You mean a client application that is talking to an
EJB? You get the UserTransaction from JNDI:
Context jndiCtx = new InitialContext();
// you may need properties for the InitialContext
UserTransaction tx = (UserTransaction)
jndiCtx.lookup("java:comp/UserTransaction");
tx.begin();
// ...
tx.commit();
// or tx.rollback();
If you are trying to get the UserTransaction from within a CMP bean, you can not - Entity beans should have declarative transactions (set in deployment descriptor).