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I have a method annotated with @Transactional. I retrieve an object from my Oracle DB, change a field, and then return from the method. I forgot to save the object, but discovered that the database gets updated anyway.
applicationContext
<tx:annotation-driven /> <bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="sessionFactory" /> </bean> my method
@Transactional public void myMethod(long id) { MyObject myObj = dao.getMstAttributeById(id); myObj.setName("new name"); //dao.update(myObj); } my question is why does MyObject get persisted to the database?
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Hibernate merge can be used to update existing values, however this method create a copy from the passed entity object and return it. The returned object is part of persistent context and tracked for any changes, passed object is not tracked. This is the major difference with merge() from all other methods.
The main difference between save and saveOrUpdate method is that save() generates a new identifier and INSERT record into the database while saveOrUpdate can either INSERT or UPDATE based upon the existence of a record.
The save() method returns the generated identifier so it has to immediately execute the SQL INSERT statement (it does not matter if we are inside or outside of a transaction) because identifiers are generated by the database during the INSERT query execution only.
Because hibernate will automatically detect changes made to persistent entities and update the database accordingly. This behaviour is documented in chapter 11 of the hibernate reference manual. The relevant part reads:
Hibernate defines and supports the following object states:
Transient - an object is transient if it has just been instantiated using the new operator, and it is not associated with a Hibernate Session. It has no persistent representation in the database and no identifier value has been assigned. Transient instances will be destroyed by the garbage collector if the application does not hold a reference anymore. Use the Hibernate Session to make an object persistent (and let Hibernate take care of the SQL statements that need to be executed for this transition).
Persistent - a persistent instance has a representation in the database and an identifier value. It might just have been saved or loaded, however, it is by definition in the scope of a Session. Hibernate will detect any changes made to an object in persistent state and synchronize the state with the database when the unit of work completes. Developers do not execute manual UPDATE statements, or DELETE statements when an object should be made transient.
Detached - a detached instance is an object that has been persistent, but its Session has been closed. The reference to the object is still valid, of course, and the detached instance might even be modified in this state. A detached instance can be reattached to a new Session at a later point in time, making it (and all the modifications) persistent again. This feature enables a programming model for long running units of work that require user think-time. We call them application transactions, i.e., a unit of work from the point of view of the user.
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