tomcat7 - how to use datasourceRealm Java -

i'm using datasourcerealm code below:

context initcontext = new initialcontext(); datasourcerealm dss = new datasourcerealm();  datasourcerealm ds =  (datasourcerealm) initcontext.lookup("java:/comp/env/infofeedproxydatabase"); // i'm getting connot cast datasource datasourcerealm 

here's configuration in tomcat:

<watchedresource>web-inf/web.xml</watchedresource> <resourcelink type="javax.sql.datasource"               name="infofeedproxydatabase"               factory="org.apache.tomcat.jdbc.pool.datasourcefactory"               driverclassname="com.mysql.jdbc.driver"               url="jdbc:mysql://localhost:3306/ppe3-1"               username="root"               password="" /> <realm classname="org.apache.catalina.realm.datasourcerealm"        datasourcename="infofeedproxydatabase"        localdatasource="true"        digest="md5"        rolenamecol="role"        usercredcol="password"        usernamecol="login"        userroletable="roles"        usertable="users" /> 

i've tried code:

 datasourcerealm dsr = new datasourcerealm();   dsr.start(); 

may know how can use datasourcerealm object in java?