1. What is ORM ?

ORM stands for object/relational mapping. ORM is the automated persistence of objects in a Java application to the tables in a relational database.

2. ORM’s flow diagram?

3. What does ORM consists of ?

An ORM solution consists of the following four pieces:

• API for performing basic CRUD operations
• API to express queries referring to classes
• Facilities to specify metadata
• Optimization facilities : dirty checking, lazy associations fetching

4. What are the ORM levels ?

The ORM levels are:

• Pure relational (stored procedure)
• Light objects mapping (JDBC)
• Medium object mapping
• Full object Mapping (composition, inheritance, Polymorphism, persistence by Reach-ability)

5. What is Hibernate?

• Hibernate 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 configuration(XML/properties) files.
• Its relieves the developer from a significant amount of relational data persistence-related programming tasks.

6. Why do you need ORM tools like hibernate?

The main advantage of ORM like hibernate is that it shields developers from messy SQL. Apart from this, ORM provides following benefits:

• Improved productivity through High-level object-oriented API, Less Java code to write, No SQL to write
• Improved performance through Sophisticated caching, Lazy loading, Eager loading
• Improved maintainability by A lot less code to write
• Improved portability as it generates database-specific SQL for you

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

7. Hibernate’s Architecture

8. What are the most common methods of Hibernate configuration?

The most common methods of Hibernate configuration are: Programmatic configuration(application.properties in Spring Boot), XML configuration (hibernate.cfg.xml)

10. What are the Core interfaces are of Hibernate framework?

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

11. What role does the Session interface play in Hibernate?

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.

12. What role does the SessionFactory interface play in Hibernate?

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();

13. What is the general flow of Hibernate communication with RDBMS?

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

14. Difference between @Entity and @Table?

@Entity(name="order") : Its a logical construct and the name will be used to name the Entity.

@Table(name="ORDER") : Its a physical construct and the name will be used to name a table in DB

15. Define cascade in relational mappings ?

cascade – enable operations to cascade to child entities when operation is performed on root entity in case of a relational mapped entities(OnetoOne, ManytoOne etc)

@OnetoMany(cascade="all")

All possible values of cascade attributes are “all|none|save-update|delete|all-delete-orphan”

16. Difference between @JoinColumn and MappedBy?

“mappedBy” attribute of mapping annotations(like @OneToOne, @OneToMany, @ManyToMany) enables bi-directional relationship. This attribute allows you to refer the associated entities from both sides. If “A” Table has association with “B” Table then you can get A from B and B from A. This also tells that entity is on this side inverse and the owner is on the other side.

@JoinColumn annotation can be used along with @OneToOne, @OneToMany, @ManyToMany etc, to indicate that this entity is the owner of the relationship (that is: the corresponding table has a column with a foreign key to the referenced table)

@Entity
public class Organization {
    @OneToMany(fetch = FetchType.EAGER, mappedBy = "employee")
    private List<Employee> employees;
}

@Entity
public class Employee {
    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "organizationId")
    private Organization organization;
}

17. What’s the difference between load() and get()?

load() vs. get() :-

18. What is the difference between and merge and update ?

• Use update() if you are sure that the session does not contain an already persistent instance with the same identifier.
• Apply merge() if you want to merge your modifications at any time without consideration of the state of the session.
• merge scenario: call to merge method on detached instance, will update it with updated value.
• detached scenario: call to update method on detached instance, will give exception org.hibernate.NonUniqueObjectException

19. What is Hibernate Query Language (HQL)?

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.

20. What are Named Queries?

Named queries are defined in the mapping xml or top of entity classes and can be invoked wherever they are required

@NamedQueries({ @NamedQuery(name = "@HQL_GET_EMPLOYEES", 
			query = "from Employee") })
class Employee { }

Invoking a named query can be done as below
Query query = session.getNamedQuery("HQL_GET_ALL_EMPLOYEES"); 

21. What are Native Queries?

Native query refers to actual sql queries (referring to actual database objects(not entity objects)). These queries are the SQL statements which are directly executed in database using a database client.

