看的电子工业出版社的《jsp宝典》,有一例程照书上的方法做始终运行出错,各位了
看的电子工业出版社的《jsp宝典》,有一例程照书上的方法做始终运行出错,求助各位了
运行环境是tomcat+mysql
开发环境是eclipse+lomboz
下面是照书上的例程改过的代码,为了让大家更容易看明白,其中//部门就是造成错误的地方:
<%@ page contentType= "text/html; charset=gb2312 " language= "java " import= "java.sql.* " errorPage= " " %>
<!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=gb2312 ">
<title> 数据库操作 </title>
</head>
//连接数据库
<%
String product_name= " ";
Connection con=null;
Statement sm=null;
ResultSet rs=null;
try{
Class.forName( "com.mysql.jdbc.Driver ").newInstance();
String url= "jdbc:mysql://localhost/mydb ";
con=DriverManager.getConnection(url, "root ", "12345 ");
sm=con.createStatement();
rs=sm.executeQuery( "select * from products ");
}catch(Exception e){
e.printStackTrace();
}
%>
//读取数据库记录
// <%
//while(rs.next()){
//product_name=rs.getString( "PRODUCT_NAME ");
//}
//%>
<%=product_name%>
<body>
</body>
</html>
如果去掉//
//读取数据库记录
// <%
//while(rs.next()){
//product_name=rs.getString( "PRODUCT_NAME ");
//}
//%>
此部分造成的错误提示如下:
HTTP Status 500 -
--------------------------------------------
type Exception report
message
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)
root cause
java.lang.NullPointerException
org.apache.jsp.index_jsp._jspService(index_jsp.java:70)
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)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.30 logs.
--------------------------------------------
Apache Tomcat/5.0.30
------解决方案--------------------
看了你的例子，感觉是没有取到数据库的连接，所以造成一下语句抛出NullPointerException
sm=con.createStatement();
加上一句System.out.println( "con: " + con);在这个语句前，检查一下这个值。
------解决方案--------------------
运行环境是tomcat+mysql
开发环境是eclipse+lomboz
下面是照书上的例程改过的代码,为了让大家更容易看明白,其中//部门就是造成错误的地方:
<%@ page contentType= "text/html; charset=gb2312 " language= "java " import= "java.sql.* " errorPage= " " %>
<!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=gb2312 ">
<title> 数据库操作 </title>
</head>
//连接数据库
<%
String product_name= " ";
Connection con=null;
Statement sm=null;
ResultSet rs=null;
try{
Class.forName( "com.mysql.jdbc.Driver ").newInstance();
String url= "jdbc:mysql://localhost/mydb ";
con=DriverManager.getConnection(url, "root ", "12345 ");
sm=con.createStatement();
rs=sm.executeQuery( "select * from products ");
}catch(Exception e){
e.printStackTrace();
}
%>
//读取数据库记录
// <%
//while(rs.next()){
//product_name=rs.getString( "PRODUCT_NAME ");
//}
//%>
<%=product_name%>
<body>
</body>
</html>
如果去掉//
//读取数据库记录
// <%
//while(rs.next()){
//product_name=rs.getString( "PRODUCT_NAME ");
//}
//%>
此部分造成的错误提示如下:
HTTP Status 500 -
--------------------------------------------
type Exception report
message
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)
root cause
java.lang.NullPointerException
org.apache.jsp.index_jsp._jspService(index_jsp.java:70)
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)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.30 logs.
--------------------------------------------
Apache Tomcat/5.0.30
------解决方案--------------------
看了你的例子，感觉是没有取到数据库的连接，所以造成一下语句抛出NullPointerException
sm=con.createStatement();
加上一句System.out.println( "con: " + con);在这个语句前，检查一下这个值。
------解决方案--------------------