Hibernate Interview Question Experience

Q.1  What is Hibernate?

Ans.

Synonym of Hibernate is Winter.  It means Spend the winter in a dormant state.

As a technology Hibernate is a open source, free Java package that makes it easy to work with relational databases. It is a pure Java object-relational mapping (ORM) and persistence framework that allows you to map plain old Java objects to relational database tables using (XML) configuration files.Its purpose is to relieve the developer from a significant amount of relational data persistence-related programming tasks.

Q2.  What is ORM?

Ans. ORM stands for Object/Relational mapping. It is the programmed and mapping of objects in a Java application in to the tables of a relational database using the meta data that describes the mapping between the objects and the database. It works by transforming the data from one representation to another.

We are familiar with model/pojo class and Relational database. Here Object means Mode/Pojo class and Relational means the databases. By hbm.xml file we represent the mapping between Object and Relational databases.

Q 3. What does an ORM solution comprises of?

Ans. Hibernate should have an language or API for performing basic CRUD (Create, Read, Update, Delete) operations on objects of persistent classes where persistent means POJO classes that you create that represent the table in database to keep the state of object alive. 

It 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 lazy association fetching, dirty checking, and other optimization functions.

Q4. What are the different levels of ORM quality?

Ans. Four levels defined for ORM quality:

• Pure relational 
• Light object mapping 
• Medium object mapping 
• Full object mapping

·         Pure relational:

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

·         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.

·         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. 

·         Full object mapping:

·         Full object mapping supports sophisticated object modeling: inheritance, polymorphism, composition 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.

 Q5. Why do you need ORM tools like hibernate?

Ans. The main advantage of ORM like hibernate is that it shields developers from messy SQL. Apart from this, ORM provides following benefits: 
► Improved productivity 
* High-level object-oriented API 
* Less Java code to write 
* No SQL to write 

► Improved performance 
* Sophisticated caching 
* Eager loading 

► Improved maintainability 
* A lot less code to write 

► Improved portability 
* ORM framework generates database-specific SQL for you

Q6. What are the benefits of ORM and Hibernate?

Ans. 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.

Q7.What is difference between Difference between save(), saveOrUpdate() and persist() in Hibernate Session

Session interface in Hibernate provides a couple of methods to move an object from new or transient state to persistent state e.g. save(), saveOrUpdate() and persist() are used to store an object into the database, but there are some significant differences between them. The Session.save() method does an INSERT to store the object into the database and it also return the identifier generated by the database. On the other hand, saveOrUpdate() can be used to reattach a detached object in Hibernate Session i.e. it can do INSERT or UPDATE depending upon whether object exists in database or not. The third method, persist() is similar to Session.save() i.e. it is used to move a transient object to the persistent state by storing it into the database but it doesn't return the database identifier. This was the basic difference between save() vs saveOrUpdate() vs persist() method and suggest when to use which method. It is also one of the frequently asked Hibernate interview questions on Java Web developer job interviews. Knowing this detail help you to do well on your interview. Btw, if you have not read already then you must read Java Persistence with Hibernate, best book to learn these kinds of subtle details about Hibernate.

Session.save() vs Session.saveOrUpdate() vs Session.Persist

As I said, all three methods save(), persist() and saveOrUpdate belongs to Hibernate's Session interface. They all are used to bring a new object into persistence context or a detached object, which was associated with a Hibernate Session in past but currently it's not associated into active persistence context. It is also one of the most popular Hibernate interview questions. Let's see couple of more differences between save(), persist() and saveOrUpdate() methods of Session interface
1.1) The main difference between save() and saveOrUpdate() method is that save() method performs an INSERT operation to store the object into the database, but INSERT will fail if the primary key is already persistent i.e. object already exists in the database. This is why, you should only call save() with an absolutely new object which doesn't have any database identifier. Calling save() with the detached object will fail. This is opposite of saveOrUpdate() method, which can do either INSERT or UPDATE SQL query depending upon whether an object exists in the database or not. The saveOrUpdate() method first executes a SELECT query to determine if it needs to do an INSERT or UPDATE operation.


1.2) Another key difference between save() and saveOrUpdate() method is that former is used to bring a transient object to persistent state but saveOurUpdate() can bring both transient (new) and detached (existing) object into persistent state. It is often used to re-attach a detached object into Session.

