EntityManager refresh() method does not refresh but resets-CodePudding

The em.refresh(person) in the following code does not work. Rather than refreshing the person with a fresh value from database, it resets (undo or discards) the changes made in the cache. I am not able to understand why?

em.getTransaction().begin();
        
Person person = em.find(Person.class, 2L); //Person[id=2L, age=23]
person.setAge(24); 
System.out.println(person.getAge()); //it prints 24
        
//Person with id=2 in database gets modified concurrently somehow,
//its age becomes 25 in PERSON table (by an SQL update for example "UPDATE person SET age=25 WHERE id=2")

em.refresh(person); // attempts to load fresh value from database with a SELECT...
System.out.println(person.getAge()); //it prints 23, rather than 25, why?
        
em.getTransaction().commit();
em.close();

Could someone help me understand this behavior by the refresh() method of EntityManager?

CodePudding user response：

If you want to see changes made from other transactions while inside a transaction you need to change the isolation level to READ_COMMITTED

<property name="hibernate.connection.isolation">TRANSACTION_READ_COMMITTED</property>

A few definitions to clarify the discussion:

Repeatable read: essentially means that within a transaction the database will see the same value, unless the data was modified within that transaction
Hibernate flush: the modifications made in the session (e.g. person.setAge(24);) are not visible by the database until they are flushed. A flush occurs when calling em.flush, committing or, typically, when executing a query such as select * from Person where name=... but not when calling refresh()
Hibernate refresh: reading the data from the database and updating the session/1st level cache with that data.

So basically:

You're modifying the age by calling setAge() but that change is not flushed, hence not visible by the database
You're updating from another session but that change is not visible either because the transactions are isolated (unless using READ_COMMITTED)
When refresh is called the database is not aware that setAge() was called and it isolates the update from another transaction, so it shows 23