Java EE (Java Enterprise Edition) General Discussion

I use tomcat 4.1.12 server.
I write simple app, that in servlet creates UserTransaction object with context.lookup(java:comp/UserTransaction),
but it gives error msg like:

Cannot create resource instance at org.apache.naming.factory.TransactionFactory.getObjectInstance(TransactionFactory.java:157)
at javax.naming.spi.NamingManager.getObjectInstance NamingManager.java:304) etc.

Do I have to configure something in web.xml or server.xml to create this transaction object?
Does anybody know why this code doesn't work?
Here is a sample code:

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Context ctx = null;
UserTransaction ut = null;
try {
ctx = new InitialContext();
ut=(UserTransaction) ctx.lookup("java:comp/UserTransaction");
ut.begin(); //start transaction
//do something
ut.commit();
}catch (NamingException ex) {
ex.printStackTrace();
} catch (Exception se) {
se.printStackTrace();
}
}