update using JPA

[Pradyut Bhattacharya]

Hi
I m using glassfish v2 and persistence in a web application.

Well i want to ask:-
How to update using em.createQuery in JTA

in detail:-
I m calling persistence codes using a normal java class file inside a web Application

I can select easily using this code: -

@PersistenceUnit
public EntityManagerFactory emf;
EntityManager em;
public List fname (String id) {
String fname = null;
List persons = null;
//private PersistenceManagerFactory persistenceManagerFactory;
try {
emf = Persistence.createEntityManagerFactory("WebApplicationSecurityPU");
em = emf.createEntityManager();
persons = em.createQuery("select r from Roleuser r").getResultList();
int i=0;
for (i=0;i<persons.size(); i++)
System.out.println("Testing n "+ i +" " + persons.get(i));
} catch(Exception e) {
System.out.println("" + e);
}
finally {
if(em != null) {
em.close();
}
}
return persons;
}

I want to update using JTA as the persistence.xml file has
transaction-type="JTA"

When i try using update using this code i get a nullPointerException without any traces in the log

@PersistenceUnit
public EntityManagerFactory emf;
EntityManager em;
Context context;
@Resource
private UserTransaction utx;
public List fname (String id) {
String fname = null;
List persons = null;
try {
emf = Persistence.createEntityManagerFactory("WebApplicationSecurityPU");
utx.begin();
em = emf.createEntityManager();
int m = em.createQuery("update Roleuser r set r.firstName = 'Jignesh I' where r.userID=9").executeUpdate();
utx.commit();
} catch(Exception e) {
System.out.println("" + e);
}
finally {
if(em != null) {
em.close();
}
}
return persons;
}

Any help
Thanks
Pradyut>