Configuring jdbcRealm in context.xml

<Context antiJARLocking="true" path="/NCellLive">
<Resource 
    name="jdbc/Gis_WebApp" 
    auth="Container"
    type="javax.sql.DataSource" 
    username="uname" 
    password="pword"
    driverClassName="com.microsoft.sqlserver.jdbc.SQLServerDriver" 
    url="jdbc:sqlserver://127.0.0.1:1433;DatabaseName=Gis_WebApp;SelectMethod=cursor;"
    maxActive="8" 
/>
    <Realm className="org.apache.catalina.realm.JDBCRealm" debug="99"
    driverName="com.microsoft.sqlserver.jdbc.SQLServerDriver"
    connectionURL="jdbc:sqlserver://127.0.0.1:1433;DatabaseName=Gis_WebApp;"
    connectionName="uname" connectionPassword="pword"
    userTable="app_user" userNameCol="username" userCredCol="password"
    userRoleTable="app_user_group" roleNameCol="groupname"/>  

</Context>

Now all data base connection is working fine but after I added realm following errors are thrown I already have sqljdbc4.jar added in library.

Jan 31, 2012 11:28:24 AM org.apache.catalina.realm.JDBCRealm authenticate
SEVERE: Exception performing authentication
java.sql.SQLException: com.microsoft.sqlserver.jdbc.SQLServerDriver
    at org.apache.catalina.realm.JDBCRealm.open(JDBCRealm.java:701)
    at org.apache.catalina.realm.JDBCRealm.authenticate(JDBCRealm.java:352)
    at org.apache.catalina.authenticator.FormAuthenticator.authenticate(FormAuthenticator.java:295)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:450)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:927)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:987)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:579)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:1805)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.ClassNotFoundException: com.microsoft.sqlserver.jdbc.SQLServerDriver

What might be the problem?

Solution

Caused by: java.lang.ClassNotFoundException: com.microsoft.sqlserver.jdbc.SQLServerDriver

tells you that the sql library cannot be found in your classpath.
It needs to be in your tomcat/lib folder.

From the doc:

To set up Tomcat to use JDBCRealm, you will need to follow these steps:

If you have not yet done so, create tables and columns in your database that conform to the requirements described above.

Configure a database username and password for use by Tomcat, that has at least read only access to the tables described above. (Tomcat will never attempt to write to these tables.)

Place a copy of the JDBC driver you will be using inside the $CATALINA_HOME/lib directory. Note that only JAR files are recognized!

Set up a element, as described below, in your $CATALINA_BASE/conf/server.xml file.

Restart Tomcat 6 if it is already running.