Hibernate Interview Questions and Answers

Hibernate Interview Questions

Following are some of the most frequently asked Hibernate interview questions in the interview, here are the answers for them.

What is Hibernate?

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.

What are the collection types in Hibernate?

There are five collection types in hibernate used for one-to-many relationship mappings.

Bag

Set

List

Array

Map

What are different states of an entity bean?

An entity bean instance can exist is one of the three states.

Transient: When an object is never persisted or associated with any session, it’s in transient state. Transient instances may be made persistent by calling save(), persist() or saveOrUpdate(). Persistent instances may be made transient by calling delete().

Persistent: When an object is associated with a unique session, it’s in persistent state. Any instance returned by a get() or load() method is persistent.

Detached: When an object is previously persistent but not associated with any session, it’s in detached state. Detached instances may be made persistent by calling update(), saveOrUpdate(), lock() or replicate(). The state of a transient or detached instance may also be made persistent as a new persistent instance by calling merge().

What is hibernate configuration file?

Hibernate configuration file contains database specific configurations and used to initialize SessionFactory. We provide database credentials or JNDI resource information in the hibernate configuration xml file. Some other important parts of hibernate configuration file is Dialect information, so that hibernate knows the database type and mapping file or class details.

What is hibernate caching? Explain Hibernate first level cache?

As the name suggests, hibernate caches query data to make our application faster. Hibernate Cache can be very useful in gaining fast application performance if used correctly. The idea behind cache is to reduce the number of database queries, hence reducing the throughput time of the application.

Hibernate first level cache is associated with the Session object. Hibernate first level cache is enabled by default and there is no way to disable it. However hibernate provides methods through which we can delete selected objects from the cache or clear the cache completely.

Any object cached in a session will not be visible to other sessions and when the session is closed, all the cached objects will also be lost.

What are the benefits of Named SQL Query?

Hibernate Named Query helps us in grouping queries at a central location rather than letting them scattered all over the code.

Hibernate Named Query syntax is checked when the hibernate session factory is created, thus making the application fail fast in case of any error in the named queries.

Hibernate Named Query is global, means once defined it can be used throughout the application.

However one of the major disadvantages of Named query is that it’s hard to debug, because we need to find out the location where it’s defined.

What is use of Hibernate Session merge() call?

Hibernate merge can be used to update existing values, however this method create a copy from the passed entity object and return it. The returned object is part of persistent context and tracked for any changes, passed object is not tracked. For example program, read Hibernate merge.

What is ORM?

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.

What is hibernate caching? Explain Hibernate first level cache?

As the name suggests, hibernate caches query data to make our application faster. Hibernate Cache can be very useful in gaining fast application performance if used correctly. The idea behind cache is to reduce the number of database queries, hence reducing the throughput time of the application.

Hibernate first level cache is associated with the Session object. Hibernate first level cache is enabled by default and there is no way to disable it. However hibernate provides methods through which we can delete selected objects from the cache or clear the cache completely.

Any object cached in a session will not be visible to other sessions and when the session is closed, all the cached objects will also be lost.

What is hibernate caching? Explain Hibernate first level cache?

As the name suggests, hibernate caches query data to make our application faster. Hibernate Cache can be very useful in gaining fast application performance if used correctly. The idea behind cache is to reduce the number of database queries, hence reducing the throughput time of the application.

Hibernate first level cache is associated with the Session object. Hibernate first level cache is enabled by default and there is no way to disable it. However hibernate provides methods through which we can delete selected objects from the cache or clear the cache completely.

Any object cached in a session will not be visible to other sessions and when the session is closed, all the cached objects will also be lost.

For better explanation, please read Hibernate First Level Cache.

What does an ORM solution comprises of?

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

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

Which design patterns are used in Hibernate framework?

Some of the design patterns used in Hibernate Framework are:

Domain Model Pattern –  An object model of the domain that incorporates both behavior and data.

Data Mapper – A layer of Mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself.

Proxy Pattern for lazy loading

Factory pattern in SessionFactory

What is Hibernate SessionFactory and how to configure it?

SessionFactory is the factory class used to get the Session objects. SessionFactory is responsible to read the hibernate configuration parameters and connect to the database and provide Session objects. Usually an application has a single SessionFactory instance and threads servicing client requests obtain Session instances from this factory.

The internal state of a SessionFactory is immutable. Once it is created this internal state is set. This internal state includes all of the metadata about Object/Relational Mapping.

SessionFactory also provide methods to get the Class metadata and Statistics instance to get the stats of query executions, second level cache details etc.

What are the advantages of Hibernate over JDBC?

