关于#java#的问题：为什么我的页面会报错两个list都能拿到值啊现在的情况是返回list页面一个可以有列表一个直接报错

为什么我的页面会报错两个list都能拿到值啊现在的情况是返回list页面一个可以有列表一个直接报错


@RequestMapping("show")
    public String show(HttpServletRequest request,HttpSession session){

        if (Integer.valueOf(String.valueOf(request.getSession().getAttribute("garden")))==2){
            ArrayList<Students> list = adminService.list();

            request.setAttribute("list",list);
        }else {
            String aname= String.valueOf(session.getAttribute("aname"));
            System.out.println(aname);
            Students list = adminService.cx(aname);
            request.setAttribute("list",list);
        }
        return "list";
    }
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@page isELIgnored="false" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<table>
    <tr>
        <td>sid</td>
        <td>sname</td>
        <td>cid</td>
        <td>cname</td>
        <td>sxf</td>
        <td>score</td>
    </tr>
    <c:forEach items="${list}" var="l">
                    <tr>
                        <td>${l.sid}</td>
                        <td>${l.sname}</td>
                        <td>${l.cid}</td>
                        <td>${l.cname}</td>
                        <td>${l.sxf}</td>
                        <td>${l.score}</td>
                    </tr>
    </c:forEach>
</table>
</body>
</html>

看下你控制台报的错



2023-09-01 09:09:04.854 ERROR 16924 --- [nio-8080-exec-4] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [An exception occurred processing JSP page /WEB-INF/jsp/list.jsp at line 26

23:         <td>sxf</td>
24:         <td>score</td>
25:     </tr>
26:     <c:forEach items="${list}" var="l">
27:                     <tr>
28:                         <td>${l.sid}</td>
29:                         <td>${l.sname}</td>


Stacktrace:] with root cause

javax.servlet.jsp.JspTagException: Don't know how to iterate over supplied "items" in &lt;forEach&gt;
    at org.apache.taglibs.standard.tag.common.core.ForEachSupport.toForEachIterator(ForEachSupport.java:274) ~[jstl-1.2.jar:1.2]
    at org.apache.taglibs.standard.tag.common.core.ForEachSupport.supportedTypeForEachIterator(ForEachSupport.java:238) ~[jstl-1.2.jar:1.2]
    at org.apache.taglibs.standard.tag.common.core.ForEachSupport.prepare(ForEachSupport.java:155) ~[jstl-1.2.jar:1.2]
    at javax.servlet.jsp.jstl.core.LoopTagSupport.doStartTag(LoopTagSupport.java:256) ~[jstl-1.2.jar:1.2]
    at org.apache.jsp.WEB_002dINF.jsp.list_jsp._jspx_meth_c_005fforEach_005f0(list_jsp.java:180) ~[na:na]
    at org.apache.jsp.WEB_002dINF.jsp.list_jsp._jspService(list_jsp.java:139) ~[na:na]
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) ~[tomcat-embed-jasper-8.5.11.jar:8.5.11]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) ~[javax.servlet-api-3.1.0.jar:3.1.0]
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:443) ~[tomcat-embed-jasper-8.5.11.jar:8.5.11]
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385) ~[tomcat-embed-jasper-8.5.11.jar:8.5.11]
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329) ~[tomcat-embed-jasper-8.5.11.jar:8.5.11]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) ~[javax.servlet-api-3.1.0.jar:3.1.0]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:728) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:470) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:395) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:316) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:170) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:315) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1325) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1069) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1008) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:974) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:866) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:687) ~[javax.servlet-api-3.1.0.jar:3.1.0]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:851) ~[spring-webmvc-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) ~[javax.servlet-api-3.1.0.jar:3.1.0]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.10.RELEASE.jar:5.0.10.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) ~[tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:493) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:800) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:806) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1498) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_131]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_131]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.34.jar:8.5.34]
    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_131]

【以下回答由 GPT 生成】

问题原因可能是在第一个if分支中，你将一个ArrayList赋值给"list"属性，而在第二个else分支中，你将一个Students对象赋值给同样的"list"属性，导致类型不兼容的错误。

解决方案： 1. 首先，确认一下报错的具体信息是什么，来确定是哪一行代码出了问题。将报错信息输出到控制台，例如使用e.printStackTrace()。

如果报错是类型不兼容的错误，可以尝试修改代码如下：

java @RequestMapping("show") public String show(HttpServletRequest request, HttpSession session){ if (Integer.valueOf(String.valueOf(session.getAttribute("garden")))==2){ ArrayList<Students> list = adminService.list(); request.setAttribute("list",list); }else { String aname= String.valueOf(session.getAttribute("aname")); System.out.println(aname); ArrayList<Students> list = new ArrayList<>(); list.add(adminService.cx(aname)); request.setAttribute("list",list); } return "list"; }

针对第二个else分支，在adminService.cx(aname)的结果后面加上ArrayList.add()方法，将结果添加到一个新的ArrayList中。

确保在JSP页面中使用的"list"属性的类型与代码中设置的类型一致，例如：

```jsp

sid

sname

cid

cname

...

${item.sid}

${item.sname}

${item.cid}

${item.cname}

... ```

确保${item}的类型是Students对象。

如果以上解决方法不能解决问题，请提供报错信息以便更好地帮助你解决问题。

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这篇博客也不错, 你可以看下java List类型传递是值还是引用？如何传递值？
您还可以看一下钟林森老师的Java实战之Redis入门到精通课程中的列表List实战之借助队列特性实现多线程发送邮件一小节, 巩固相关知识点
除此之外, 这篇博客: [java]集合类详细总结中的 3.1.1 List 实现类的特点 部分也许能够解决你的问题。

如果你已经解决了该问题, 非常希望你能够分享一下解决方案, 写成博客, 将相关链接放在评论区, 以帮助更多的人 ^-^

关于#java#的问题：为什么我的页面会报错 两个list都能拿到值啊 现在的情况是返回list页面一个可以有列表 一个直接报错

关于#java#的问题：为什么我的页面会报错两个list都能拿到值啊现在的情况是返回list页面一个可以有列表一个直接报错