• JPA : entityManager.createNativeQuery()
• Hibernate (Non-JPA implementation): session.createSQLQuery()

22. Explain Criteria API

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();

Difference between Projections and Restrictions

Projections are used to define which part of an entity you want criteria query to retrieve.

criteria.setProjection(Projections.property("name"));

Criteria above when executed will fetch only the name column from the Employee table.

Restrictions, on the other hand, used to define what subset of the data(properties) you want your criteria query to fetch. Restrictions fill the FROM and WHERE part in the query.

criteria.add(Restrictions.eq("department", "Sales"));
criteria.setMaxResults(5);
criteria.addOrder(Order.asc("name"));

23. Define Hibernate Template?

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.

24. What are the benefits does HibernateTemplate provide?

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.

25. How do you switch between relational databases without code changes?

Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined.

hibernate.dialect=org.hibernate.dialect.MySQL5Dialect

26. How can we see the Hibernate generated SQL statements on console 

In Hibernate configuration file set as follows (application.properties in Spring Boot)

hibernate.show_sql= true

27. What are derived properties?

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.

28. What is Component mapping in Hibernate?

• 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

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

sorted collection vs. order collection :-

31. What is the advantage of Hibernate over jdbc?

Hibernate Vs. JDBC:-

32. What are the Collection types in Hibernate ?

Bag ,Set, List, Array, Map

33. What are the ways to express joins in HQL?

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.

34. Define cascade and inverse option in one-many mapping?

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?

35. What is Hibernate proxy?

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. Example: session.load() method returns a proxy object.

36. How can Hibernate be configured to access an instance variable directly and not through a setter method ?

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.

37. How can a whole class be mapped as immutable?

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.

38. What is the use of dynamic-insert and dynamic-update attributes in a class mapping?

• 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.

39. What do you mean by fetching strategy ?

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.

Example: @OneToMany(fetch=fetchType.LAZY)
         @ManyToOne(fetch=fetchType.EAGER)

40. What is automatic dirty checking?

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.

41. What is transactional write-behind?

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.

42. What are Callback interfaces?

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.

43. What are the types of Hibernate instance states ?

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

44. What are the differences between EJB 3.0 & Hibernate

45.What are the types of inheritance models in Hibernate?

There are three types of inheritance models in Hibernate:

• Table per class hierarchy

@Inheritance(strategy=InheritanceType.SINGLE_TABLE), @DiscriminatorColumn on the parent class @DiscriminatorValue annotations on childclass are used for mapping table per hierarchy strategy. In case of table per hierarchy, only one table is required to map the inheritance hierarchy. Here, an extra column (also known as discriminator column) is created in the table to identify the class.

• Table per concrete class

@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) annotation in the parent class and @AttributeOverrides annotation in the subclasses. Disadvantage of this is that duplicate columns are created in sub class tables

• Table per subclass

Tables are created as per persistent classes but they are treated using primary and foreign key. So there will not be any duplicate column in the relation. We need to specify @Inheritance(strategy=InheritanceType.JOINED) in the parent class and @PrimaryKeyJoinColumn annotation in the subclasses.

46. What is purpose of session.flush() method?

48. Second level cache?

Second level cache can be implemented by following poviders

• EH Cache
• OS Cache
• Swarm Cache
• JBoss Cache

Each of the above provides different cache usage functionality. There are four ways to use second level cache.

1. read-only: caching will work for read only operation.
2. nonstrict-read-write: caching will work for read and write but one at a time.
3. read-write: caching will work for read and write, can be used simultaneously.
4. transactional: caching will work for transaction.

@Cacheable,  @Cache(usage=CacheConcurrencyStrategy.READ_ONLY)  are used on top of Entity classes and below properties needs to be added to the application

cache.use_second_level_cache=true   
cache.region.factory_class=org.hibernate.cache.ehcache.EhCacheRegionFactory

49. What is optimistic locking and how it is implemented?

In long running transaction scenarios, optimistic locking becomes a necessity to ensure there is no performance hit. Without optimistic locking, tables could be locked for long duration avoiding concurrent access.

Two approaches to optimistic locking are timestamp and version number.

• version number is preferred over the timestamp approach.
• It is implemented using @Version annotation. Entity manager use the VERSION_COLUMN to detect conflicting updates.

    @Version
    @Column(name="VERSION_COLUMN")
    public Integer getVersion() {}