曾经碰到如下错误：

org.apache.jasper.JasperException: javax.servlet.ServletException: javax.servlet.jsp.JspTagException: Don't know how to iterate over supplied "items" in <forEach> org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:491) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:401) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260) javax.servlet.http.HttpServlet.service(HttpServlet.java:717)

后来经过检查代码，发现是在JSP文件中，JSTL的标签  <c:forEach> 里的items属性被赋予了一个无法遍历的对象。下面是重现此问题的代码：

User.java

讯享网package zc.csdn; import java.util.List; import java.util.ArrayList; import java.io.Serializable; public class User implements Serializable{ / * 序列化ID */ private static final long serialVersionUID = L; / * 主键ID */ private Integer id; / * 用户名 */ private String userName; / * 书单 */ private List<Book> books = new ArrayList<Book>(); public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getUserName() { return userName; } public void setUserName(String userName) { this.userName = userName; } public List<Book> getBooks() { return books; } public void setBooks(List<Book> books) { this.books = books; } }

Book.java

讯享网

package zc.csdn; import java.io.Serializable; public class Book implements Serializable { / * 序列化ID */ private static final long serialVersionUID = -L; / * 主键ID */ private Integer id; / * 书名 */ private String bookName; public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getBookName() { return bookName; } public void setBookName(String bookName) { this.bookName = bookName; } } 

讯享网<%@ page language="java" import="java.util.*,zc.csdn.*" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";

User user = new User();
user.setId(1);
user.setUserName("lisi");

Book book1 = new Book();
book1.setId(1);
book1.setBookName("孙子兵法");

Book book2 = new Book();
book2.setId(2);
book2.setBookName("百万英镑");

List<Book> books = new ArrayList<Book>();
books.add(book1);
books.add(book2);

user.setBooks(books);

request.setAttribute("user", user);
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    
    <title>My JSP 'index.jsp' starting pagesfsdf</title>
	<meta http-equiv="pragma" content="no-cache">
	<meta http-equiv="cache-control" content="no-cache">
	<meta http-equiv="expires" content="0">    
	<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
	<meta http-equiv="description" content="This is my page">
	<!--
	<link rel="stylesheet" type="text/css" href="styles.css">
	-->
  </head>
  
  <body>
    用户${user.userName}的书单。 <br>
    <table border="1">
    	<tr>
    		<th>书名</th>
    	</tr>
    	<c:forEach items="${user}" var="item">
	    	<tr>
	    		<td>${item.bookName}</td>
	    	</tr>
	    </c:forEach>
    </table>
  </body>
</html>

如果要改正这个错误，只需把那一行改成    <c:forEach items="${user.books}" var="item">   即可。由于 user.book  是一个 List ，所以 <c:forEach> 可以正常遍历它。