Couple of clarifications -
A lazily loaded FIELD ACCESS collection is a null value when initially
accessed if the Collection is EMPTY (I said "null" incorrectly below).
The test I have shows this behavior for a newly persisted Entity during
the same transaction where em.persist(entity) is called. This is with a
LAZY loaded collection.
On 4/9/2009 9:26 AM, Paul Copeland wrote:
Hi Craig -
My experience is not what you are describing. A lazily loaded FIELD
ACCESS collection is a null value when initially accessed if the
Collection is null (possibly a PROPERTY ACCESS collection behaves
differently as mentioned by Pinaki , I haven't tested that).
To repeat what is below -
getMyPcList()
returns null if the Collection is empty unless you initialize the
value with "new ArrayList()". This is what my testing shows with
1.2.1 - I wish it weren't this way since that might it make it
possible to use the Collections.unmodifiedList() idiom (as it is that
idiom has unreliable behavior). If the experts are pretty sure that
I am wrong about this then I definitely want to investigate it
further. I'd like to hear more.
I don't think you have given a reason to require initializing the
Collection at construction time or at first access -- there are
reasonable aesthetic and performance arguments either way.
- Paul
On 4/9/2009 7:01 AM, Craig L Russell wrote:
Hi Paul,
I like to think of entities as POJOs first, so I can test them
without requiring them to be persistent. So if you want code to be
able to add elements to collections, the collections must not be null.
If you construct the field as null and then "lazily" instantiate an
empty collection, then anyway you end up with an empty collection the
first time you access the field. And constructing an empty collection
should not be even a blip on your performance metric.
Considering everything, I still recommend that you instantiate an
empty collection when you construct an entity.
Craig
On Apr 8, 2009, at 10:21 AM, Paul Copeland wrote:
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
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!
