Java JSP getAttribute() 返回 null
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JSP getAttribute() returning null
提问by Alex Botev
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:
我的小服务程序：
<%@ 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:
网页.xml：
<?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. Anyone have any ideas?
我知道我应该使用 JSTL 并且我是，但这是我检查它不是 JSTL 问题而是一些 java 问题的方法。谁有想法？
PS: If I do just <%= request %>I get org.apache.catalina.connector.RequestFacade@58c3fbebso the problem is not in not casting the result to String.
And if I do on the 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?
PS：如果我不只是<%= request %>我得到org.apache.catalina.connector.RequestFacade@58c3fbeb这样的问题是不是在不投结果为String。
如果我在System.out.println(req);我得到的 servlet 上做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. When I deployed it on with maven compiled WAR file on the tomcat it worked okay.
结果：事实证明，出于某种原因，IDE 正在做一些奇怪的事情，并在转发中引入了这个问题。当我在 tomcat 上使用 maven 编译的 WAR 文件部署它时，它工作正常。
回答by JNL
You are not typecasting it to String.
request.getAttribute()will return an Object.
您没有将其类型转换为 String。
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
或者
<%= (String)request.getAttribute("colNames") %>
Why are you using forEachhere? You just need to display a Stringright?
Also, shouldn't var="col"be ------>var = "colNames"
你为什么用forEach在这里？你只需要显示一个String吧？另外，不应该var="col"是------>var = "colNames"
<tr>
<c:forEach items="${param.colNames}" var="col">
<td>${col}</td>
</c:forEach>
</tr>
回答by ChadNC
You need to use,
你需要使用，
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/响应将其持久化，然后将其从 JSP 页面上的会话中拉出。
回答by wiTcH_doCtoR
For me it was an issue with the tomcat server 7. I updated it to 8.5 and its working fine now.
对我来说，这是 tomcat 服务器 7 的问题。我将其更新到 8.5，现在工作正常。
