Hibernate Interview Questions

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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) 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 are the Core interfaces are of Hibernate framework?

A) People who read this also read:
The five core interfaces are used in just about every Hibernate application. Using these interfaces, you can store and retrieve persistent objects and control transactions.
* Session interface
* SessionFactory interface
* Configuration interface
* Transaction interface
* Query and Criteria interfaces

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 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) 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) How do you map Java Objects with Database tables?

* 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.

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’s the difference between load() and get()?

A) load() vs. get()

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 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) How do you define sequence generated primary key in hibernate?

A) Using tag.
Example:-

SEQUENCE_NAME



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 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) What do you mean by Named – SQL query?

A) Named SQL queries are defined in the mapping xml document and called wherever required.
example:

SELECT emp.EMP_ID AS {emp.empid},
emp.EMP_ADDRESS AS {emp.address},
emp.EMP_NAME AS {emp.name}
FROM Employee EMP WHERE emp.NAME LIKE :name

Invoke Named Query :

List people = session.getNamedQuery(”empdetails”)
.setString(”TomBrady”, name)
.setMaxResults(50)
.list();

Q) How do you invoke Stored Procedures?

A) { ? = call selectAllEmployees() }


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) 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) 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) 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) 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

Q) 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) Different States of Hibernate Objects

* Transient Objects - Object instantiated using the new Operator aren't immediately persistent. Their state is Transient, which means they aren't associated with any database tables row and so their state is lost as soon as they're no longer referenced by any other object

* Persistant Objects- is an Entity instance with a database identity. Means a persistent and managed instance has a primary key value set as its database identifier. They are always associated with a persistence context

*Removed Objects- when object is scheduled for deletion

*Detached Objects - When object is no longer guaranteed to be synchronized with DB state. They are no longer attached to persistence context.