Here is an object's lifecycle and state transition diagram in Hibernate which shows that save() and saveOrUpdate() can move an object from transient to persistent state.

1.3) Coming to persist() method, the difference between save() and persist() method is that former returns the generated database identifier, a Serializable object but persist() method doesn't return anything. It's return type is void.

For example:

System.out.println(session.save(aCoin));

will print the generated primary key, but the following line will throw a compile time error because persist()'s return type is void.

System.out.println(session.persist(aCoin));

That's all about difference between save(), saveOrUpdate() and persist() method of Hibernate Session interface. Now you know the basic difference between them and also learned when to use the save(), saveOrUpdate() and persist() method. Use the save() method to store new object into database and when you need the generated database identifier, otherwise use persist() method. You can use saveOrUpdate() to reattach a detached object into Hibernate Session.

Q.8. What is the difference between get and load in Hibernate?

get vs load is one of the most frequently asked Hibernate Interview question, since correct understanding of both get() and load() is require to effectively using Hibernate. Main difference between get and load is that, get will hit the database if object is not found in the cache and returned completely initialized object, which may involve several database call while load() method can return proxy, if object is not found in cache and only hit database if any method other than getId() is called. This can save a lot of performance in some cases.

Q.9 What is named SQL query in Hibernate?

This Hibernate Interview question is related to query functionality provided by Hibernate. Named queries are SQL queries which are defined in mapping document using <sql-query> tag and called using Session.getNamedQuery() method. Named query allows you to refer a particular query by the name you provided, by the way, you can define named query in hibernate either by using annotations or XML mapping file, as I said above. @NameQuery is used to define single named query and @NameQueries is used to define multiple named query in hibernate.

Q.10. What is SessionFactory in Hibernate? is SessionFactory thread-safe?

Another common Interview questions related to Hibernate framework. SessionFactory, as the name suggest, is a factory to hibernate Session objects. SessionFactory is often built during start-up and used by application code to get session object. It acts as a single data store and it's also thread safe so that multiple threads can use the same SessionFactory. Usually, a Java JEE application has just one SessionFactory, and individual threads, which are servicing client’s request obtain hibernate Session instances from this factory, that’s why any implementation of SessionFactory interface must be thread-safe. Also, the internal state of SessionFactory, which contains all metadata about Object/Relational mapping is immutable and can not be changed once created.

Q.11 What is Session in Hibernate? Can we share single Session among multiple threads in Hibernate?

This is usually asked as follow-up question of previous Hibernate Interview question. After SessionFactory its time for Session. Session represent a small unit of work in Hibernate, they maintain a connection with the database and they are not thread-safe, it means you can not share Hibernate Session between multiple threads. Though Session obtains database connection lazily it's good to close session as soon as you are done with it.

Q.12 What is the difference between sorted and ordered collection in hibernate?

This is one of the easy Hibernate interview questions you ever face. A sorted collection is sorted in memory by using java comprator while an ordered collection uses database's order by clause for ordering. For large data set it's better to use ordered collection to avoid any Out of memory in java, by trying to sort them in memory.

Q.13 What is the difference between transient, a persistent and detached object in Hibernate?

In Hibernate, Object can remain in three state transient, persistent or detached.  An object which is associated with Hibernate session is called persistent object. Any change in this object will reflect in the database based on your flush strategy i.e. automatic flush whenever any property of object change or explicit flushing by calling Session.flush() method. On the other hand, if an object which is earlier associated with Session, but currently not associated with it are called detached object. 

You can reattach detached object to any other session by calling either update() or saveOrUpdate() method on that session. Transient objects have newly created an instance of persistence class, which is never associated with any Hibernate Session. Similarly, you can call persist() or save() methods to make a transient object persistent. Just remember, here transient doesn’t represent transient keyword in jav, which is an altogether different thing.

Q14. What is hibernate configuration file?.

Hibernate requires to know in advance where to find the mapping information that defines how your Java classes relate to the database tables. Hibernate also requires a set of configuration settings related to database and other related parameters. All such information is usually supplied as a standard Java properties file called hibernate.properties, or as an XML file named hibernate.cfg.xml.

