Q) What is Hibernate?
A) Hibernate is a powerful, high performance object/relational persistence and query service. This lets the users to develop persistent classes following object-oriented principles such as association, inheritance, polymorphism, composition, and collections.

Q) What is ORM?
A) ORM stands for Object/Relational mapping. It is the programmed and translucent perseverance of objects in a Java application in to the tables of a relational database using the metadata that describes the mapping between the objects and the database. It works by transforming the data from one representation to another.

Q) What does an ORM solution comprises of?
A) It should have an API for performing basic CRUD (Create, Read, Update, Delete) operations on objects of persistent classes
*Should have a language or an API for specifying queries that refer to the classes and the properties of classes
*An ability for specifying mapping metadata
*It should have a technique for ORM implementation to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions

Q) What are the different levels of ORM quality?
A) There are four levels defined for ORM quality.
i. Pure relational
ii. Light object mapping
iii. Medium object mapping
iv. Full object mapping

Q) What is a pure relational ORM?
A) The entire application, including the user interface, is designed around the relational model and SQL-based relational operations.

Q) What is a meant by light object mapping?
A) The entities are represented as classes that are mapped manually to the relational tables. The code is hidden from the business logic using specific design patterns. This approach is successful for applications with a less number of entities, or applications with common, metadata-driven data models. This approach is most known to all.

Q) What is a meant by medium object mapping?
A) The application is designed around an object model. The SQL code is generated at build time. And the associations between objects are supported by the persistence mechanism, and queries are specified using an object-oriented expression language. This is best suited for medium-sized applications with some complex transactions. Used when the mapping exceeds 25 different database products at a time.

Q) What is meant by full object mapping?
A) Full object mapping supports sophisticated object modeling: composition, inheritance, polymorphism and persistence. The persistence layer implements transparent persistence; persistent classes do not inherit any special base class or have to implement a special interface. Efficient fetching strategies and caching strategies are implemented transparently to the application.

Q) What are the benefits of ORM and Hibernate?
A) There are many benefits from these. Out of which the following are the most important one.
i. Productivity - Hibernate reduces the burden of developer by providing much of the functionality and let the developer to concentrate on business logic.
ii. Maintainability - As hibernate provides most of the functionality, the LOC for the application will be reduced and it is easy to maintain. By automated object/relational persistence it even reduces the LOC.
iii. Performance - Hand-coded persistence provided greater performance than automated one. But this is not true all the times. But in hibernate, it provides more optimization that works all the time there by increasing the performance. If it is automated persistence then it still increases the performance.
iv. Vendor independence - Irrespective of the different types of databases that are there, hibernate provides a much easier way to develop a cross platform application.

Q) How does hibernate code looks like?
A) Session session = getSessionFactory().openSession(); Transaction tx = session.beginTransaction();
MyPersistanceClass mpc = new MyPersistanceClass ("Sample App"); session.save(mpc);
tx.commit();
session.close();
The Session and Transaction are the interfaces provided by hibernate. There are many other interfaces besides this.

Q) What are Callback interfaces?
A) These interfaces are used in the application to receive a notification when some object events occur. Like when an object is loaded, saved or deleted. There is no need to implement callbacks in hibernate applications, but they're useful for implementing certain kinds of generic functionality.

Q) What the Core interfaces are of hibernate framework?
A) There are many benefits from these. Out of which the following are the most important one.
i.Session Interface - This is the primary interface used by hibernate applications. The instances of this interface are lightweight and are inexpensive to create and destroy. Hibernate sessions are not thread safe.
ii.SessionFactory Interface - This is a factory that delivers the session objects to hibernate application. Generally there will be a single SessionFactory for the whole application and it will be shared among all the application threads.
iii.Configuration Interface - This interface is used to configure and bootstrap hibernate. The instance of this interface is used by the application in order to specify the location of hibernate specific mapping documents.
iv.Transaction Interface - This is an optional interface but the above three interfaces are mandatory in each and every application. This interface abstracts the code from any kind of transaction implementations such as JDBC transaction, JTA transaction.
v. Query and Criteria Interface - This interface allows the user to perform queries and also control the flow of the query execution.

Q) Show Hibernate overview?
A)
     

Q) What is a hibernate xml mapping document and how does it look like?
A) In order to make most of the things work in hibernate, usually the information is provided in an xml document. This document is called as xml mapping document. The document defines, among other things, how properties of the user defined persistence classes' map to the columns of the relative tables in database.
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
< hibernate-mapping>
< class name="sample.MyPersistanceClass" table="MyPersitaceTable"> < id name="id" column="MyPerId">
< generator class="increment"/>
< /id>
< property name="text" column="Persistance_message"/>
< many-to-one name="nxtPer" cascade="all" column="NxtPerId"/> </class>
< /hibernate-mapping>
Everything should be included under < hibernate-mapping> tag. This is the main tag for an xml mapping document.

