java-hibernate: What is the difference between merge() and update() in hibernate?

update() and merge() both methods are used to convert the object which is in detached state into persistence state in hibernate. But there is a little difference. Let us see which method will be used in what situation.

Update():- update() is used to save the data when you are sure that the session does not contains an already persistent instance with the same identifier.

Merge():- merge() is used when you want to save your modifications at any time with out knowing about the state of a session.


Let Us Consider an Example:
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1. ------
2. -----
3. SessionFactory factory = cfg.buildSessionFactory();
4. Session session1 = factory.openSession();
5.
6. Student s1 = null;
7. Object o = session1.get(Student.class, new Integer(101));
8. s1 = (Student)o;
9. session1.close();
10.
11. s1.setMarks(97);
12.
13. Session session2 = factory.openSession();
14. Student s2 = null;
15. Object o1 = session2.get(Student.class, new Integer(101));
16. s2 = (Student)o1;
17. Transaction tx=session2.beginTransaction();
18.
19. session2.merge(s1);


Explanation:
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See from line numbers 6 to 9, we just loaded one object s1 into session1 cache and closed session1 at line number 9, so object s1 in the session1 cache will be destroyed as session1 cache will expires when ever we say session1.close()

Now s1 object will be in some RAM location, not in the session1 cache, here s1 is in detached state, and at line number 11 we modified that detached object s1, now if we call update() method then hibernate will throws an error, because we can update the object in the session only.

So we opened another session [session2] at line number 13, and again loaded the same student object from the database, but with name s2.

so in this session2, we called session2.merge(s1); now into s2 object s1 changes will be merged and saved into the database.

we hope you are clear about it, actually update and merge methods will come into significance when ever we loaded the same object again and again into the database, like above.

Update method will just updates the record and if record doesn´t exist in the DB table then it will through the exception. But merge method will not through the exception but it will create the new record if already record is not there to update..