I will consider XML formatted file hibernate.cfg.xml to specify required Hibernate properties in my examples. Most of the properties take their default values and it is not required to specify them in the property file unless it is really required. This file is kept in the root directory of your application's classpath.

Hibernate Properties:

Following is the list of important properties you would require to configure for a databases in a standalone situation:

S.N.	Properties and Description
1	hibernate.dialect This property makes Hibernate generate the appropriate SQL for the chosen database.
2	hibernate.connection.driver_class The JDBC driver class.
3	hibernate.connection.url The JDBC URL to the database instance.
4	hibernate.connection.username The database username.
5	hibernate.connection.password The database password.
6	hibernate.connection.pool_size Limits the number of connections waiting in the Hibernate database connection pool.
7	hibernate.connection.autocommit Allows autocommit mode to be used for the JDBC connection.

If you are using a database along with an application server and JNDI then you would have to configure the following properties:

S.N.	Properties and Description
1	hibernate.connection.datasource The JNDI name defined in the application server context you are using for the application.
2	hibernate.jndi.class The InitialContext class for JNDI.
3	hibernate.jndi.<JNDIpropertyname> Passes any JNDI property you like to the JNDI InitialContext.
4	hibernate.jndi.url Provides the URL for JNDI.
5	hibernate.connection.username The database username.
6	hibernate.connection.password The database password.

Hibernate with MySQL Database:

MySQL is one of the most popular open-source database systems available today. Let us create hibernate.cfg.xml configuration file and place it in the root of your application's classpath. You would have to make sure that you have testdb database available in your MySQL database and you have a user test available to access the database.

The XML configuration file must conform to the Hibernate 3 Configuration DTD, which is available from http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd.

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE hibernate-configuration SYSTEM

"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

   <session-factory>

   <property name="hibernate.dialect">

      org.hibernate.dialect.MySQLDialect

   </property>

   <property name="hibernate.connection.driver_class">

      com.mysql.jdbc.Driver

   </property>

   <!-- Assume test is the database name -->

   <property name="hibernate.connection.url">

      jdbc:mysql://localhost/test

   </property>

   <property name="hibernate.connection.username">

      root

   </property>

   <property name="hibernate.connection.password">

      root123

   </property>

   <!-- List of XML mapping files -->

   <mapping resource="Employee.hbm.xml"/>

</session-factory>

</hibernate-configuration>

The above configuration file includes <mapping> tags which are related to hibernate-mapping file and we will see in next chapter what exactly is a hibernate mapping file and how and why do we use it. Following is the list of various important databases dialect property type:

Database	Dialect Property
DB2	org.hibernate.dialect.DB2Dialect
HSQLDB	org.hibernate.dialect.HSQLDialect
HypersonicSQL	org.hibernate.dialect.HSQLDialect
Informix	org.hibernate.dialect.InformixDialect
Ingres	org.hibernate.dialect.IngresDialect
Interbase	org.hibernate.dialect.InterbaseDialect
Microsoft SQL Server 2000	org.hibernate.dialect.SQLServerDialect
Microsoft SQL Server 2005	org.hibernate.dialect.SQLServer2005Dialect
Microsoft SQL Server 2008	org.hibernate.dialect.SQLServer2008Dialect
MySQL	org.hibernate.dialect.MySQLDialect
Oracle (any version)	org.hibernate.dialect.OracleDialect
Oracle 11g	org.hibernate.dialect.Oracle10gDialect
Oracle 10g	org.hibernate.dialect.Oracle10gDialect
Oracle 9i	org.hibernate.dialect.Oracle9iDialect
PostgreSQL	org.hibernate.dialect.PostgreSQLDialect
Progress	org.hibernate.dialect.ProgressDialect
SAP DB	org.hibernate.dialect.SAPDBDialect
Sybase	org.hibernate.dialect.SybaseDialect
Sybase Anywhere	org.hibernate.dialect.SybaseAnywhereDialect

Q.15 Hibernate mapping file?.

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE hibernate-mapping PUBLIC

 "-//Hibernate/Hibernate Mapping DTD//EN"

 "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>

   <class name="Employee" table="EMPLOYEE">

      <meta attribute="class-description">

         This class contains the employee detail.