Q) What are Extension interfaces?
A) When the built-in functionalities provided by hibernate is not sufficient enough, it provides a way so that user can include other interfaces and implement those interfaces for user desire functionality. These interfaces are called as Extension interfaces.

Q) What are the Extension interfaces that are there in hibernate?
A) There are many extension interfaces provided by hibernate.
ProxyFactory interface - used to create proxies
ConnectionProvider interface - used for JDBC connection management TransactionFactory interface - Used for transaction management
Transaction interface - Used for transaction management
TransactionManagementLookup interface - Used in transaction management. Cahce interface - provides caching techniques and strategies
CacheProvider interface - same as Cache interface
ClassPersister interface - provides ORM strategies
IdentifierGenerator interface - used for primary key generation
Dialect abstract class - provides SQL support

Q) What are different environments to configure hibernate?
A) 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.

Q) What is the file extension you use for hibernate mapping file?
A) 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.

Q) What do you create a SessionFactory?
A) 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().

Q) What is meant by Method chaining?
A) 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();

Q) What does hibernate.properties file consist of?
A) 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

Q) What should SessionFactory be placed so that it can be easily accessed?
A) 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.

Q) What are POJOs?
A) 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.

Q) What is object/relational mapping metadata?
A) 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.

Q) What are the different types of property and class mappings?
A) * 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"/>

Q) What is HQL?
A) 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.

Q) What is Attribute Oriented Programming?
A) 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.

Q) What are the different methods of identifying an object?
A) 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.

Q) What are the different approaches to represent an inheritance hierarchy?
A) i. Table per concrete class.
ii. Table per class hierarchy.
iii. Table per subclass.

Q) What are managed associations and hibernate associations?
A) Associations that are related to container management persistence are called managed associations. These are bi-directional associations. Coming to hibernate associations, these are unidirectional.

Q) Why do you need ORM tools like hibernate?
A) The main advantage of ORM like hibernate is that it shields developers from messy SQL. Apart from this, ORM provides following benefits:
* Improved productivity
o High-level object-oriented API
o Less Java code to write
o No SQL to write
* Improved performance
o Sophisticated caching
o Lazy loading
o Eager loading
* Improved maintainability
o A lot less code to write
* Improved portability
o ORM framework generates database-specific SQL for you

Q) What Does Hibernate Simplify?
A) Hibernate simplifies:
* Saving and retrieving your domain objects
* Making database column and table name changes
* Centralizing pre save and post retrieve logic
* Complex joins for retrieving related items
* Schema creation from object model

Q) What is the difference between hibernate and spring JDBC template? List any advantages and disadvantages
A) Spring provides hibernate template and it has many advantages like
1) It removes boiler plate code like getting connection from data source, try/catch block for closing connection. So that developer can focus on writing business logic rather then writing boilier plate code every where.
2) Spring hibernateTemplate also throws RunTime exception compared to checkd exception which allows to remove writing try/catch block in each DAO.
3) It also gives richer template class, using which developer can write query code easily. This template class also allows to get session explicitly, so if developer wants to get session object and work on it, then it's possible.

Q) What is the difference between and merge and update?
A) In COBOL merging means combining records of two or more sequential files. This can be acheived through MERGE VERB. After merging all the records can be stored in seperate output file. In output file all the records are sorted. Update means we can modify the records of any file. This can be achieved through REWRITE VERB.

Q) what is the difference between hibernate and Spring?
A) Hibernate is ORM tool used for data persistency.Spring is a framework for enterprise applications with default APIs for presentation, middle tiers and presistence layers and allows to integrate with various presentations, middle tier and presistence APIs like Hibernate, Struts, Ibatis, JMS, MQ series etc.

Q) What are the most common methods of Hibernate configuration?
A) The most common methods of Hibernate configuration are:
* Programmatic configuration
* XML configuration (hibernate.cfg.xml)

Q) Why Hibernate is advantageous over Entity Beans & JDBC?
A) Hibernate and EJB have different purpose.EJB is for writing business logic as well and provide Database independency just as hibernate provide for us.EJB infact use the container services(EJB container) like transaction, securiry etc as well.Which is preferred over HIbernate and Ejb is actually depend on the application requirement. EJB have greater complexity then Hibernate But in many heavily functionality web applications it is preferred than hibernate. Other Then this you can use Ejb where EJB container is present as in case of tomcat it is not present still yet.So many web apllication which uses free tomcat server use hibernate for database independency.

