Skip to main content

The difference betwen Hibernate saveOrUpdate and merge Method



SaveOrUpdate:

session.saveOrUpdate( someDetachedCat );

This is used to Reattach detached data. Reattachment is the process of taking an incoming entity instance that is in detached state and re-associating it with the current persistence context.

The method name update is a bit misleading here. It does not mean that an SQL UPDATE is immediately performed. It does, however, mean that an SQL UPDATE will be performed when the persistence context is flushed since Hibernate does not know its previous state against which to compare for changes. Unless the entity is mapped with select-before-update, in which case Hibernate will pull the current state from the database and see if an update is needed.

Provided the entity is detached, update and saveOrUpdate operate exactly the same.


Merge:
Cat theManagedInstance = session.merge( someDetachedCat );


Merging is the process of taking an incoming entity instance that is in detached state and copying its data over onto a new instance that is in managed state.


Comments

Popular posts from this blog

Stretch a row if data overflows in jasper reports

It is very common that some columns of the report need to stretch to show all the content in that column. But  if you just specify the property " stretch with overflow' to that column(we called text field in jasper report world) , it will just stretch that column and won't change other columns, so the row could be ridiculous. Haven't find the solution from internet yet. So I just review the properties in iReport one by one and find two useful properties(the bold highlighted in example below) which resolve the problems.   example:
<band height="20" splitType="Stretch"> <textField isStretchWithOverflow="true" pattern="" isBlankWhenNull="true"> <reportElement stretchType="RelativeToTallestObject" mode="Opaque" x="192" y="0" width="183" height="20"/> <box leftPadding="2"> <pen lineWidth="0.25"/> …

JasperReports - Configuration Reference

Spring - Operations with jdbcTemplate

This class manages all the database communication and exception handling using a java.sql.Connection that is obtained from the provided DataSource. JdbcTemplate is a stateless and threadsafe class and you can safely instantiate a single instance to be used for each DAO.


Use of Callback Methods
JdbcTemplate is based on a template style of programming common to many other parts of Spring. Some method calls are handled entirely by the JdbcTemplate, while others require the calling class to provide callback methods that contain the implementation for parts of the JDBC workflow. This is another form of Inversion of Control. Your application code hands over the responsibility of managing the database access to the template class. The template class in turn calls back to your application code when it needs some detail processing filled in. These callback methods are allowed to throw a java.sql.SQLException, since the framework will be able to catch this exception and use its built-in excepti…