatcher("WEB-INF/views/Bio.jsp").forward(request, response);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        System.out.println("Post Bio");
        Bio bio = new Bio();
        if(request.getAttribute("age")==null) {
            System.out.println("age is null");
        }
        HttpSession session = request.getSession();
        String age = (String) request.getAttribute("age");
        bio.setAge(Integer.parseInt(age));
        bio.setHeight(Double.parseDouble(request.getAttribute("height").toString()));
        bio.setHeightunit(request.getAttribute("heightunit").toString());
        bio.setWeight(Double.parseDouble(request.getAttribute("weight").toString()));
        bio.setWeightunit(request.getAttribute("weightunit").toString());
        session.setAttribute("bio", bio);
        response.sendRedirect("Address");
    }

}

我可以从Name.jsp 页面转到Bio.jsp 页面并访问NameServlet 中的属性，但是只要我在Bio.jsp 页面中输入值并尝试将其重定向到应用程序中的下一页，我就会运行进入空指针异常。下面的堆栈跟踪。

May 16, 2020 5:35:53 PM org.apache.catalina.startup.Catalina start
INFO: Server startup in [3,151] milliseconds
Get Name
Post Name
Get Bio
Post Bio
age is null
May 16, 2020 5:38:40 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.servelets.BioServlet] in context with path [/DisplayAddress] threw exception
java.lang.NumberFormatException: null
    at java.base/java.lang.Integer.parseInt(Integer.java:614)
    at java.base/java.lang.Integer.parseInt(Integer.java:770)
    at com.servelets.BioServlet.doPost(BioServlet.java:50)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
    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:202)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
    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:690)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)

request.getAttribute("age") 来自Bio.jsp 页面的null，我将非常感谢任何帮助。

【问题讨论】：

标签： java jsp tomcat servlets

【解决方案1】：

我意识到我使用的是 Request.getAttribute 而不是 Request.getParameter，我更新了我的 BioServlet 文件中的代码并且它有效，更新的代码如下所示。

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        System.out.println("Post Bio");
        Bio bio = new Bio();
        if(request.getParameter("age")==null) {
            System.out.println("age is null");
        }
        HttpSession session = request.getSession();
        String age = (String) request.getParameter("age");
        bio.setAge(Integer.parseInt(age));
        bio.setHeight(Double.parseDouble(request.getParameter("height").toString()));
        bio.setHeightunit(request.getParameter("heightunit").toString());
        bio.setWeight(Double.parseDouble(request.getParameter("weight").toString()));
        bio.setWeightunit(request.getParameter("weightunit").toString());
        session.setAttribute("bio", bio);
        response.sendRedirect("Address");
    }

【讨论】：