Q) What is the main difference between Entity Beans and Hibernate ?
A) 1) In Entity Bean at a time we can interact with only one data Base. Where as in Hibernate we can able to establishes the connections to more than One Data Base. Only thing we need to write one more configuration file.
2) Entity Beans does not support OOPS concepts where as Hibernate does.
3) Hibernate supports multi level cacheing, where as Entity Beans doesn't.

Q) How does Hibrnate maintain relations of RDBMS?
A) Those are .. 1 to many, many to many, many to one.. relations. these relational mappings are done by using the hibernates utilities such as list, set, bags, object... In detail Go thru... www.hibernate.org

Q) What is database persistence?How well it is implemented in Hibernate
A) Hibernate has an extremely sophisticated granular two-level cache architecture. It is possible to enable or disable the use of the (second-level) process or cluster level cache for a particular class or collection role. There is support for pluggable cache implementations, including EHCache, JBossCache, SwarmCache, Tangosol Coherence Cache. There is also a granular query result set cache. All this flexibility comes at the cost of complexity and can occasionally be tricky for new users. it will helps hibernate to do the persitence work more feasible

Q) What are the important tags of hibernate.cfg.xml?
A) An Action Class is an adapter between the contents of an incoming HTTP rest and the corresponding business logic that should be executed to process this rest.

Q) What role does the Session interface play in Hibernate?
A) The Session interface is the primary interface used by Hibernate applications. It is a single-threaded, short-lived object representing a conversation between the application and the persistent store. It allows you to create query objects to retrieve persistent objects.
Session session = sessionFactory.openSession();
Session interface role:
* Wraps a JDBC connection
* Factory for Transaction
* Holds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier

Q) What role does the SessionFactory interface play in Hibernate?
A) The application obtains Session instances from a SessionFactory. There is typically a single SessionFactory for the whole application-created during application initialization. The SessionFactory caches generate SQL statements and other mapping metadata that Hibernate uses at runtime. It also holds cached data that has been read in one unit of work and may be reused in a future unit of work
SessionFactory sessionFactory = configuration.buildSessionFactory();

Q) What is Hibernate Query Language (HQL)?
A) Hibernate offers a query language that embodies a very powerful and flexible mechanism to query, store, update, and retrieve objects from a database. This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL.

Q) What is the difference between and merge and update ?
A) Use update() if you are sure that the session does not contain an already persistent instance with the same identifier, and merge() if you want to merge your modifications at any time without consideration of the state of the session.

Q) What's the difference between load() and get()?
A)
load() :-
Only use the load() method if you are sure that the object exists.
load() method will throw an exception if the unique id is not found in the database.
load() just returns a proxy by default and database won't be hit until the proxy is first invoked.
get() :-
If you are not sure that the object exists, then use one of the get() methods.
get() method will return null if the unique id is not found in the database.
get() will hit the database immediately.

Q) What does it mean to be inverse?
A) It informs hibernate to ignore that end of the relationship. If the one-to-many was marked as inverse, hibernate would create a child->parent relationship (child.getParent). If the one-to-many was marked as non-inverse then a child->parent relationship would be created.

Q) Explain Criteria API
A) Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.
Example :
List employees = session.createCriteria(Employee.class)
.add(Restrictions.like("name", "a%") )
.add(Restrictions.like("address", "Boston")).addOrder(Order.asc("name") ).list();

Q) Define HibernateTemplate?
A) org.springframework.orm.hibernate.HibernateTemplate is a helper class which provides different methods for querying/retrieving data from the database. It also converts checked HibernateExceptions into unchecked DataAccessExceptions.

Q) How do you switch between relational databases without code changes?
A) Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined.

Q) What are the benefits does HibernateTemplate provide?
A) The benefits of HibernateTemplate are :
* HibernateTemplate, a Spring Template class simplifies interactions with Hibernate Session.
* Common functions are simplified to single method calls.
* Sessions are automatically closed.
* Exceptions are automatically caught and converted to runtime exceptions.

Q) Define cascade and inverse option in one-many mapping?
A) cascade - enable operations to cascade to child entities.
cascade="all|none|save-update|delete|all-delete-orphan"
inverse - mark this collection as the "inverse" end of a bidirectional association.
inverse="true|false"
Essentially "inverse" indicates which end of a relationship should be ignored, so when persisting a parent who has a collection of children, should you ask the parent for its list of children, or ask the children who the parents are?

