Pinaki -
I tried your suggestion of not initializing the value of myPcList and I
get a null pointer exception when adding to an empty list.
I noticed your example was for Property access and Russell (and I) were
talking about Field access. Do you agree that it is necessary to
initialize an empty list when using Field access?
On Craig's advice to always construct a new ArrayList(), why is that
necessary instead of just constructing it in the getter when it tests to
null? Otherwise you are constructing an ArrayList that is unnecessary
when the List is NOT empty (usually) and also unnecessary in the case of
LAZY loading if the List is never accessed (perhaps also a frequent
case). In some applications you might create lots of these objects and
normal optimization is to avoid calling constructors unnecessarily.
Just want to be clear about whether it is necessary.
- Paul
On 4/8/2009 9:43 AM, Paul Copeland wrote:
Thanks Pinaki -
I think you are saying that at some point the proxy object does
replace the local List. Is that right?
I have seen that model - if (myPcList == null) myPcList = new
ArrayList() - in various examples (not sure where now). Thanks for
clearing that up. But then Craig Russell contradicts you in his reply
(below) where he recommends always initializing the Collection in the
constructor (which seems like a performance anti-pattern of wasted
constructor calls since usually it will be replaced by the proxy).
Are you and Craig saying opposite things here?
In my testing when the List is empty - (myPcList == null) - does
indeed evaluate to true.
getMyPcList().add(new MyPcObject())
Therefore I thought the above would cause a null pointer exception
when the List is empty. You say that won't happen so I'll give it a try!
- Paul
On 4/8/2009 3:16 AM, Pinaki Poddar wrote:
Hi,
According to JPA spec:
"If there are no associated entities for a multi-valued relationship
of an entity fetched from the database,
the persistence provider is responsible for returning an empty
collection as the value of the relationship."
That is what OpenJPA does. So the application do not need to return
an empty list for a null (initialized) list.
OpenJPA proxies all returned collections. So application code can
simply do the following
// In the domain class
private List<MyPcObject> myPcList = null; // never explictly initialized
@OneToMany (mappedBy="ownerSide", fetch=FetchType.LAZY,
cascade=CascadeType.PERSIST)
public List<Promotion> getMyPcList() {
return myPcList; // return as it is
}
// In the application
List<Promotion> list = owner.getMyPcList();
assertNotNull(list);
assertTrue(java.util.List.class.isInstance(list));
assertNotSame(java.util.ArrayList.class, list.getClass());
list.add(new MyPcObject());
owner.setMyPcList(list);
On Apr 7, 2009, at 11:10 PM, Paul Copeland wrote:
Can OpenJPA replace a Collection when it is loaded?
With the code below when the list is initially empty you need to
create a List (ArrayList) so you can add elements to it. When I
persisted new objects on the ManyToOne side and added them to the
List that worked. But the first time the List was loaded it seemed
to replace my ArrayList with the newly loaded data and made an
older reference to the ArrayList stale (no longer updated when
more elements were added to myPcList). This was all in one
transaction.
So now I wonder if the initial null List is a special case or if
OpenJPA might replace the Collection anytime it decides to load it
again. Anyone know the answer?
If the list is persistent and the class is enhanced, the collection
will always reflect what's in the database.
If I don't create an initial ArrayList how can I add elements when
the List is empty?
I'd recommend always having a non-empty list. Initialize it in the
constructor to an empty list and don't check it after that.
Here's what it would look like:
@OneToMany (mappedBy="ownerSide", fetch=FetchType.LAZY,
cascade=CascadeType.PERSIST)
private List<MyPcObject> myPcList = new ArrayList<MyPcObject>();
List<Promotion> getMyPcList()
{
return myPcList;
}
Craig
Craig L Russell
Architect, Sun Java Enterprise System http://db.apache.org/jdo
408 276-5638 mailto:craig.russ...@sun.com
P.S. A good JDO? O, Gasp!
-----
Pinaki Poddar http://ppoddar.blogspot.com/
http://www.linkedin.com/in/pinakipoddar
OpenJPA PMC Member/Committer
JPA Expert Group Member
