Re: ORM or JDBC?
Arved Sandstrom wrote:
One exercise I show newcomers to JPA is to persist some entities, then
deliberately detach them. Still in the same transaction I show them the
"contains" method to find out what's managed and what's not, and when.
While the new entities are _unmanaged_, the exercise continues along the
lines of making changes to the new entities. Finally there is a merge,
which in the context of JTA leads to a final commit. The tyros see that
changes made to _unmanaged_ entities are actually reflected in the SQL.
I've frequently encountered programmers who have used JPA for some
years, who never realized that changes to unmanaged JPA entities are
every bit as valid as changes to managed entities, provided that the
entities get merged at some point. You see this thinking too when you
meet programmers, and there's no shortage of them, who think that
merge==save.
This is one of the reasons - there are others - why I no longer think
JPA entities should be freely used in presentation and business logic.
Not so much because of inserts and deletes, but because of uncontrolled
updates.
Again, your point is valid.
I have a terminology quibble, though it is potentially at the root of your
newbies' trouble with detached entities. You aren't making changes to
unmanaged entities that get reflected in the SQL. You did make changes to
unmanaged entities, but those changes didn't reflect in the SQL until they
were managed again.
This is actually the huge strength of the merge. It's a shame that it turns
into a problem.
That's by the wayside, though, to the primary point that you don't want
unmanaged entities to touch an EntityManager outside the data layer, nor that
letting even unmanaged entities out of their playpen is dangerous.
The key point is the one you made upthread: JPA entities are not POJOs. That
is their strength, of course, but with great power comes great responsibility.
Maybe I just like JPA so much because I grok the mindset and tend to avoid the
kinds of mistakes you describe. I'm far from expert with it, but I don't come
close to the kinds of problems that ORMs are renowned for.
And I *hated* ORMs before JPA came along.
Still do - hate the pre-JPA ORMs that is.
--
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg