Hibernate Interview Questions

What are different environments to configure hibernate?

There are mainly two types of environments in which the configuration of hibernate application differs.
i.    Managed environment - In this kind of environment everything from database connections, transaction boundaries, security levels and all are defined. An example of this kind of environment is environment provided by application servers such as JBoss, Weblogic and WebSphere.
ii.    Non-managed environment - This kind of environment provides a basic configuration template. Tomcat is one of the best examples that provide this kind of environment.

What is the file extension you use for hibernate mapping file?

The name of the file should be like this : filename.hbm.xml
The filename varies here. The extension of these files should be ".hbm.xml".
This is just a convention and it's not mandatory. But this is the best practice to follow this extension.

What do you create a SessionFactory?

Configuration cfg = new Configuration();
cfg.addResource("myinstance/MyConfig.hbm.xml");
cfg.setProperties( System.getProperties() );
SessionFactory sessions = cfg.buildSessionFactory();
First, we need to create an instance of Configuration and use that instance to refer to the location of the configuration file. After configuring this instance is used to create the SessionFactory by calling the method buildSessionFactory().

What is meant by Method chaining?

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();

What does hibernate.properties file consist of?

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

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

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.

What are POJOs?

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 then simple java classes.

What is object/relational mapping metadata?

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.

What is HQL?

HQL stands for Hibernate Query Language. Hibernate allows the user to express queries in its own portable SQL extension and this is called as HQL. It also allows the user to express in native SQL.

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"/>

What is Attribute Oriented Programming?

XDoclet has brought the concept of attribute-oriented programming to Java. Until JDK 1.5, the Java language had no support for annotations; now XDoclet uses the Javadoc tag format (@attribute) to specify class-, field-, or method-level metadata attributes. These attributes are used to generate hibernate mapping file automatically when the application is built. This kind of programming that works on attributes is called as Attribute Oriented Programming.

What are the different methods of identifying an object?

There are three methods by which an object can be identified.
i.    Object identity -Objects are identical if they reside in the same memory location in the JVM. This can be checked by using the = = operator.
ii.    Object equality - Objects are equal if they have the same value, as defined by the equals( ) method. Classes that don't explicitly override this method inherit the implementation defined by java.lang.Object, which compares object identity.
iii.    Database identity - Objects stored in a relational database are identical if they represent the same row or, equivalently, share the same table and primary key value.

What are the different approaches to represent an inheritance hierarchy?

1. Table per concrete class.
2. Table per class hierarchy.
3. Table per subclass.

What are managed associations and hibernate associations?

Associations that are related to container management persistence are called managed associations. These are bi-directional associations. Coming to hibernate associations, these are unidirectional

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