急求jsp servlet 中这个错误是怎么为事？

怎么都在Servlet中做，不觉得拥挤呢。
String username = request.getParameter("username");
String password = request.getParameter("password");
System.out.println("username = " + username + ", password = " + password);打印看看有没有值？

恩··刚学jsp，就用servlet来练习练习··
打印出来了，没有错！····我去Google里找找了，也许是缺小什么架包什么的···2010-1-12 12:53:00 org.apache.catalina.startup.HostConfig checkResources
信息: Reloading context [/blog]
sa
sa
org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot load JDBC driver class 'com.mysql.jdbc.Driver'
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1136)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.getConnection(BasicDataSource.java:880)
at org.apache.commons.dbutils.QueryRunner.prepareConnection(QueryRunner.java:195)
at org.apache.commons.dbutils.QueryRunner.query(QueryRunner.java:306)
at cn.com.jobedu.blog.UserServlet.login(UserServlet.java:51)
at cn.com.jobedu.blog.UserServlet.doPost(UserServlet.java:31)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:263)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:584)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:169)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1130)
... 19 more
2010-1-12 12:53:05 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet UserServlet threw exception
java.lang.NullPointerException
at cn.com.jobedu.blog.UserServlet.login(UserServlet.java:55)
at cn.com.jobedu.blog.UserServlet.doPost(UserServlet.java:31)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:263)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:584)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)

提示信息都那么清楚了，UserServlet.java第55行空指针。

org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot load JDBC driver class 'com.mysql.jdbc.Driver'
把你的数据库驱动加进去

在数据库端执行下sql，是否能得到数据？

错误的行数都有了，还找不到吗？login方法。都给出来了55行，同志，看你这个代码不是qr是空，就是list是空，调试一下吧

cn.com.jobedu.blog.UserServlet.login(UserServlet.java:55) 找找UserServlet 第55行是不是有空引用

楼主第二次提供的错误信息表明没有MySQL的JDBC驱动。到下面的网站上下载一个吧。http://dev.mysql.com/downloads/connector/j/5.1.html

mysql的驱动找不到哦！是放在lib文件夹下吗

看看楼主下面的代码：         List list = null;
        try {
            list = (List) qr.query(sql, params,new BeanListHandler(User.class));
        } catch (SQLException e) {
            e.printStackTrace();
        }
        // 此时list可能为null，所以导致下面的语句（list.size()）抛出NullPointsException。
        if (list.size() > 0) {
            user = (User) list.get(0);
            HttpSession session = request.getSession();
            session.setAttribute("user", user);
            request.getRequestDispatcher("/admin/admin.jsp").forward(request,
                    response);
        } else {
            request.setAttribute("message", "用户名或者密码不正确!");
            request.getRequestDispatcher("/admin/login.jsp").forward(request,
                    response);
        }有两点需要改进：
第1：QueryRunner的query(String sql, Object[] params, ResultSetHandler<T> rsh)方法已经被Deprecated。应该使用
query(String sql, ResultSetHandler<T> rsh, Object... params)因此楼主的代码（上例第三行）应改为：
list = (List) qr.query(sql, new BeanListHandler(User.class), params);第2：对结果集应做空为判断，如：
if (list != null && list.size() > 0) {
   ...
}else {
   ...
}

我加了包··怎么还是报错哦··我就想不明白了··
我把包拷在lib文件夹下了···

问题我搞定了···我总结下··
首先，我用的连接池来连接数据库的，所以，mysql架包放到项目的lib目录下，也是报错的，
所以，要想不报错，就得把驱动架包放到tomcat的lib目录下··但是，为什么呢？这个我也想不清楚，望各位指点···

数据库连接池是tomcat来管理的，所以需要，你可以自己写jdbc连接测试一下，不用连接池，就不需要把包放到tomcat目录下了。

调试易

急求jsp servlet 中这个错误是怎么为事？

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