今天使用Tomcat连接数据库时出现了Cannot create JDBC driver of class '' for connect URL 'null' 错误，程序是没有问题的。 最后查找发现是Tomcat版本的问题，原来的版本是Tomcat5.0，现在是5.5，二者server.xml中对数据源的配置发生了一些变化。

Tomcat5.0中：
 <Resource name="jdbc/myapp"
                     auth="Container"
                     type="javax.sql.DataSource"/>

        <ResourceParams name="jdbc/myapp">
          <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>30</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 15 seconds. An Exception is thrown if
               this timeout is exceeded.  Set to -1 to wait indefinitely.
          -->

          <parameter>
            <name>maxWait</name>
            <value>15000</value>
          </parameter>

          <!-- MySQL dB username and password for dB connections  -->
          <parameter>
           <name>username</name>
           <value>root</value>
          </parameter>
          <parameter>
           <name>password</name>
           <value></value>
          </parameter>

          <!-- Class name for mm.mysql JDBC driver -->
          <parameter>
             <name>driverClassName</name>
             <value>org.gjt.mm.mysql.Driver</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:mysql://localhost:3306/myapp?autoReconnect=true</value>
          </parameter>
        </ResourceParams>

Tomcat5.5中：
  <Resource name="jdbc/myapp"
               type="javax.sql.DataSource"
      password="root"
      driverClassName="org.gjt.mm.mysql.Driver"
      maxIdle="30"
      maxWait="15000"
      username="root"
      url="jdbc:mysql://localhost:3306/myapp?autoReconnect=true"
      maxActive="30"/>

重启Tomcat，提示消失。
这个错误的出现可能还有其他很多种的原因，这个只是其中一个。