Definitely if you intend to use UserTransaction I would absolutely go
with a TRANSACTION scoped unit and then use @PersistenceContext
EntityManager. Once you call joinTransaction you end up paying the
cost of a JTA transaction but don't get any of the benefits. One of
those benefits would make it impossible for you to make the mistake
you made. Very good learning experience though, and you're lucky you
found this quickly!
In this particular code that you posted you're creating two
EntityManagers in the same JTA transaction and they are competing
against each other. As I mentioned, EntityManagers created from a
RESOURCE_LOCAL/EntityManagerFactory do not cooperate with anyone.
Each EntityManager represents a cache and has it's own non-shared
database connection, etc.
So here's what the implications of that are, for simplification
reasons we'll say the accountNumber variable is 12345:
- Method1: you create a private database Cache, we'll call it
PersistenceContext A (i.e. you create an EntityManager)
- Method1: a JTA transaction is begin via UserTransaction.begin();
- Method1: you call joinTransaction on PersistenceContext A
instructing it to flush it's cache when the JTA transaction ends.
- Method1: you ask PersistenceContext A to find Account 12345,
PersistenceContext A does yet have a copy of that date, thus it will
pull the data from the database and into the cache of
PersistenceContext A.
- Method1: updates to Account 12345 are done against the *cached
copy* in PersistenceContext A.
- Method2: you create a *second* private database Cache,
PersistenceContext B (i.e. you create an EntityManager)
- Method2: you call joinTransaction on PersistenceContext B
instructing it to flush it's cache when the JTA transaction ends.
- Method2: you ask PersistenceContext B to find Account 12345,
PersistenceContext B does yet have a copy of that date, thus it will
pull the data from the database and into the cache of
PersistenceContext B. You now have *two* separate and competing
copies of Account 1234 in the transaction, in in the cache of
PersistenceContext A and one in the cache of PersistenceContext B.
- Method2: updates to Account 12345 are done against the *cached
copy* in PersistenceContext B.
- Method1: more up to Account 12345 are done against the *cached
copy* in PersistenceContext A.
- Method1: the JTA transaction is committed and PersistenceContext A
and PersistenceContext B are each individually notified to flush their
caches.
At this point there's no guarantee who will win, A or B. But based on
your results it's pretty clear A got to the database before B,
therefore B was the last person to update the record. I.e.:
- PersistenceContext A flushed it's cache and executed "ACCOUNT
12345 BALANCE = 7000"
- Then PersistenceContext B flushed it's cache and executed "ACCOUNT
12345 BALANCE = 6000"
So the moral of this story is that the concept of a database cache is
an extremely important concept to be aware of. Without a copy of the
data in memory (i.e. a cache) when you call account.getBalance() the
persistence provider would have to go read the value from the database
every time. This would obviously be a big waste of resources. The
other side of having a cache is that when you call
account.setBalance(5000) it also doesn't hit the database (usually).
When the cache is "flushed" the data in it is sent to the database via
as many SQL updates, inserts and deletes as are required. That is the
basics of java persistence of any kind all wrapped in a nutshell. If
you can understand that, you're good to go in nearly any persistence
technology java has to offer.
With this in mind, the explanation I posted before will likely take on
a whole new light. Here's a slightly reworded/abbreviated version:
A RESOURCE_LOCAL persistence unit: The implication of the
RESOURCE_LOCAL is that the EntityManager instance you create from the
factory *is* the entire persistence context, cache and all. For
illustrative purposes it's safe to imagine big "cache" hashmap as a
field inside the EntityManager instance you created. If you want
someone else to do persistent operations with the same persistence
context you need to find a way to get them the exact EntityManager
instance you created (not a second EntityManager they create).
A TRANSACTION persistence unit: With TRANSACTION, everyone in the
transaction shares the same EntityMangaer. As a vendor what we do
underneath the covers is give you an EntityManager wrapper that points
to nothing. Then when a transaction is started (via UserTransaction
or a container transaction) we use the EntityManagerFactory to create
an EntityManager instance and make all the wrappers point to it. When
the transaction commits, the cache associated with the internal
EntityManager instance is flushed and the EntityManager instance is
discarded and all EntityManager wrappers again point to nothing. This
is essentially why you need a transaction in progress to use a
EntityManager with JTA scope and TRANSACTION unit, because the
EntityManager you hold onto doesn't represent a real EntityManager
rather just a "fake" EntityManager that directs all calls you make on
it to the container's internal EntityManager that is associated with
the JTA transaction you're in. If there is no JTA transaction in
progress, there is no internal EntityManager and the EnityManager
wrappers therefore point nowhere.
