Servlet.service（）拋出NullPointerException

Question

老實說，我對此感到困惑，為什么這會引發這個特殊異常。 如果我有直接指向另一個JSP的servlet，它就可以正常工作。 這是我得到的錯誤的堆棧跟蹤

17:13:51,190 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/ProjectServlet].[jsp]] (http-localhost-127.0.0.1-8080-2) Servlet.service() for servlet jsp threw exception: java.lang.NullPointerException
    at org.apache.jsp.createProfile_jsp._jspService(createProfile_jsp.java:74)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) [jbossweb-7.0.13.Final.jar:]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:326) [jbossweb-7.0.13.Final.jar:]
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:253) [jbossweb-7.0.13.Final.jar:]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:840) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:622) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:560) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:488) [jbossweb-7.0.13.Final.jar:]
    at ecollaborator.Servlet.CreateAccountServlet.gotoPage(CreateAccountServlet.java:55) [classes:]
    at ecollaborator.Servlet.CreateAccountServlet.doPost(CreateAccountServlet.java:45) [classes:]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:754) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [jboss-servlet-api_3.0_spec-1.0.0.Final.jar:1.0.0.Final]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161) [jbossweb-7.0.13.Final.jar:]
    at org.jboss.as.jpa.interceptor.WebNonTxEmCloserValve.invoke(WebNonTxEmCloserValve.java:50) [jboss-as-jpa-7.1.1.Final.jar:7.1.1.Final]
    at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:153) [jboss-as-web-7.1.1.Final.jar:7.1.1.Final]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [jbossweb-7.0.13.Final.jar:]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:368) [jbossweb-7.0.13.Final.jar:]
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [jbossweb-7.0.13.Final.jar:]
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:671) [jbossweb-7.0.13.Final.jar:]
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:930) [jbossweb-7.0.13.Final.jar:]
    at java.lang.Thread.run(Unknown Source) [rt.jar:1.7.0]

現在，我有一個頁面，提示用戶創建一個帳戶，JSP如下所示。 它稱為createAccount.jsp

    <%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>

<% String type = (String) request.getParameter("type");%>

<!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=ISO-8859-1">
<b><font size="6"><center>E-Collaborator</center></font></b>
</head>
<body>
<form name='createAccount' action="CreateAccountServlet" method="post">
    <table>
        <tr>
            <td>Create Account<td>
        </tr>
        <tr>
            <td>Username:</td><td><input type="text" name="username" /></td>
        </tr>
        <tr>
            <td>Password:</td><td><input type="password" name="password" /></td>
        </tr>
        <tr>
            <td>Type:</td>
            <td>
                <select id="acctype" name="acctype">
                <option></option>
                <option>Collaborator</option>
                <option>Consumer</option>
                <option>ServiceProvider</option>
                </select> 
            </td>
        </tr>
    </table>
    <input type="submit" value="Submit"/>
    <input type = "hidden" name="createAccount" value="true">
</form>
</body>
</html>

它被轉發到該Servlet，在dispatcher.forward命令上，該錯誤在gotoPage方法中引發錯誤。

package ecollaborator.Servlet;

import java.io.IOException;

import javax.ejb.EJB;
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 ecollaborator.Entity.*;
import ecollaborator.Session.*;

@WebServlet("/CreateAccountServlet")
public class CreateAccountServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    @EJB
    private Admin admin;

    public CreateAccountServlet() {
        super();
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
    {
        boolean isForwarded = false;

        String createAccount = request.getParameter("createAccount");
        if(createAccount != null && createAccount.equals("true"))
        {

            String username = (String)request.getParameter("username");
            String password = (String)request.getParameter("password");
            String type = (String)request.getParameter("acctype");

            User u = new User(username,password,type);
            admin.createAccount(u);


            if(!isForwarded)
            {
                request.setAttribute("type", type);
                isForwarded = gotoPage("/createProfile.jsp", request, response);
            }
        }

    }

    private boolean gotoPage(String address, HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
        {
                RequestDispatcher dispatcher = getServletContext().getRequestDispatcher(address);
                dispatcher.forward(request, response);
                return true;
        }

}

如何修復此代碼，以便避免遇到此錯誤？ 我的其他servlet使用相同的gotoPage函數並轉發到JSP。

Answer 1

嘗試將getParameter替換為jsp中的getAttribute

<% String type = (String) request.getAttribute("type");%>

並刪除以下行：

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>

並檢查

getParameter和getAttribute是不同的方法

http://docs.oracle.com/cd/E17802_01/webservices/webservices/docs/1.6/api/javax/servlet/ServletRequest.html

Answer 2

在viewpage.jsp jst上，檢查是否使用Iterator方法將ArrayList對象與Iterator引用itr關聯。