Java 获取Arraylist以填写选项列表（从模型到Servlet再到Jsp）

我有一个模型、servlet和jsp页面。我想给选项列表数组列表中的所有选项，但由于某种原因，我得到了一个空指针异常

有人能帮我找出哪里出了问题吗？下面是我的代码：

JSP 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>

在jsp呈现选项中，如下所示：

State:<br>
        <select>
            <%
            edu.witc.Assignment03.model.States states = request.getAttribute("states");
            if(states!=null){   
            for (String state : states.getStates()) { 
                   out.println("<option>"+state+"</option>");
               }
             }else{
                 System.out.print("states is null");
             }
             %>
        </select><br>

状态：

---

注意：

---

最后但并非最不重要的一点是，请避免jsp中的脚本let使用

，因为您在哪里获得空指针异常。只需上传异常堆栈跟踪就可以了！我上传了代码下面的异常，浏览器中请求的url是什么？当nullpointer引发..@Rembo时，很抱歉，我不理解您的问题。好的，我替换了服务器中的内容，按照您所说的设置属性，但我仍然得到一个nullpointer@user3457789空指针异常在什么时候出现？您在哪个url中发出请求？@user3457789我想您是直接运行jsp的吧？谢谢@Rembo。这是一个非常详细的答案。我真的很感激。

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;
    }
}

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)

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>

 request.setAttribute("states", states);

State:<br>
        <select>
            <%
            edu.witc.Assignment03.model.States states = request.getAttribute("states");
            if(states!=null){   
            for (String state : states.getStates()) { 
                   out.println("<option>"+state+"</option>");
               }
             }else{
                 System.out.print("states is null");
             }
             %>
        </select><br>