获取Arraylist填写选项列表（模型到Servlet到Jsp）

时间：2014-03-29 17:59:41

标签： java jsp java-ee servlets arraylist

我有一个模型，servlet和jsp页面。我想给数组列表中的所有选项提供选项列表但由于某种原因我得到一个空指针异常。

有人可以帮我弄清楚我出错的地方以下是我的代码：

JSP 

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
    <%@ page import="java.util.ArrayList" %>
<!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">
<title>Customer Management</title>
</head>
<body>
    <form action="/customerManagment" method="post">
        First Name:<br>
        <input type="text" name="firstName"/><br>
        Last Name:<br>
        <input type="text" name="lastName"/><br>
        Email:<br>
        <input type="text" name="email"/><br>
        Phone:<br>
        <input type="text" name="phone"/><br>
        Phone Type:<br>

        Street Address:<br>
        <input type="text" name="streetAddress"/><br>
        Apartment Number:<br>
        <input type="text" name="apartmentNumber"/><br>
        City:<br>
        <input type="text" name="city"/><br>
        State:<br>
        <select>
            <option><%
            ArrayList<edu.witc.Assignment03.model.States> states = (java.util.ArrayList)request.getAttribute("states");
               for (edu.witc.Assignment03.model.States state : states) { 
                   state.getStates();
               }%></option>
        </select><br>

        <input type="submit" value="submit">
        </form>

的servlet 

import java.util.List;    
import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
//import javax.servlet.annotation.WebServlet;
//import javax.servlet.http.HttpServlet;
//import javax.servlet.http.HttpServletRequest;
//import javax.servlet.http.HttpServletResponse;




import edu.witc.Assignment03.model.Customer;
import edu.witc.Assignment03.model.Phone;
import edu.witc.Assignment03.model.States;

/*
 * Not thread-safe. For illustration purpose only
 */
@WebServlet(name = "CustomerServlet", urlPatterns = { 
        "/customerManagement"})
public class CustomerServlet extends HttpServlet {
    private static final long serialVersionUID = -20L;

    private List<edu.witc.Assignment03.model.States> states = new ArrayList<States>();
    private List<edu.witc.Assignment03.model.Customer> customers = new ArrayList<Customer>();

    public void init() throws ServletException {
       States state = new States();
        states.add(state);

    }

 private void addCustomer(HttpServletResponse response, HttpServletRequest request)//redirect to index
           throws IOException, ServletException {
    String url = "/customerManagement.jsp";
        request.setAttribute("customers", customers);
        request.getRequestDispatcher(url).forward(request,response);
    }

    private void editCustomer(HttpServletResponse response, HttpServletRequest request)//redirect to index
           throws IOException, ServletException {
        String url = "/customerManagement.jsp";
        request.setAttribute("customers", customers);
    request.getRequestDispatcher(url).forward(request,response);
    }

    private void sendCustomerList(HttpServletResponse response, HttpServletRequest request)//redirect to index
            throws IOException, ServletException {
        String url = "/index.jsp";
        request.setAttribute("customers", customers);
        request.getRequestDispatcher(url).forward(request,response);                                           
    }

    private Customer getCustomer(int customerId) {
        for (Customer customer : customers) {
            if (customer.getCustomerId() == customerId) {
                return customer;
            }
        }
        return null;
    }

    private void sendEditCustomerForm(HttpServletRequest request, 
            HttpServletResponse response) throws IOException, ServletException {

