Erro ao conectar com MySQL

SEVERE: >>org.zkoss.zk.ui.UiException: org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of class '' for connect URL 'null'
>>org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of class '' for connect URL 'null'
[SQL: 0, 08001]
>>java.sql.SQLException: No suitable driver
>>	at java.sql.DriverManager.getDriver(DriverManager.java:243)
>>	at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:773)
>>	at org.apache.tomcat.dbcp.dbcp.BasicDataSource.getConnection(BasicDataSource.java:540)
>>	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
>>	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
>>	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
>>	at java.lang.reflect.Method.invoke(Method.java:585)
>>	at bsh.Reflect.invokeMethod(Unknown Source)

<?xml version="1.0" encoding="UTF-8"?>
<Server>
  <Listener className="org.apache.catalina.core.AprLifecycleListener"/>
  <Listener className="org.apache.catalina.mbeans.GlobalResourcesLifecycleListener"/>
  <Listener className="org.apache.catalina.storeconfig.StoreConfigLifecycleListener"/>
  <Listener className="org.apache.catalina.mbeans.ServerLifecycleListener"/>
  <GlobalNamingResources>
    <Environment
      name="simpleValue"
      type="java.lang.Integer"
      value="30"/>
    <Resource
      name="jdbc/adcDB"
      type="javax.sql.DataSource"
      password="admin"
      driverClassName="org.gjt.mm.mysql.Driver"
      maxIdle="100"
      maxWait="10000"
      validationQuery="select count(*) from adctab1"
      username="root"
      url="jdbc:mysql://localhost:3306/adc"
      maxActive="200"/>
    <Resource
      auth="Container"
      description="User database that can be updated and saved"
      name="UserDatabase"
      type="org.apache.catalina.UserDatabase"
      pathname="conf/tomcat-users.xml"
      factory="org.apache.catalina.users.MemoryUserDatabaseFactory"/>
  </GlobalNamingResources>
  <Service
      name="Catalina">
    <Connector
        port="8080"
        redirectPort="8443"
        minSpareThreads="25"
        connectionTimeout="20000"
        maxThreads="150"
        maxSpareThreads="75">
    </Connector>
    <Connector
        port="8009"
        redirectPort="8443"
        protocol="AJP/1.3">
    </Connector>
    <Engine
        defaultHost="localhost"
        name="Catalina">
      <Realm className="org.apache.catalina.realm.UserDatabaseRealm"/>
      <Host
          appBase="webapps"
          name="localhost">
      </Host>
    </Engine>
  </Service>
</Server>

<resource-ref>
  <res-ref-name>jdbc/adcDB</res-ref-name>
  <res-type>javax.sql.DataSource</res-type>
  <res-auth> Container</res-auth>
</resource-ref>