问题是这样的。
第22章的进入论坛界面可以显示，当点击进入论坛是出错。在页面上显示的是
java.lang.NullPointerException
com.toolsbean.DB.getRs(DB.java:71)
com.dao.CategoryDao.getCategoryMap(CategoryDao.java:133)
com.servlet.IndexServlet.doPost(IndexServlet.java:27)
com.servlet.IndexServlet.doGet(IndexServlet.java:20)
javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
com.filter.CookieFilter.doFilter(CookieFilter.java:43)
com.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:24)

在控制器里面显示的是
加载数据库驱动失败！
java.lang.ClassNotFoundException: com.microsoft.sqlserver.jdbc.SQLServerDriver
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1387)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1233)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at com.toolsbean.DB.<init>(DB.java:20)
at com.dao.CategoryDao.getCategoryMap(CategoryDao.java:131)
at com.servlet.IndexServlet.doPost(IndexServlet.java:27)
at com.servlet.IndexServlet.doGet(IndexServlet.java:20)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.filter.CookieFilter.doFilter(CookieFilter.java:43)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:24)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
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:191)
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:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)
创建数据库连接失败！
java.sql.SQLException: No suitable driver
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at com.toolsbean.DB.getCon(DB.java:31)
at com.toolsbean.DB.doPstm(DB.java:50)
at com.dao.CategoryDao.getCategoryMap(CategoryDao.java:132)
at com.servlet.IndexServlet.doPost(IndexServlet.java:27)
at com.servlet.IndexServlet.doGet(IndexServlet.java:20)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.filter.CookieFilter.doFilter(CookieFilter.java:43)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:24)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
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:191)
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:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)
2011-3-16 15:59:22 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet index threw exception
java.lang.NullPointerException
at com.toolsbean.DB.getRs(DB.java:71)
at com.dao.CategoryDao.getCategoryMap(CategoryDao.java:133)
at com.servlet.IndexServlet.doPost(IndexServlet.java:27)
at com.servlet.IndexServlet.doGet(IndexServlet.java:20)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.filter.CookieFilter.doFilter(CookieFilter.java:43)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.filter.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:24)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
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:191)
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:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)
我是使用的数据库是SQL Server2005（补丁都打了，和数据库应该没有关系，因为别的章节的数据库在2005中可以运行），数据库驱动msbase.jar mssqlserver.jar msutil.jar已经正确放到lib中
在com.toolsbean.DB里的代码是这样
package com.toolsbean;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DB {
private Connection con;
private PreparedStatement pstm;
private String user="king";
private String password="123";
private String className="com.microsoft.sqlserver.jdbc.SQLServerDriver";
private String url="jdbc:sqlserver://localhost:1433;DatabaseName=db_forum";
//jdbc:sqlserver://127.0.0.1:1433;DatabaseName=spc
/** 构造方法，在该方法中加载数据库驱动  */
public DB(){
try{
Class.forName(className);
}catch(Exception e){
System.out.println("加载数据库驱动失败！");
e.printStackTrace();
}
}

/**创建数据库连接*/
public Connection getCon(){
if(con==null){
try {
con=DriverManager.getConnection(url,user,password);
//con.setAutoCommit(true);
} catch (SQLException e) {
System.out.println("创建数据库连接失败！");
con=null;
e.printStackTrace();
}
}
return con;
}

/**
 *@功能：对数据库进行增、删、改、查操作
 *@参数：sql为SQL语句；params为Object数组，里面存储的是为sql表示的SQL语句中"?"占位符赋值的数据 
 */
public void doPstm(String sql,Object[] params){
if(sql!=null&&!sql.equals("")){
if(params==null)
params=new Object[0];
getCon();
if(con!=null){
try{
System.out.println(sql);
pstm=con.prepareStatement(sql,ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
for(int i=0;i<params.length;i++)
pstm.setObject(i+1,params[i]);
pstm.execute();
}catch(SQLException e){
System.out.println("doPstm()方法出错！");
e.printStackTrace();
}
}
}
}

/**
 * @功能：获取调用doPstm()方法执行查询操作后返回的ResultSet结果集
 * @返回值：ResultSet
 */
public ResultSet getRs() throws SQLException{
return pstm.getResultSet();
}
/**
 * @功能：获取调用doPstm()方法执行更新操作后返回影响的记录数
 * @返回值：int
 * @throws SQLException
 */
public int getCount() throws SQLException{
return pstm.getUpdateCount();
}
/**
 * @功能：释放PrepareStatement对象与Connection对象
 */
public void closed(){
try{
if(pstm!=null)
pstm.close();
}catch(SQLException e){
System.out.println("关闭pstm对象失败！");
e.printStackTrace();
}
try{
if(con!=null){
con.close();
}
}catch(SQLException e){
System.out.println("关闭con对象失败！");
e.printStackTrace();
}
}
}


麻烦帮我看看这个是为什么？