-David
On Jun 12, 2008, at 5:02 AM, Phani Madgula wrote:
Thanks for providing useful. Now the intricacies are becoming clear
indeed !!
I am trying to guess what could/should be the outcome of the
following scenario.
***************
@PersistenceUnit(unitName="AccountUnit")
EntityManagerFactory emf;
[..]
private void method1(int accountNumber, PrintWriter out)
throws Exception{
try{
EntityManager em = emf.createEntityManager();
Context ctx = new InitialContext();
UserTransaction ut =
(UserTransaction)ctx.lookup("java:comp/UserTransaction");
ut.begin();
em.joinTransaction();
Account account = em.find(Account.class, accountNumber);
account.setBalance(5000);
method2(accountNumber,out);
account.setBalance(7000); (this update is missing in the
table)
ut.commit();
}catch(Exception e){
throw e;
}
}
private void method2(int accountNumber, PrintWriter out)throws
Exception {
try{
EntityManager em = emf.createEntityManager();
em.joinTransaction();
Account account = em.find(Account.class, accountNumber);
account.setBalance(6000);
}catch(Exception e){
throw e;
}
}
********************************
What I observed from the above code is account.setBalance(7000);
(this update is missing in the table). The value of balance column
will be 6000. The last update in the method (to 7000) misses.
Thanks
Phani
On Thu, Jun 12, 2008 at 7:44 AM, David Blevins
<[EMAIL PROTECTED]> wrote:
On Jun 9, 2008, at 1:43 AM, Phani Madgula wrote:
Hi,
I have tried to play with <jta-datasource> and <non-jta-datasource>
as follows.
I have the following peristence.xml in a web application.
[...]
<persistence-unit name="Tutorial" transaction-type="RESOURCE_LOCAL">
[...]
In the servlet, I have the following code
[...]
@PersistenceContext(unitName="Tutorial")
private EntityManager em;
.....
....
UserTransaction ut;
try{
Context ctx = new InitialContext();
//ut = (UserTransaction)ctx.lookup("java:comp/
UserTransaction");
//ut.begin();
//Uncomment EntityTransaction
EntityTransaction et = em.getTransaction();
et.begin();
Book book = new Book("DDDDD","John",
1.0,"Joe","1934834-23823","Phani");
em.persist(book);
et.commit();
//ut.commit();
}catch(Exception e){
e.printStackTrace();
throw new ServletException (e);
}
***********************
I get the following error
***********************
java.lang.IllegalStateException : You can not call getTransaction on
a container managed EntityManager
[...]
Please note that I am using "RESOURCE_LOCAL" persistence unit.
You're in the right direction. Try modifying this part of your
servlet as follows:
[cut this part]
@PersistenceContext(unitName="Tutorial")
private EntityManager em;
[redo as]
@PersistenceUnit(unitName="Tutorial")
private EntityManagerFactory emf;
The use of RESOURCE_LOCAL definitely requires you to use the
EntityTransaction API as you were attempting to do. It also
requires you to use the EntityManagerFactory to get your
EntityManager. The implication of the RESOURCE_LOCAL is that the
EntityManager instance you create from the factory *is* the entire
persistence context, cache and all (EntityManger instance ==
PersistenceContext, terminology wise). For illustrative purposes
it's safe to imagine big "cache" hashmap as a field inside the
EntityManager instance you created. It's not entirely how most
providers do it, but the technical ramifications are the same. One
of the ramifications is that if you want someone else to do
persistent operations with the same persistence context you need to
find a way to get them the EntityManager instance you created. From
a spec perspective we (EJB 3.0 spec hat on) could have allowed you
to reference a RESOURCE_LOCAL unit via an EntityManager reference,
but the trick is that each reference would wind up creating a new
EntityManager instance, cache and all, and you could make a really
big mess; seemed better to just force you to create the instance
yourself and share it however you feel best so that there could be
no misunderstandings.