Q) What is the general flow of Hibernate communication with RDBMS?
A) The general flow of Hibernate communication with RDBMS is :
* Load the Hibernate configuration file and create configuration object. It will automatically load all hbm mapping files
* Create session factory from configuration object
* Get one session from this session factory
* Create HQL Query
* Execute query to get list containing Java objects

Q) 30.What is the difference between sorted and ordered collection in hibernate?
A)sorted collection vs. order collection :-
Sorted collection :
*A sorted collection is sorting a collection by utilizing the sorting features provided by the Java collections framework. The sorting occurs in the memory of JVM which running Hibernate, after the data being read from database using java comparator.
*If your collection is not large, it will be more efficient way to sort it.
Order collection :
*Order collection is sorting a collection by specifying the order-by clause for sorting this collection when retrieval.
*If your collection is very large, it will be more efficient way to sort it .

Q) If you want to see the Hibernate generated SQL statements on console, what should we do?
A) In Hibernate configuration file set as follows:true

Q) What are derived properties?
A) The properties that are not mapped to a column, but calculated at runtime by evaluation of an expression are called derived properties. The expression can be defined using the formula attribute of the element.

Q) How do you map Java Objects with Database tables?
A) * First we need to write Java domain objects (beans with setter and getter). The variables should be same as database columns.
* Write hbm.xml, where we map java class to table and database columns to Java class variables.
Example :
name="userName" not-null="true" type="java.lang.String"/>
name="userPassword" not-null="true" type="java.lang.String"/>

Q) What are the Collection types in Hibernate ?
A)* Bag
* Set
* List
* Array
* Map

Q) What are the ways to express joins in HQL?
A) HQL provides four ways of expressing (inner and outer) joins:-
* An implicit association join
* An ordinary join in the FROM clause
* A fetch join in the FROM clause.
* A theta-style join in the WHERE clause.

Q) What is Hibernate proxy?
A) The proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies which implement the named interface. The actual persistent object will be loaded when a method of the proxy is invoked.

Q) How can Hibernate be configured to access an instance variable directly and not through a setter method ?
A) By mapping the property with access="field" in Hibernate metadata. This forces hibernate to bypass the setter method and access the instance variable directly while initializing a newly loaded object.

Q) How can a whole class be mapped as immutable?
A) Mark the class as mutable="false" (Default is true),. This specifies that instances of the class are (not) mutable. Immutable classes, may not be updated or deleted by the application.

Q) What is the use of dynamic-insert and dynamic-update attributes in a class mapping?
A) Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.
* dynamic-update (defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed
* dynamic-insert (defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.

Q) What do you mean by fetching strategy ?
A) A fetching strategy is the strategy Hibernate will use for retrieving associated objects if the application needs to navigate the association. Fetch strategies may be declared in the O/R mapping metadata, or over-ridden by a particular HQL or Criteria query.

Q) What is automatic dirty checking?
A) Automatic dirty checking is a feature that saves us the effort of explicitly asking Hibernate to update the database when we modify the state of an object inside a transaction.

Q) What is transactional write-behind?
A) Hibernate uses a sophisticated algorithm to determine an efficient ordering that avoids database foreign key constraint violations but is still sufficiently predictable to the user. This feature is called transactional write-behind.

Q) What are Callback interfaces?
A) Callback interfaces allow the application to receive a notification when something interesting happens to an object-for example, when an object is loaded, saved, or deleted. Hibernate applications don't need to implement these callbacks, but they're useful for implementing certain kinds of generic functionality.

Q) What are the types of Hibernate instance states ?
A) Three types of instance states:
* Transient -The instance is not associated with any persistence context
* Persistent -The instance is associated with a persistence context
* Detached -The instance was associated with a persistence context which has been closed - currently not associated

Q) What are the differences between EJB 3.0 & Hibernate
A) Hibernate Vs EJB 3.0 :-
Hibernate :-
Session-Cache or collection of loaded objects relating to a single unit of work
XDoclet Annotations used to support Attribute Oriented Programming
Defines HQL for expressing queries to the database
Supports Entity Relationships through mapping files and annotations in JavaDoc
Provides a Persistence Manager API exposed via the Session, Query, Criteria, and Transaction API
Provides callback support through lifecycle, interceptor, and validatable interfaces
Entity Relationships are unidirectional. Bidirectional relationships are implemented by two unidirectional relationships

