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Re: [eclipselink-users] Caching and Relational Objects 
If you use EntityManager.find() it will get a cache hit.  If you use
EntityManager.createQuery().getSingleResult(), by default it will query the
database, unless you set the Query hint,
"eclipselink.cache-usage"="CheckCacheByPrimaryKey".

>> * What is the expected/accepted usage pattern with regards to cache, is
>> the use case of creating objects in memory with only primary key fields
>> invalid/incorrect?

It is invalid to create objects that are existing, especially with only an
id.  You can you the EntityManager.getReference() method to get an object
reference.

>> * Does eclipselink make use of the hashcode/equals implemented in domain
>> objects (TimesheetPeriod) for its caching. I am guessing not as eclipse
>> link seems to use primary key equality (via ClassDescriptor instances)
>> and not the hashcode/equals combination for its cache (based on the code
>> I stepped
through).

No. EclipseLink never uses the object's hashCode() or equals() in its
caching, or in general.




Bhaskar Maddala wrote:
> 
>>> The cache will cache anything you read, so if you read it again (by
> primary key) you avoid accessing the database, and (by query) avoid
> rebuilding the object and its relationships.
> 
> If I understood this right, there is no way to prevent the database query
> (as in the case of using get) when using the "Query" objects, the cache
> only
> avoid the translation of tabular data to objects, correct?
> 
> On the other questions, I must have done a real bad job of explaining ..
> :)
> Hopefully this will help (I am using eclipselink as my EJB3 persistence
> framework to a relational datastore)
> 
> Problem 1:
> I get NullPointerException in my application due to entities that
> eclipselink returns when I retrieve (get or Query) them. The entities
> returned from eclipselink are objects it has cached (I figured this out by
> stepping into the eclipselink code). The entities in the cache have null
> values for attributes/fields which I know are constrained by the database
> schema to be not null, leading me to not do a null check and ultimately to
> a
> NPE
> 
> I tried to figure out how the entities got into the cache (with a bad
> state)
> and tracked it down to the saving of another entity having a relation with
> the one in bad state
> 
> I have a One-To-Many relationship between TSDATES - RSRHOUR, represented
> earlier by TimesheetPeriod and ResourceHour respectively. Saving (in this
> case it is a create or presist from your response) ResourceHour resulted
> in
> TimesheetPeriod being added to the eclipselink cache.
> 
> My code that resulted in TimesheetPeriod being added to the cache (Note :
> Timesheet period instance is already in the database, when creating
> ResourceHour, I just need to provide a relational object reference as
> RSRCHOUR has a non null foreign key constraint)
> 
> TimesheetPeriod tp = new TimesheetPeriod();
> tp.setTimesheetPeriodId(Integer.valueOf(timesheetTask.getTsId())); // only
> setting the key field, ignoring other fields, as I am only using this for
> foreign key reference
> 
> ResourceHour hour = new ResourceHour();
> hour.setTimesheetPeriod(tp); // sets the TimesheetPeriod on ResourceHour
> m_hoursDao.save(hour);  // Create/Persist the resource hour
> 
> Starting with an empty cache for TimesheetPeriod prior to the persist call
> above, I end up with the following cache state after the persist
> 
> UnitOfWorkIdentityMap for: TimesheetPeriod
> Key: [10]    Identity Hash Code: 20015579    Object:
> TimesheetPeriod[TimesheetPeriodId: 10 StartDate: null EndDate: null
> 
> Now when I load the TimesheetPeriod with TimesheetPeriodId (primary key
> @Id)
> 10, eclipselink returns the object in cache, with StartDate and EndDate
> null. My solution to fixing this was to replace the 2 lines of code
> 
> TimesheetPeriod tp = new TimesheetPeriod();
> tp.setTimesheetPeriodId(Integer.valueOf(timesheetTask.getTsId())); // only
> setting the key field, ignoring other fields, as I am only using this for
> foreign key reference
> 
> with
> 
> TimesheetPeriod tp
> =m_timesheetPeriodDao.get(Integer.valueOf(timesheetTask.getTsId()));
> 
> this results in the object in the database being loaded into the cache.
> The
> rest of the code remains the same.
> 
> UnitOfWorkIdentityMap for: TimesheetPeriod
> Key: [10]    Identity Hash Code: 20015579    Object:
> TimesheetPeriod[TimesheetPeriodId: 10 StartDate: 01/01/09 EndDate:
> 01/31/09
> 
> 
> Now back to the questions
> * What is the expected/accepted usage pattern with regards to cache, is
> the
> use case of creating objects in memory with only primary key fields
> invalid/incorrect?
>    Should I be using
> 
>   TimesheetPeriod tp = new TimesheetPeriod();
>   tp.setTimesheetPeriodId(Integer.valueOf(timesheetTask.getTsId()));
>   ResourceHour hour = new ResourceHour();
>   hour.setTimesheetPeriod(tp);
>   m_hoursDao.save(hour);
> 
>   or
> 
>   TimesheetPeriod tp
> =m_timesheetPeriodDao.get(Integer.valueOf(timesheetTask.getTsId()));
>   ResourceHour hour = new ResourceHour();
>   hour.setTimesheetPeriod(tp);
>   m_hoursDao.save(hour);
> 
>   or something else.
> 
> * Does eclipselink make use of the hashcode/equals implemented in domain
> objects (TimesheetPeriod) for its caching. I am guessing not as eclipse
> link
> seems to use primary key equality (via ClassDescriptor instances) and not
> the hashcode/equals combination for its cache (based on the code I stepped
> through).
> 
> Thank you for the reply.
> 
> -Bhaskar
> 
> 


-----
---
http://wiki.eclipse.org/User:James.sutherland.oracle.com James Sutherland 
http://www.eclipse.org/eclipselink/
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