CAUTION!
Using @Modifying(clearAutomatically=true) will drop any pending updates on the managed entities in the persistence context spring states the following :
Doing so triggers the query annotated to the method as an updating
query instead of selecting one. As the EntityManager might contain
outdated entities after the execution of the modifying query, we do
not automatically clear it (see the JavaDoc of EntityManager.clear()
for details), since this effectively drops all non-flushed changes
still pending in the EntityManager. If you wish the EntityManager to
be cleared automatically, you can set the @Modifying annotation’s
clearAutomatically attribute to true.
Fortunately, starting from Spring Boot 2.0.4.RELEASE Spring Data added flushAutomatically flag (https://jira.spring.io/browse/DATAJPA-806) to auto flush any managed entities on the persistence context before executing the modifying query check reference https://docs.spring.io/spring-data/jpa/docs/2.0.4.RELEASE/api/org/springframework/data/jpa/repository/Modifying.html#flushAutomatically
So the safest way to use @Modifying is :
@Modifying(clearAutomatically=true, flushAutomatically=true)
What happens if we don't use those two flags??
Consider the following code :
repo {
@Modifying
@Query("delete User u where u.active=0")
public void deleteInActiveUsers();
}
Scenario 1 why flushAutomatically
service {
User johnUser = userRepo.findById(1); // store in first level cache
johnUser.setActive(false);
repo.save(johnUser);
repo.deleteInActiveUsers();// BAM it won't delete JOHN right away
// JOHN still exist since john with active being false was not
// flushed into the database when @Modifying kicks in
// so imagine if after `deleteInActiveUsers` line you called a native
// query or started a new transaction, both cases john
// was not deleted so it can lead to faulty business logic
}
Scenario 2 why clearAutomatically
In following consider johnUser.active is false already
service {
User johnUser = userRepo.findById(1); // store in first level cache
repo.deleteInActiveUsers(); // you think that john is deleted now
System.out.println(userRepo.findById(1).isPresent()) // TRUE!!!
System.out.println(userRepo.count()) // 1 !!!
// JOHN still exists since in this transaction persistence context
// John's object was not cleared upon @Modifying query execution,
// John's object will still be fetched from 1st level cache
// `clearAutomatically` takes care of doing the
// clear part on the objects being modified for current
// transaction persistence context
}
So if - in the same transaction - you are playing with modified objects before or after the line which does @Modifying, then use clearAutomatically & flushAutomatically if not then you can skip using these flags
BTW this is another reason why you should always put the @Transactional annotation on service layer, so that you only can have one persistence context for all your managed entities in the same transaction.
Since persistence context is bounded to hibernate session, you need to know that a session can contain couple of transactions see this answer for more info https://stackoverflow.com/a/5409180/1460591
The way spring data works is that it joins the transactions together (known as Transaction Propagation) into one transaction (default propagation (REQUIRED)) see this answer for more info https://stackoverflow.com/a/25710391/1460591
To connect things together if you have multiple isolated transactions (e.g not having a transactional annotation on the service) hence you would have multiple sessions following the way spring data works hence you have multiple persistence contexts (aka 1st level cache) that means you might delete/modify an entity in a persistence context even with using flushAutomatically the same deleted/modified entity might be fetched and cached in another transaction's persistence context already, That would cause wrong business decisions due to wrong or un-synced data.