        String url = "/customerManagement.jsp";
        request.setAttribute("customers", customers);
        request.getRequestDispatcher(url).forward(request,response);
    }


    public void doGet(HttpServletRequest request, 
            HttpServletResponse response)
            throws ServletException, IOException {
        String uri = request.getRequestURI();
        if (uri.endsWith("/customer")) {
            sendCustomerList(response, request);
        } else if (uri.endsWith("/editCustomer")) {
            sendEditCustomerForm(request, response);
        }           
    }

    public void doPost(HttpServletRequest request, 
            HttpServletResponse response)
            throws ServletException, IOException {
        // update customer
        int customerId = 0;
        try {
            customerId = 
                    Integer.parseInt(request.getParameter("id"));
        } catch (NumberFormatException e) {
        }
        Customer customer = getCustomer(customerId);
        if (customer != null) {
            customer.setFirstName(request.getParameter("firstName"));
            customer.setLastName(request.getParameter("lastName"));
            customer.setEmail(request.getParameter("email"));
            customer.setPhone(request.getParameter("phone"));
            customer.setAddress(request.getParameter("address"));
            customer.setCity(request.getParameter("city"));
            customer.setState(request.getParameter("states"));
            customer.setZip(request.getParameter("zip"));
        }
        addCustomer(response, request);
    }
}

模型

package edu.witc.Assignment03.model;

import java.util.ArrayList;
import java.util.List;

public class States {

    private List<String> state = new ArrayList<>();{

    state.add("Alabama");
    state.add("Alaska"); 
    state.add("Arizona"); 
    state.add("Arkansas"); 
    state.add("California"); 
    state.add("Colorado"); 
    state.add("Connecticut"); 
    state.add("Delaware"); 
    state.add("Florida"); 
    state.add("Georgia"); 
    state.add("Hawaii"); 
    state.add("Idaho"); 
    state.add("Illinois"); 
    state.add("Indiana"); 
    state.add("Iowa"); 
    state.add("Kansas"); 
    state.add("Kentucky"); 
    state.add("Louisiana"); 
    state.add("Maine"); 
    state.add("Maryland"); 
    state.add("Massachusetts"); 
    state.add("Michigan"); 
    state.add("Minnesota"); 
    state.add("Mississippi"); 
    state.add("Missouri"); 
    state.add("Montana"); 
    state.add("Nebraska"); 
    state.add("Nevada"); 
    state.add("New Hampshire"); 
    state.add("New Jersey"); 
    state.add("New Mexico"); 
    state.add("New York"); 
    state.add("North Carolina"); 
    state.add("North Dakota"); 
    state.add("Ohio"); 
    state.add("Oklahoma"); 
    state.add("Oregon"); 
    state.add("Pennsylvania"); 
    state.add("Rhode Island"); 
    state.add("South Carolina"); 
    state.add("South Dakota"); 
    state.add("Tennessee"); 
    state.add("Texas"); 
    state.add("Utah"); 
    state.add("Vermont"); 
    state.add("Virginia"); 
    state.add("Washington"); 
    state.add("West Virginia"); 
    state.add("Wisconsin"); 
    state.add("Wyoming");
    }

