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From Paul Copeland <t...@jotobjects.com>
Subject Re: Does OpenJPA replace Collections?
Date Wed, 08 Apr 2009 16:43:01 GMT
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.Russell@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
>   


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