With a TRANSACTION unit you are in fact required to lookup or have
injected the EntityManager and are not allowed to use the
EntityManagerFactory or the EntityTransaction API. As a vendor what
we do underneath the covers is give you an EntityManager wrapper
that points to nothing. Then when a transaction is started (via
UserTransaction or a container transaction) we use the
EntityManagerFactory to create an EntityManager instance and make
all the wrappers point to it. When the transaction commits, the
internal EntityManager instance is discarded (cache goes bye-bye)
and wrappers again point to nothing. This is essentially why you
need a transaction in progress to use a EntityManager with JTA scope
and TRANSACTION unit. With an EXTENDED PersistenceContext (again,
available only with a TRANSACTION unit) things work pretty much the
same except that the internal EntityManager instance is *not*
discarded and the end of transactions (cache stays in memory and can
be used again in another transaction), instead it lives as long as
the Stateful session bean holding it.
My questions are
1. Should we always use UserTransaction object irrespective of
<jta-datasource> or <non-jta-datasource> in JEE environment??
2. How is the use of <non-jta-datsource> different from the use
<jta-datasource> in JEE environment??
3. In EJBs, when ContainerManaged Transactions are used, there is no
need to use JTA. Is that correct??
4. Only in J2SE environments, we can use EntityTransaction object. is
that correct??
Hopefully the above explanation answers these questions (good
questions, BTW). If anything is still unclear, definitely don't
hesitate to ask for more information or for something to be simply
reworded (can be hard trying to find the right way to explain these
things).
-David
On 5/28/08, David Jencks <[EMAIL PROTECTED]> wrote:
1. I hope you named the file openejb-jar.xml
2. jta-datasource and non-jta-datasource have to contain the names of
the datasources in the geronimo plan used for the pool, NOT some jndi
name you might also map them to. The datasources don't need to be
bound in jndi in order to be used for jpa
3. The non-jta-datasource must be a datasource that really has no
transaction support, using the <no-transaction/> element instead of
<local-transaction/> or <xa-transaction>... in the connector plan.
With derby I find it necessary to have a non-jta-datasource if any ddl
is needed or if openjpa is generating the primary keys. I don't know
if you can get away without one for other databases. If you want to
experiment, leave out the non-jta-datasource rather than duplicating
the jta-datasource contents.
hope this helps
david jencks
On May 27, 2008, at 4:09 PM, zeros wrote:
Good evening:
I'm newbie with EJB3.0. I want to configure the persistence.xml
to have
entities managed with JPA. I have the next configuration files:
OPENJPA-JAR.XML
<openejb-jar xmlns="http://www.openejb.org/xml/ns/openejb-jar-2.1";
xmlns:nam="http://geronimo.apache.org/xml/ns/naming-1.1";
xmlns:pkgen="http://www.openejb.org/xml/ns/pkgen-2.0";
xmlns:sec="http://geronimo.apache.org/xml/ns/security-1.1";
xmlns:sys="http://geronimo.apache.org/xml/ns/deployment-1.2";>
<sys:environment>
<sys:moduleId>
<sys:groupId>o2o.marketing</sys:groupId>
<sys:artifactId>EJB</sys:artifactId>
<sys:version>1.0.8</sys:version>
<sys:type>jar</sys:type>
</sys:moduleId>
<sys:dependencies>
<sys:dependency>
<sys:groupId>console.dbpool</sys:groupId>
<sys:artifactId>marketing</sys:artifactId>
<sys:version>1.0</sys:version>
<sys:type>rar</sys:type>
</sys:dependency>
</sys:dependencies>
</sys:environment>
</openejb-jar>
And I have also persistence.xml
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence
"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd";>
<persistence-unit name="marketing">
<description>Entity Beans for User</description>
<provider>org.apache.openjpa.persistence.PersistenceProviderImpl</
provider>
<jta-data-source>java:comp/env/marketing</jta-data-source>
<non-jta-data-source>java:comp/env/marketing</non-jta-data-
source>
<mapping-file>META-INF/orm.xml</mapping-file>
<properties />
</persistence-unit>
</persistence>
The error which I'm having is the next one: Unable to resolve
reference
"JtaDataSourceWrapper" and Unable to resolve reference
"NonJtaDataSourceWrapper", which are basically the same error.
I think this is produced because I'm not mapping the Datasocurce to
the
DBPool. Geronimo returns to me examples to do it for web, but no
example to
do it for an EJB.
Please could you help me?
WATCH OUT! I'm talking about entities and EJB3.0, not entiti beans and
EJB2.0
Thanks in advance
SERGIO
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