Some of the important advantages of Hibernate framework over JDBC are:

-Hibernate removes a lot of boiler-plate code that comes with JDBC API, the code looks more cleaner and readable.

-Hibernate supports inheritance, associations and collections. These features are not present with JDBC API.

-Hibernate implicitly provides transaction management, in fact most of the queries can’t be executed outside transaction. In JDBC API, we need to write code for transaction management using commit and rollback. Read more at JDBC Transaction Management.

-JDBC API throws SQLException that is a checked exception, so we need to write a lot of try-catch block code. Most of the times it’s redundant in every JDBC call and used for transaction management. Hibernate wraps JDBC exceptions and throw JDBCException or HibernateException un-checked exception, so we don’t need to write code to handle it. Hibernate built-in transaction management removes the usage of try-catch blocks.

-Hibernate Query Language (HQL) is more object oriented and close to java programming language. For JDBC, we need to write native sql queries.

-Hibernate supports caching that is better for performance, JDBC queries are not cached hence performance is low.

-Hibernate provide option through which we can create database tables too, for JDBC tables must exist in the database.

-Hibernate configuration helps us in using JDBC like connection as well as JNDI DataSource for connection pool. This is very important feature in enterprise application and completely missing in JDBC API.

-Hibernate supports JPA annotations, so code is independent of implementation and easily replaceable with other ORM tools. JDBC code is very tightly coupled with the application.

What is Hibernate Validator Framework?

Data validation is integral part of any application. You will find data validation at presentation layer with the use of Javascript, then at the server side code before processing it. Also data validation occurs before persisting it, to make sure it follows the correct format.

Validation is a cross cutting task, so we should try to keep it apart from our business logic. That’s why JSR303 and JSR349 provide specification for validating a bean by using annotations. Hibernate Validator provides the reference implementation of both these bean validation specs. Read more at Hibernate Validation Example.

What is difference between Hibernate Session get () and load () method?

Hibernate session comes with different methods to load data from database. get and load are most used methods, at first look they seems similar but there are some differences between them.

get() loads the data as soon as it’s called whereas load() returns a proxy object and loads data only when it’s actually required, so load() is better because it support lazy loading.

Since load() throws exception when data is not found, we should use it only when we know data exists.

We should use get() when we want to make sure data exists in the database.

For clarification regarding the differences, please read Hibernate get vs load.

What are the different levels of ORM quality?

There are four levels defined for ORM quality.

Pure relational

Light object mapping

Medium object mapping

Full object mapping

What is a pure relational ORM?

The entire application, including the user interface, is designed around the relational model and SQL-based relational operations.

Name some important interfaces of Hibernate framework?

Some of the important interfaces of Hibernate framework are:

SessionFactory (org.hibernate.SessionFactory): SessionFactory is an immutable thread-safe cache of compiled mappings for a single database. We need to initialize SessionFactory once and then we can cache and reuse it. SessionFactory instance is used to get the Session objects for database operations.

Session (org.hibernate.Session): Session is a single-threaded, short-lived object representing a conversation between the application and the persistent store. It wraps JDBC java.sql.Connection and works as a factory for org.hibernate.Transaction. We should open session only when it’s required and close it as soon as we are done using it. Session object is the interface between java application code and hibernate framework and provide methods for CRUD operations.

Transaction (org.hibernate.Transaction): Transaction is a single-threaded, short-lived object used by the application to specify atomic units of work. It abstracts the application from the underlying JDBC or JTA transaction. A org.hibernate.Session might span multiple org.hibernate.Transaction in some cases.

What is a meant by light object mapping?

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.

What is a meant by medium object mapping?

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.

What is meant by full object mapping?

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.

What are the benefits of ORM and Hibernate?

There are many benefits from these. Out of which the following are the most important one.

Productivity – Hibernate reduces the burden of developer by providing much of the functionality and let the developer to concentrate on business logic.

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.

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.

Vendor independence – Irrespective of the different types of databases that are there, hibernate provides a much easier way to develop a cross platform application.

How does hibernate code looks like?

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.

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 different states of an entity bean?

An entity bean instance can exist is one of the three states.

Transient: When an object is never persisted or associated with any session, it’s in transient state. Transient instances may be made persistent by calling save (), persist() or saveOrUpdate(). Persistent instances may be made transient by calling delete ().

Persistent: When an object is associated with a unique session, it’s in persistent state. Any instance returned by a get() or load() method is persistent.

Detached: When an object is previously persistent but not associated with any session, it’s in detached state. Detached instances may be made persistent by calling update (), saveOrUpdate(), lock() or replicate(). The state of a transient or detached instance may also be made persistent as a new persistent instance by calling merge ().

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