It is the JPA second level cache. This should clarify -
http://weblogs.java.net/blog/guruwons/archive/2006/09/understanding_t.html
On 2/23/2011 3:45 PM, emiddio-frontier wrote:
> i am learning JPA -- an example from Core JavaServer Faces 3rd
> edition, with JPA2.0 examples
> is not reading the database before each update -- it does the first time.
>
> a 2nd app using JDBC from the same book -- does read the database each
> time the query
> is submitted.
>
> both web applications do the same thing functionally
>
> there are 4 versions of the Application in the book, jdbc, jpa, jpa
> with stateless session bean, jpa
> with stateful session bean.
>
> all 4 web apps read a row from the db selected by "user", read a
> "password" field and if the password
> check passes, update the "login count" field.
>
> when running any of the 4 web apps -- while having different apps open
> in different web browser tabs,
> each successful login increments the loginCount column field-- as it
> should. i use netbeans and sql queries
> to see the database values after each web page click. using Derby with
> netbeans.
>
> But only the jdbc app will always "see" the updated loginCount field
> before it updates it -- while the JPA apps
> only see "their" last updated value -- it seems JPA is using some kind
> of cache and not reading the db when told
> to with the query command -- as if assuming only1 app (itself) is
> using the db.
>
> again -- i am learning but have been unable to find any info in books
> i've been studying addressing
> this particular issue. -- Core JavaServer Faces 3rd edition, Beginning
> Java EE6 Platform with Glassfish V3, or
> The Java EE6 Tutorial Basic Concepts 4th edition.
>
> code that does update in JPA snipped below
>
> public void doLogin() throws NotSupportedException, SystemException,
> RollbackException, HeuristicMixedException,
> HeuristicRollbackException {
> EntityManager em = emf.createEntityManager();
> try {
> utx.begin();
> em.joinTransaction();
> boolean committed = false;
> try {
> Query query = em.createQuery(
> "SELECT c FROM Credentials c WHERE c.username
> = :username").
> setParameter("username", name);
> @SuppressWarnings("unchecked")
> List<Credentials> result = query.getResultList();
>
> if (result.size() == 1) {
> Credentials c = result.get(0);
> if (c.getPasswd().equals(password)) {
> loggedIn = true;
> count = c.incrementLoginCount();
> //geb
> em.merge(c);
> }
> }
> utx.commit();
> committed = true;
> } finally {
> if (!committed) {
> utx.rollback();
> }
> }
> } finally {
> em.close();
> }
> }
>
> and
>
> public int incrementLoginCount() {
> loginCount++;
> return loginCount;
> }
> contained in the Entity class Credentials.
>
> can someone help educate me on what i'm not understanding here.
>
> thanks
>
> gary
>
>
>