    public List<String> getStates(){
        return this.state;
    }
}

空指针异常（单击客户按钮后）

Mar 29, 2014 2:42:50 PM org.apache.catalina.core.AprLifecycleListener init
INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: C:\Program Files\Java\jre7\bin;C:\WINDOWS\Sun\Java\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\Program Files (x86)\Intel\iCLS Client\;C:\Program Files\Intel\iCLS Client\;C:\windows\system32;C:\windows;C:\windows\System32\Wbem;C:\windows\System32\WindowsPowerShell\v1.0\;C:\Program Files\Intel\Intel(R) Management Engine Components\DAL;C:\Program Files\Intel\Intel(R) Management Engine Components\IPT;C:\Program Files (x86)\Intel\Intel(R) Management Engine Components\DAL;C:\Program Files (x86)\Intel\Intel(R) Management Engine Components\IPT;C:\Program Files (x86)\Intel\OpenCL SDK\2.0\bin\x86;C:\Program Files (x86)\Intel\OpenCL SDK\2.0\bin\x64;C:\Program Files\Lenovo\Bluetooth Software\;C:\Program Files\Lenovo\Bluetooth Software\syswow64;c:\Program Files (x86)\Microsoft SQL Server\100\Tools\Binn\;c:\Program Files\Microsoft SQL Server\100\Tools\Binn\;c:\Program Files\Microsoft SQL Server\100\DTS\Binn\;C:\Program Files\Microsoft SQL Server\110\Tools\Binn\;C:\Program Files (x86)\Microsoft SQL Server\110\Tools\Binn\;C:\Program Files\Microsoft SQL Server\110\DTS\Binn\;C:\Program Files (x86)\Microsoft SQL Server\110\Tools\Binn\ManagementStudio\;C:\Program Files (x86)\Microsoft SQL Server\110\DTS\Binn\;C:\Program Files (x86)\nodejs\;C:\Program Files (x86)\QuickTime\QTSystem\;C:\Program Files\Microsoft\Web Platform Installer\;C:\Program Files (x86)\Microsoft ASP.NET\ASP.NET Web Pages\v1.0\;C:\Program Files (x86)\Windows Kits\8.0\Windows Performance Toolkit\;%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;%SYSTEMROOT%\System32\WindowsPowerShell\v1.0\;C:\RailsInstaller\Git\cmd;C:\RailsInstaller\Ruby1.9.3\bin;C:\Users\esder_000\AppData\Roaming\npm;.
Mar 29, 2014 2:42:50 PM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:Justin_EJ_Assignment03_15400579' did not find a matching property.
Mar 29, 2014 2:42:50 PM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["http-bio-8080"]
Mar 29, 2014 2:42:50 PM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["ajp-bio-8009"]
Mar 29, 2014 2:42:50 PM org.apache.catalina.startup.Catalina load
INFO: Initialization processed in 756 ms
Mar 29, 2014 2:42:50 PM org.apache.catalina.core.StandardService startInternal
INFO: Starting service Catalina
Mar 29, 2014 2:42:50 PM org.apache.catalina.core.StandardEngine startInternal
INFO: Starting Servlet Engine: Apache Tomcat/7.0.52
Mar 29, 2014 2:42:51 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["http-bio-8080"]
Mar 29, 2014 2:42:51 PM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["ajp-bio-8009"]
Mar 29, 2014 2:42:51 PM org.apache.catalina.startup.Catalina start
INFO: Server startup in 772 ms
Mar 29, 2014 2:42:55 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [jsp] in context with path [/Justin_EJ_Assignment03_15400579] threw exception [java.lang.NullPointerException] with root cause
java.lang.NullPointerException
    at org.apache.jsp.customerManagement_jsp._jspService(customerManagement_jsp.java:94)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:315)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

1 个答案:

答案 0 :(得分：1)

看这里：

State:<br>
        <select>
            <option><%
            ArrayList<edu.witc.Assignment03.model.States> states = (java.util.ArrayList)request.getAttribute("states");
               for (edu.witc.Assignment03.model.States state : states) { 
                   state.getStates();
               }%></option>
        </select><br>

此处request.getAttribute("states");将为null，因为您的servlet中没有request.setAttribute("states", states);。导致java.lang.NullPointerException ..

java.lang.NullPointerException的可能性：

  • 如果直接运行jsp页面（ ctrl + F11 ）。此处states对象将无法在请求中使用。
  • 如果states对象为null，那么在jsp中你有foreach循环来迭代states这将导致异常

您使用了两个ArrayList to hold only州`：

  1. 在States课程中： private List<String> state = new ArrayList<>();
  2. 和CustomerServlet servlet： private List<edu.witc.Assignment03.model.States> states = new ArrayList<States>();

    3. 其中一个ArrayList就足够了。

    最终解决方案

    从CustomerServlet班级更改：

    private List<edu.witc.Assignment03.model.States> states = new ArrayList<States>();

    到

    private edu.witc.Assignment03.model.States states = new States();

    然后将其添加到请求中，以便{1}}可用于jsp，doGet（）或任何应添加states请求的地方，如：

    states

    并在你的jsp渲染选项中，如：

     request.setAttribute("states", states);

    注意：

    最后但并非最不重要的是请避免在jsp中使用script-lets而是使用jstl to render easily

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