NullPointerException 空指针异常

HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:372)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
Bean.EncodingFilter.doFilter(EncodingFilter.java:23)
root cause java.lang.NullPointerException
org.apache.jsp.showProduct_jsp._jspService(showProduct_jsp.java:83)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
Bean.EncodingFilter.doFilter(EncodingFilter.java:23)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.28 logs.
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解决方案 »

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<%@ page language="java" import="java.util.*,com.Wease.struts.form.*"
pageEncoding="gbk"%>
<%@ taglib uri="http://struts.apache.org/tags-bean" prefix="bean"%>
<%@ taglib uri="http://struts.apache.org/tags-html" prefix="html"%>
<%@ taglib uri="http://struts.apache.org/tags-logic" prefix="logic"%>
<%@ taglib uri="http://struts.apache.org/tags-tiles" prefix="tiles"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme() + "://"
+ request.getServerName() + ":" + request.getServerPort()
+ path + "/";
ArrayList proList = (ArrayList) request.getAttribute("proList");
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>"> <title>产品信息</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">
 </head>
<body>
<form action="">
<table> <%
for (int i = 0; i < proList.size(); i++) {
AddProductForm product = (AddProductForm) proList.get(i);
%>
<tr>
<td>
产品名称：<%=product.getProductName()%><br>
</td>
</tr>
<tr>
<td>
产品描述：<%=product.getProductInco()%><br>
</td>
</tr>
<tr>
<td>
产品图片：
<img width="150px" height="100px"
src="<%=basePath%>productImg/<%=product.getProductImg()%>" />
<br>
</td>
</tr>
<%
}
%>
</table>
</form>
</body>
</html>
request.setAttribute("proList",你要保存的值);
并且你要保存的值不为null
这些是不是都有了啊?
  action 里面这样写的:
ProductDAO proDao = new ProductDAO();
List proList = proDao.getPoductInfo();
request.setAttribute("proList", proList);
return mapping.findForward("show");
DAO方法:
public List getPoductInfo() {
Connection con = getCon();
ArrayList proList = new ArrayList();
try {
Statement st = con.createStatement();
ResultSet rs = st
.executeQuery("select productName,productInfo,productImg,ProductFunction,ProductGood,ProductParameter from product");
while (rs.next()) {
AddProductForm product = new AddProductForm();
product.setProductName(rs.getString(1));
product.setProductInco(rs.getString(2));
product.setProductImg(rs.getString(3));
product.setProductFunction(rs.getString(4));
product.setProductGood(rs.getString(5));
product.setProductParameter(rs.getString(6));
proList.add(product);
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection(con);
}
return proList;
}
action : ProductDAO proDao = new ProductDAO();
List proList = proDao.getPoductInfo();
request.setAttribute("proList", proList);
return mapping.findForward("show");DAO :public List getPoductInfo() {
Connection con = getCon();
ArrayList proList = new ArrayList();
try {
Statement st = con.createStatement();
ResultSet rs = st
.executeQuery("select productName,productInfo,productImg,ProductFunction,ProductGood,ProductParameter from product");
while (rs.next()) {
AddProductForm product = new AddProductForm();
product.setProductName(rs.getString(1));
product.setProductInco(rs.getString(2));
product.setProductImg(rs.getString(3));
product.setProductFunction(rs.getString(4));
product.setProductGood(rs.getString(5));
product.setProductParameter(rs.getString(6));
proList.add(product);
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection(con);
}
return proList;
}JSP:
<%@ page language="java" import="java.util.*,com.Wease.struts.form.*"
pageEncoding="gbk"%>
<%@ taglib uri="http://struts.apache.org/tags-bean" prefix="bean"%>
<%@ taglib uri="http://struts.apache.org/tags-html" prefix="html"%>
<%@ taglib uri="http://struts.apache.org/tags-logic" prefix="logic"%>
<%@ taglib uri="http://struts.apache.org/tags-tiles" prefix="tiles"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme() + "://"
+ request.getServerName() + ":" + request.getServerPort()
+ path + "/";
ArrayList proList = (ArrayList) request.getAttribute("proList");
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>"> <title>产品信息</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">
 </head>
<body>
<form action="">
<table> <%
for (int i = 0; i < proList.size(); i++) {
AddProductForm product = (AddProductForm) proList.get(i);
%>
<tr>
<td>
产品名称：<%=product.getProductName()%><br>
</td>
</tr>
<tr>
<td>
产品描述：<%=product.getProductInco()%><br>
</td>
</tr>
<tr>
<td>
产品图片：
<img width="150px" height="100px"
src="<%=basePath%>productImg/<%=product.getProductImg()%>" />
<br>
</td>
</tr>
<%
}
%>
</table>
</form>
</body>
</html>
数据库有数据
action里面加个断点,看下proList是不是空的吧,nullpointException都是有数据空导致的
看看proList是否已经实例化.LZ可是在代码中用后台输出语句System.out.println(" ***** ");进行分析,看看到底那里是NULL