Cannot load JDBC driver class 'null'

[zyzyzcyzl]

把数据库连接写在jsp中能访问数据库，但写在server.xml中tomcat启动时错误为：
[ksystem] DEBUG [main] ClickstreamListener.<init>(28) | ClickstreamLogger constr
ucted
[ksystem] ERROR [main] JDBCExceptionReporter.logExceptions(46) | Cannot load JDB
C driver class 'null'
[ksystem] ERROR [main] JDBCExceptionReporter.logExceptions(46) | Cannot load JDB
C driver class 'null'
Cannot load JDBC driver class 'null'
java.lang.NullPointerException
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:164)
at org.apache.commons.dbcp.BasicDataSource.createDataSource(BasicDataSou
rce.java:523)。。。。。

server.xml的配置为：
<Realm className="org.apache.catalina.realm.JDBCRealm" debug="99"
driverName="net.sourceforge.jtds.jdbc.Driver"
connectionURL="jdbc:jtds:sqlserver://localhost/shtzdb;charset=gb2312;autoReconnect=true"
connectionName="sa" connectionPassword="sa"
userTable="tb_sys_user" userNameCol="user_code" userCredCol="password"
userRoleTable="tb_sys_user_role" roleNameCol="role_code" />
<Resource name="jdbc/ksystem" auth="Container" type="javax.sql.DataSource"/>
<ResourceParams name="jdbc/ksystem">
<parameter>
<name>factory</name>
<value>org.apache.commons.dbcp.BasicDataSourceFactory</value>
</parameter>
<!-- Maximum number of dB connections in pool. Make sure you
configure your mysqld max_connections large enough to handle
all of your db connections. Set to 0 for no limit.
-->
<parameter>
<name>maxActive</name>
<value>100</value>
</parameter>
<!-- Maximum number of idle dB connections to retain in pool.
Set to 0 for no limit.
-->
<parameter>
<name>maxIdle</name>
<value>30</value>
</parameter>
<!-- Maximum time to wait for a dB connection to become available
in ms, in this example 10 seconds. An Exception is thrown if
this timeout is exceeded. Set to -1 to wait indefinitely.
-->
<parameter>
<name>maxWait</name>
<value>10000</value>
</parameter>
<!-- MySQL dB username and password for dB connections -->
<parameter>
<name>username</name>
<value>sa</value>
</parameter>
<parameter>
<name>password</name>
<value>sa</value>
</parameter>
<!-- Class name for JDBC driver -->
<parameter>
<name>driverClassName</name>
<value>net.sourceforge.jtds.jdbc.Driver</value>
</parameter>
<!-- Autocommit setting. This setting is required to make
Hibernate work. Or you can remove calls to commit(). -->
<parameter>
<name>defaultAutoCommit</name>
<value>true</value>
</parameter>
<!-- The JDBC connection url for connecting to your MySQL dB.
The autoReconnect=true argument to the url makes sure that the
mm.mysql JDBC Driver will automatically reconnect if mysqld closed the
connection. mysqld by default closes idle connections after 8 hours.
-->
<parameter>
<name>url</name>
<value>jdbc:jtds:sqlserver://localhost/shtzdb;charset=gb2312;autoReconnect=true</value>
</parameter>
<!-- Recover abandoned connections -->
<parameter>
<name>removeAbandoned</name>
<value>true</value>
</parameter>
<!-- Set the number of seconds a dB connection has been idle
before it is considered abandoned.
-->
<parameter>
<name>removeAbandonedTimeout</name>
<value>60</value>
</parameter>
<!-- Log a stack trace of the code which abandoned the dB
connection resources.
-->
<parameter>
<name>logAbandoned</name>
<value>true</value>
</parameter>
</ResourceParams>