How do I catch the constraint violation exception from EclipseLink?

Question

I am using EclipseLink in my web application, and I am having a hard time gracefully catching and handling Exceptions it generates. I see from this thread what seems to be a si

Answer 1

2019-12-18

As its a very well viewed question and I just had a very similar issue with EclipseLink, in a Maven multi module web application running on Weblogic 12c server and using JTA, I am going to post my solution here, hoping to save a couple hours for someone.

In the persistence.xml we are having:

< property name="eclipselink.persistence-context.flush-mode"
    value="commit" />

The REST resource class was marked with @Transactional, meaning that the transaction starts at the point when the request has been received by the related method of the resource class, and it ends when this method returns.
JTA used for managing the transactions.

Now, JTA commit time happens to occur AFTER the resource class's method returns (with a response to the REST client).

Which subsequently means that:

Even though you had a very proper setup to catch the Exception, you cannot, as exceptions like SQLIntegrityConstraintViolationException occur only AFTER your INSERT/UPDATE/DELETE query
--that has been sitting all this time in your JPA provider cache--,
now finally sent to the database.

Which happens just after the resource class's method returns, and at that point, all the exceptions has been skipped already.

Since no query sent == no exception occured at the time when the execution ran through the try{...}catch(Exception e){...} lines, you were not able to catch it,
but at the end, you will see the exception in the server's log.

Solution:
I had to manually call flush() on EntityManager to force flush and the exception to occur at the proper time and line (basically in the try block) to be able to catch it, handle it, and allow my REST method to return with my intended response.

The final caught exception in the log (I have masked some not related info):

javax.persistence.PersistenceException: Exception [EclipseLink-4002] (Eclipse Persistence Services - x.x.x.v00000000-0000000): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: java.sql.SQLIntegrityConstraintViolationException: ORA-00001: unique constraint (XXXXX.UNIQUE_KEY_NAME) violated

Related pseudo code:

    try {
            repository.update(entity);
            repository.getEntityManager().flush();
        } catch (Exception e ) {
            log.info(e.toString());  
            ...
        }