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From Dain Sundstrom <d...@iq80.com>
Subject Re: Can I reuse instances?
Date Mon, 09 Apr 2007 18:10:51 GMT
On Apr 8, 2007, at 1:56 PM, Craig L Russell wrote:

> Hi Dain,
>
> I haven't looked in detail at the life cycle of CMP beans in a  
> couple of years, but in general you can't simply keep the state of  
> the underlying Entities through the life cycle. CMP beans are  
> pooled and reused in transaction contexts and you have to load the  
> state at specific points in the life cycle.

It depends on the commit option the container is using.  In commit  
option A, you assume the cmp engine is the sole user of the database,  
so you don't need to load.  Normally people use commit option B where  
you keep instances activated with a specific primary key and reload  
the data in each tx.

> Using the primary key stored in the CMP bean to do em.find at the  
> appropriate time is the obvious way to take advantage of the em  
> second level cache. To the extent that this is not efficient, we  
> should fix it in the JPA layer not the CMP layer.

I would prefer to keep as much of the CMP stuff on the CMP side as  
possible so the JPA implementation can focus on JPA issues.   One of  
the assumption of JPA is that entities are light weight and cheap to  
create, so you take the safe route and construct a new one when every  
you need an instance.  In CMP the assumption is that entity instances  
are expensive to create, so less safe route and you pool them.   
Reusing instances is really a CMP problem, but I don't think it can  
be implemented without the help of the JPA engine.

-dain



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