      </meta>

      <id name="id" type="int" column="id">

         <generator class="native"/>

      </id>

      <property name="firstName" column="first_name" type="string"/>

      <property name="lastName" column="last_name" type="string"/>

      <property name="salary" column="salary" type="int"/>

   </class>

</hibernate-mapping>

You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping document in the file Employee.hbm.xml. Let us see little detail about the mapping elements used in the mapping file:

·        The mapping document is an XML document having <hibernate-mapping> as the root element which contains all the <class> elements.

·        The <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.

·        The <meta> element is optional element and can be used to create the class description.

·        The <id> element maps the unique ID attribute in class to the primary key of the database table. The name attribute of the id element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.

·        The <generator> element within the id element is used to automatically generate the primary key values. Set the class attribute of the generator element is set to native to let hibernate pick up either identity, sequence or hilo algorithm to create primary key depending upon the capabilities of the underlying database.

·        The <property> element is used to map a Java class property to a column in the database table. The name attribute of the element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.

Q.16 What are different types of caches available in Hibernate? 
This is another common Hibernate interview question. Hibernate provides the out-of-box caching solution but there are many caches e.g. first level cache, second level cache and query cache. First level cache is maintained at Session level and cannot be disabled but the second level cache is required to be configured with external cache provider like EhCache.

Q.17. Does Hibernate Session interface is thread-safe in Java? 
No, Session object is not thread-safe in Hibernate and intended to be used with-in single thread in the application.

Q.18.What is different between Session and Sessionfactory in Hibernate? 

 This is another popular Hibernate interview question, mostly at a telephonic round of interviews. The main difference between Session and SessionFactory is that former is a single-threaded, short-lived object while later is Immutable and shared by all Session. It also lives until the Hibernate is running. Another difference between Session and SessionFactory is that former provides first level cache while SessionFactory provides the Second level cache.

Q.19. What is criterion query in hibernate? 
Criteria is a simplified API for retrieving entities by composing Criterion objects also known as Criterion query. This is a very convenient approach for functionality like "search" screens where you can filter data on multiple conditions as shown in the following example:

List books = session.createCriteria(Book.class)

.add(Restrictions.like("name", "dilip%") )

.add(Restrictions.like("published by java", "2017"))

.addOrder(Order.asc("salary") )

.list();

This can be a tough question if you are not using Hibernate on a daily basis, I have interviewed several Java developers who have used Hibernate but doesn't know about Criterion query or API.

Q20. What is the difference between save() and saveOrUpdate() method of Hibernate? 
Though both save() and saveOrUpdate() method is used to store object into Database, the key difference between them is that save can only INSERT records but saveOrUpdate() can either INSERT or UPDATE records.

Q.21What is difference between getCurrentSession() and openSession() in Hibernate? 
An interesting Hibernate interview question as you might have used both getCurrentSession() and openSession() to obtain an instance o

Session object.

Q.22 What is Hibernate Query Language (HQL)? 
Hibernate query language, HQL is an object-oriented extension to SQL. It allows you to query, store, update, and retrieve objects from a database without using SQL. This question is also similar to the earlier question about Criterion query, Java developers who have not used Hibernate extensively will not know much about features like HQL and Criterion.

Q.23 When do you use merge() and update() in Hibernate? 
This is one of the tricky Hibernate interview questions. You should use update() if you are sure that the Hibernate session does not contain an already persistent instance with the same id and use merge() if you want to merge your modifications at any time without considering the state of the session.

Q.24 How do you log SQL queries issued by the Hibernate framework in Java application?
You can use the show_sql property to log SQL queries issued by the Hibernate framework, Just add the following line in your Hibernate configuration file:

<property name=”show_sql”> true </property>

Q25. What are the three states of a Hibernate Persistence object can be? 
The Hibernate persistent or entity object can live in following three states:
1) transient
2) persistent
3) detached



New objects created in Java program but not associated with any hibernate Session are said to be in the transient state. On the other hand, an object which is associated with a Hibernate session is called Persistent object. While an object which was earlier associated with Hibernate session but currently it's not associate is known as a detached object. You can call save() or persist() method to store those object into the database and bring them into the Persistent state. Similarly, you can re-attach a detached object to hibernate sessions by calling either update() or saveOrUpdate() method.



Note:-please read this all Question carefully and take job… thnks..