Cannot create resource instance in Tomcat7
===

I'm trying to configure connection pool in Tomcat 7. Here is code:
part of server.xml:
 

    <GlobalNamingResources>
    <!-- Editable user database that can also be used by
         UserDatabaseRealm to authenticate users
    -->
    <Resource name="UserDatabase" auth="Container"
              type="org.apache.catalina.UserDatabase"
              description="User database that can be updated and saved"
              factory="org.apache.catalina.users.MemoryUserDatabaseFactory"
              pathname="conf/tomcat-users.xml" />
	
			  
    <Resource  
   	name="jdbc/testDataSource"                                  
   	auth="Container"                                            
   	type="javax.sql.DataSource"                                   
   	driverClassName="com.mysql.jdbc.Driver"                      
   	url="jdbc:mysql://localhost:3306/delta_server"                
   	username="test" password="test"                              
   	validationQuery="select count (*) from session_contexts"              
	/>
  </GlobalNamingResources>


web.xml configuration:

    <resource-ref> 
        <description> 
            Sample JNDI DataSource resource reference 
        </description> 
        <res-ref-name>jdbc/testDataSource</res-ref-name>                   
        <res-type>java.sql.DatSource</res-type>                           
        <res-auth>Container</res-auth>                                     
    </resource-ref> 

and access from jsp page:

    
            Context initialContext = new InitialContext();
            Context envContext = (Context) initialContext.lookup("java:/comp/env");
            conn = (Connection)  envContext.lookup("jdbc/testDataSource");



But unfortunately I get exception:

    javax.naming.NamingException: Cannot create resource instance
	org.apache.naming.factory.ResourceFactory.getObjectInstance(ResourceFactory.java:146)
	javax.naming.spi.NamingManager.getObjectInstance(NamingManager.java:321)
	org.apache.naming.NamingContext.lookup(NamingContext.java:826)
	org.apache.naming.NamingContext.lookup(NamingContext.java:145)
	org.apache.naming.NamingContext.lookup(NamingContext.java:814)
	org.apache.naming.NamingContext.lookup(NamingContext.java:159)
	org.apache.jsp.index_jsp._jspService(index_jsp.java:91)
	org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:433)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:389)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:333)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:722)


How can I fix it?
Thank you. 