Difference Between Merge And Update Methods In Hibernate

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

Let Us Take An Example

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SessionFactory factory = cfg.buildSessionFactory();
Session session1 = factory.openSession();

Student s1 = null;

Object o = session1.get(Student.class, new Integer(101)); s1 = (Student)o;

session1.close();

s1.setMarks(97);

Session session2 = factory.openSession();

Student s2 = null; Object o1 = session2.get(Student.class, new Integer(101)); s2 = (Student)o1; Transaction tx=session2.beginTransaction();

session2.merge(s1);

Explanation

• See from line numbers 6 – 9, we just loaded one object s1 into session1 cache and closed session1 at line number 9, so now 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

Hope you are clear…, actually update and merge methods will come into picture when ever we loaded the same object again and again into the database, like above.

Folks i have been changed the values in the source code.