从Servlet无法在JSP中打印ArrayList
[英]Cant print ArrayList in JSP from Servlet
问题描述
So I have this simple list made in a java servlet, and I would like to display it within a JSP page. 
因此，我有一个用Java servlet制作的简单列表，我想在JSP页面中显示它。 Servlet code: Servlet代码：
public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException{
ArrayList<String> myList = new ArrayList<String>();
myList.add("cat");
myList.add("dog");
myList.add("frog");
request.setAttribute("list", myList);
String nextJSP = "/index.jsp";
RequestDispatcher dispatcher = getServletContext().getRequestDispatcher(nextJSP);
dispatcher.forward(request,response);
}
} }
However it won't print in the following JSP file: 但是，它不会在以下JSP文件中打印：
<%@page import="java.io.*" %>
<%@page import="java.net.*" %>
<%@page import="java.util.*" %>
<%@page import="java.util.List" %>
<%@page import="java.util.ArrayList" %>
<%@page language="java" import="myPackage.*" %>
<!DOCTYPE html>
<html>
<body>
<h1>My First Heading</h1>
<p>My first paragraph.</p>
<% List<String> myList = (ArrayList<String>)
request.getAttribute("list"); %>
<% out.println(myList); %>
</body>
</html>
Any help would be appreciated! 任何帮助，将不胜感激！
3 个解决方案
解决方案1
1 已采纳 2018-04-24 01:44:19
The HTTP 500 error is come from this line: HTTP 500错误来自以下行：
<% List<String> myList = (ArrayList<String>)
request.getAttribute("list"); %>
It has extra () ,remove it and make it as below,then the HTTP 500 will diappear,instead you will got a warning Type safety:Unchecked cast from Object to ArrayList<String> ,but it doesn't matter 它具有多余的() ，将其删除，并使其如下所示，然后HTTP 500就会消失，相反，您会收到警告Type safety:Unchecked cast from Object to ArrayList<String>强制转换Type safety:Unchecked cast from Object to ArrayList<String> ，但这没关系
For how to print the ArrayList,if you have import JSTL tag in your jsp page,you can do it as below,previous answer has metioned: 对于如何打印ArrayList，如果您在jsp页面中导入了JSTL标记，则可以按照以下步骤进行操作，前面的答案已提及：
<c:forEach var="li" items="${list}">
<c:out value="${li}"/>
</c:forEach>
If you do not want to use JSTL , you can use java code to print it: 如果您不想使用JSTL ，则可以使用Java代码来打印它：
<% for(int i=0;i<myList.size();i++){
out.println(myList.get(i));
} %>
解决方案2
0 2018-04-24 00:46:03
Okay, your are having a HTTP 500 error message and I would suspect that it is coming from the following part of your code: 好的，您收到的是HTTP 500错误消息，我怀疑它来自代码的以下部分：
<% out.println(myList); %>
Keep in mind that the out used here is not System.out and does not behave the same way. 请记住，此处使用的输出不是System.out，并且行为方式也不相同。 Instead try (replace the line mentionned above by the following): 而是尝试（用以下内容替换上面提到的行）：
<% Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
out.println(<iterator.next());
}
%>
解决方案3
0 2018-04-24 01:16:00
You could use core tag foreach as 您可以使用核心标记foreach作为
<c:forEach var="list" items="${YourList}">
<c:out value="${list}"/>
</c:forEach>
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