hibernate concurrency threading issue

we are using spring/hibernate for our application.. i have a cron job which picks new comments to update the parentage relation where a comment can be parented by another comments.. the cron picks the comments in chunks.. my issue is suppose i have 10 comments.. 1,2,3...10

the cron will process comments in chunks

thread-1 (1..5)

thread-2 (6-10)

// now in hibernate I'm using 
@PersistenceContext(name = "pu1", type = PersistenceContextType.TRANSACTION)
protected EntityManager em;

so thread-1 reads from parent table to check if there is any parents for the current chunk of comments.. then it inserts default values to parent table.

the issues is what thread-1 inserts into database parent table is not visible to thread-2 and vice versa.

// we are using,, but this is not working
commentsService.getEntityManger().flush();

// the only way to see the changes either by adding 
commentsService.getEntityManger().getTransaction().commit() // this will throw exception (transaction is not active)

// OR by adding 
Thread.sleep(3000); //before reading from parent table.

// from logs "select * from parent" for each thread.
// by this time comment 648 was inserted to parent 
2015-02-19 04:54:09,289 DEBUG [pool-2-thread-1] (CommentsServiceImpl.java:679) - Parent Table:: 23424977 for Comment 648
2015-02-19 04:54:09,289 DEBUG [pool-2-thread-1] (CommentsServiceImpl.java:679) - Parent Table:: comment_id: 648 for Comment 648

// by this time comment 649 was inserted to parent
// comment number 649 is inserted to parent table, but thread-2 can't see comment number 648 which is supposed to be available in parent table
2015-02-19 04:54:09,292 DEBUG [pool-2-thread-2] (CommentsServiceImpl.java:679) - Parent Table:: comment_id: 23424977 for Comment 649
2015-02-19 04:54:09,292 DEBUG [pool-2-thread-2] (CommentsServiceImpl.java:679) - Parent Table:: comment_id: 649 for Comment 649