JSP getAttribute（）返回null
[英]JSP getAttribute() returning null
问题描述
So in my Servlet I have the following: 
所以在我的Servlet中我有以下内容：
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
resp.setContentType("text/html");
req.setAttribute("colNames","ka");
req.setAttribute("items", new String[]{});
//System.out.println(req.getAttribute("colNames"));
req.getRequestDispatcher("/index.jsp").forward(req,resp);
}
My Servlet: 我的Servlet：
<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page isELIgnored="false"%>
<!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>NewGem OrderInfo</title>
<script src="sorttable.js"></script>
</head>
<body>
<%= request.getAttribute("colNames") %>
<table id="table" class="sortable">
<tr>
<c:forEach items="${param.colNames}" var="col">
<td>${col}</td>
</c:forEach>
</tr>
<c:forEach items="${param.items}" var="row">
<tr>
<c:forEach items="${row.elements()}" var="value">
<td>${value}</td>
</c:forEach>
</tr>
</c:forEach>
</table>
</body>
</html>
web.xml: web.xml中：
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
id="WebApp_ID" version="2.5">
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<display-name>EntityDumpServlet</display-name>
<welcome-file-list>
<welcome-file>dump</welcome-file>
</welcome-file-list>
<servlet>
<servlet-name>EntityDumpServlet</servlet-name>
<servlet-class>
com.jpmorgan.d1.ptg.web.EntityDumpServlet
</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>EntityDumpServlet</servlet-name>
<url-pattern>/dump</url-pattern>
</servlet-mapping>
</web-app>
So I'm just running the get, have only this servlet nothing else. 所以我只是运行get，只有这个servlet没有别的。
I know that I should use JSTL and I am, but this was my way of checking that it was not a JSTL problem but some what a java problem. 我知道我应该使用JSTL而且我是，但这是我检查它不是JSTL问题的方式，而是一些java问题。 Anyone have any ideas? 有人有主意吗？
PS: If I do just <%= request %> I get org.apache.catalina.connector.RequestFacade@58c3fbeb so the problem is not in not casting the result to String. PS：如果我只是<%= request %>我得到org.apache.catalina.connector.RequestFacade@58c3fbeb所以问题不在于没有将结果转换为String。
And if I do on the servlet System.out.println(req); 如果我在servlet System.out.println(req); I get org.apache.catalina.connector.RequestFacade@4ac37ce2 , which means for some reason the request passed and received are different? 我得到org.apache.catalina.connector.RequestFacade@4ac37ce2 ，这意味着由于某种原因，传递和接收的请求是不同的？
Result: It turned out that for some reason the IDE is doing some weird stuff and introduced this problem in the forwarding. 结果：事实证明，由于某种原因，IDE正在做一些奇怪的事情，并在转发中引入了这个问题。 When I deployed it on with maven compiled WAR file on the tomcat it worked okay. 当我在tomcat上使用maven编译的WAR文件部署它时，它运行正常。
3 个解决方案
解决方案1
7 2013-08-28 14:29:54
You are not typecasting it to String. 你没有将它类型转换为String。 request.getAttribute() will return an Object . request.getAttribute()将返回一个Object 。
Try using this and see if it works: 尝试使用它，看看它是否有效：
String value = (String)request.getAttribute("colnames");
Or 要么
<%= (String)request.getAttribute("colNames") %>
Why are you using forEach here? 你为什么在这里使用forEach ？ You just need to display a String right? 你只需要显示一个String吗？ Also, shouldn't var="col" be ------> var = "colNames" 另外， var="col"不应该是------> var = "colNames"
<tr>
<c:forEach items="${param.colNames}" var="col">
<td>${col}</td>
</c:forEach>
</tr>
解决方案2
1 2013-08-29 11:38:50
You need to use, 你需要用，
request.getSession().setAttribute("colNames",yourObject);
To persist it through the request/response and then pull it out of the session on your JSP page. 通过请求/响应持久化它，然后将其从JSP页面上的会话中拉出来。
解决方案3
0 2018-04-08 16:27:20
对我来说这是tomcat服务器7的一个问题。我将它更新为8.5并且它现在工作正常。
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