Hibernate Interview Questions - Three

Q1. Should all the mapping files of hibernate have .hbm.xml extension to work properly?

Ans: No, having .hbm.xml extension is a convention and not a requirement for hibernate mapping file names. We can have any extension for these mapping files. But this is the best practice to follow this extension.

Q2. How do we create session factory in hibernate?

Ans: To create a session factory in hibernate, an object of configuration is created first which refers to the path of configuration file and then for that configuration, session factory is created as given in the example below:

Configuration config = new Configuration();
config.addResource("myinstance/configuration.hbm.xml");
config.setProperties( System.getProperties() );
SessionFactory sessions = config.buildSessionFactory();

Q3. What is meant by Method chaining?

Ans: Method chaining is a programming technique that is supported by many hibernate interfaces. This is less readable when compared to actual java code. And it is not mandatory to use this format. Look how a SessionFactory is created when we use method chaining.

SessionFactory sessions = new Configuration()
.addResource(“myinstance/MyConfig.hbm.xml”)
.setProperties( System.getProperties() )
.buildSessionFactory();

Q4. What does hibernate.properties file consist of?

Ans: This is a property file that should be placed in application class path. So when the Configuration object is created, hibernate is first initialized. At this moment the application will automatically detect and read this hibernate.properties file.

hibernate.connection.datasource = java:/comp/env/jdbc/AuctionDB
hibernate.transaction.factory_class = net.sf.hibernate.transaction.JTATransactionFactory
hibernate.transaction.manager_lookup_class = net.sf.hibernate.transaction.JBossTransactionManagerLookup
hibernate.dialect = net.sf.hibernate.dialect.PostgreSQLDialect

Q5. What should SessionFactory be placed so that it can be easily accessed?

Ans: As far as it is compared to J2EE environment, if the SessionFactory is placed in JNDI then it can be easily accessed and shared between different threads and various components that are hibernate aware. You can set the SessionFactory to a JNDI by configuring a property hibernate.session_factory_name in the hibernate.properties file.

Q6. What are POJOs and what’s their significance?

Ans: POJO stands for plain old java objects. These are just basic JavaBeans that have defined setter and getter methods for all the properties that are there in that bean. Besides they can also have some business logic related to that property. Hibernate applications works efficiently with POJOs rather than simple java classes. Use of POJOs instead of simple java classes results in an efficient and well-constructed code.

Q7. What is ORM metadata?

Ans: ORM tools require a metadata format for the application to specify the mapping between classes and tables, properties and columns, associations and foreign keys, Java types and SQL types. This information is called the object/relational mapping metadata. It defines the transformation between the different data type systems and relationship representations.

Q8. What’s HQL?

Ans: HQL is the query language used in Hibernate which is an extension of SQL. HQL is very efficient, simple and flexible query language to do various types of operations on relational database without writing complex database queries.

Q9. What are the different types of property and class mappings?

• Typical and most common property mapping
<property name=”description” column=”DESCRIPTION” type=”string”/>
Or 
<property name=”description” type=”string”>
<column name=”DESCRIPTION”/>
</property>
• Derived properties
<property name=”averageBidAmount” formula=”( select AVG(b.AMOUNT) from BID b
where b.ITEM_ID = ITEM_ID )” type=”big_decimal”/>
• Typical and most common property mapping
<property name=”description” column=”DESCRIPTION” type=”string”/>
• Controlling inserts and updates
<property name=”name” column=”NAME” type=”string”
insert=”false” update=”false”/>