EJB 3.0 :-
Persistence Context-Set of entities that can be managed by a given EntityManager is defined by a persistence unit
Java 5.0 Annotations used to support Attribute Oriented Programming
Defines EJB QL for expressing queries
Support Entity Relationships through Java 5.0 annotations
Provides and Entity Manager Interface for managing CRUD operations for an Entity
Provides callback support through Entity Listener and Callback methods
Entity Relationships are bidirectional or unidirectional

Q) What are the types of inheritance models in Hibernate?
A) There are three types of inheritance models in Hibernate:
* Table per class hierarchy
* Table per subclass
* Table per concrete class

Q) What is component mapping in Hibernate?
A) * A component is an object saved as a value, not as a reference
* A component can be saved directly without needing to declare interfaces or identifier properties
* Required to define an empty constructor
* Shared references not supported
Example:

Component Mapping:

public class Address{
private String street;
private int postalCode;
private String city;
//no-arg constructor + get set
}
public class Person{
//other properties
private Address address;
//get and set methods
}

Component Mapping and Property Mapping :

< class name ="...example.Person table="persons"" >
<!-- other properties >
< component name="address" >
< property name="street"/>
< property name ="postalCode"/>
< property name ="city"/>
</component>
</class >

Q) What is the advantage of Hibernate over jdbc?
A) Hibernate Vs. JDBC :-
JDBC :
*With JDBC, developer has to write code to map an object model's data representation to a relational data model and its corresponding database schema.
*With JDBC, the automatic mapping of Java objects with database tables and vice versa conversion is to be taken care of by the developer manually with lines of code.
*JDBC supports only native Structured Query Language (SQL). Developer has to find out the efficient way to access database, i.e. to select effective query from a number of queries to perform same task.
*Application using JDBC to handle persistent data (database tables) having database specific code in large amount. The code written to map table data to application objects and vice versa is actually to map table fields to object properties. As table changed or database changed then it's essential to change object structure as well as to change code written to map table-to-object/object-to-table.
*With JDBC, it is developer's responsibility to handle JDBC result set and convert it to Java objects through code to use this persistent data in application. So with JDBC, mapping between Java objects and database tables is done manually.
*With JDBC, caching is maintained by hand-coding.
*In JDBC there is no check that always every user has updated data. This check has to be added by the developer.

   

Hibernate :
*Hibernate is flexible and powerful ORM solution to map Java classes to database tables. Hibernate itself takes care of this mapping using XML files so developer does not need to write code for this.
*Hibernate provides transparent persistence and developer does not need to write code explicitly to map database tables tuples to application objects during interaction with RDBMS.
*Hibernate provides a powerful query language Hibernate Query Language (independent from type of database) that is expressed in a familiar SQL like syntax and includes full support for polymorphic queries. Hibernate also supports native SQL statements. It also selects an effective way to perform a database manipulation task for an application.
*Hibernate provides this mapping itself. The actual mapping between tables and application objects is done in XML files. If there is change in Database or in any table then the only need to change XML file properties.
*Hibernate reduces lines of code by maintaining object-table mapping itself and returns result to application in form of Java objects. It relieves programmer from manual handling of persistent data, hence reducing the development time and maintenance cost.
*Hibernate, with Transparent Persistence, cache is set to application work space. Relational tuples are moved to this cache as a result of query. It improves performance if client application reads same data many times for same write. Automatic Transparent Persistence allows the developer to concentrate more on business logic rather than this application code.
*Hibernate enables developer to define version type field to application, due to this defined field Hibernate updates version field of database table every time relational tuple is updated in form of Java class object to that table. So if two users retrieve same tuple and then modify it and one user save this modified tuple to database, version is automatically updated for this tuple by Hibernate. When other user tries to save updated tuple to database then it does not allow saving it because this user does not have updated data.

Q) What is the need for Hibernate xml mapping file?
A) Hibernate mapping file tells Hibernate which tables and columns to use to load and store objects. Typical mapping file look as follows:

DTD : - <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

Class element : -
< hibernate-mapping>
< class name="abc.example.Event" table="events" >

Identifier mapping & generation :-
< id name="id" column="event_id">
< generator class=""native />
</id>

Property Mapping :-
< property name="title" not-null="true" unique="true"/>
< property name="date" type="date" column="event_date"/>

Unidirectional Many-to-Many association mappping:-
< set name="persons" table="event_persons" >
< key column="event_id" />
< many-to-many column ="person_id" class="abc.example.Person"/>
</set>
</class>
</hibernate-mapping>