Re: J2EE - entities - When do JPA entity units get saved into the database

Owen Jacobson <>
Mon, 28 Apr 2008 07:04:03 -0700 (PDT)
On Apr 28, 3:31 am, Taras_96 <> wrote:

i all,

I'm trying to gain a better understanding of some J2EE concepts, in
particular entities. AFAIK, if you make changes to an object using a
CMP Entity Bean, those changes are written to the database

Customer customer =
// ... obtain a remote
//reference to the bean
// get the customer's address
Address addr = customer.getAddress();
// change the zip code = "56777";
// update the customer's address
customer.setAddress(addr); <- the address field in the database has
now been updated

In 'Java EE with Glassfish application server', the author writes:

customer3 = entityManager.find(Customer.class, 4L);

What is the purpose of the 'persist' method, and how is it different
to the 'merge' method? Notice that in the EJB 2.1 code, I didn't have
to explicitly tell the code to update the DB, this was done

Furthermore, the book uses a Session Bean to implement a DAO to the
JPA unit:

public interface CustomerDao
public void saveCustomer(Customer customer);
public Customer getCustomer(Long customerId);
public void deleteCustomer(Customer customer);


what is the purpose of the 'saveCustomer' method? Do you have to
explicitly tell the code to save any changes made to objects that are
stored in a database?


Customer cust = dao.getCustomer(1);

I thought the point of CMP was so that you didn't have to explicitly
save objects; that any changes made to the objects in the code were
automatically reflected in the database representation of the object.
The 2.1 code automatically updates the appropriate field, whereas in
the 3.0 code you have to explicitly call save - in 3.0 do you have to
explicitly save changes to the database, whereas in 2.1 this was done
for you by using the Entity Bean?



It's unfortunate (but understandable) that the EJB 3 persistence spec
reused the term "entity", as JPA entities and EJB 2 Entity Beans have
almost nothing to do with one another beyond "they map to the

EJB 2 entity beans are full-blown remote objects; when you return an
entity bean from an EJB, the code receiving it may actually receive a
remote stub pointing to a server object. JPA entities are merely
serialized as-is and returned across remote calls. (For local calls,
neither EJB 2 entity beans nor JPA entities are remoted, so none of
this is relevant.)

Both JPA entities and EJB 2 entity beans will automatically persist
field changes back to the database if the changes occur within the
same transaction that the entity was loaded in AND if the object has
not passed across a remote method call (even within the same JVM).
EJB 2 entity beans will also automatically persist changes back even
after the transaction has completed or when referenced across a remote
interface, at the cost of making every method call on the entity a
remote method.

The EntityManager merge method takes a JPA entitiy that has been
allowed to escape its original context, either by being passed or
returned across a remote interface or by surviving past the end of the
transaction that originally loaded it, and "reattaches" it to the
database (in the process persisting changes from the entity into the
database and vice-versa). The persist method does what it says on the
tin: it stores the entity in the database as-is (and attaches it to
the database within the transaction).


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