Hibernate - Interview Questions and Answers for 'Hibernate' | Search Interview Question

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Q31. How many SessionFactory and Session objects are created ?

Ans. Single SessionFactory object and multiple session objects for opening different session. Hibernate creates new Session object per thread.

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Q32. What is the use of hbm2ddl Configuration in Hibernate ?

Hibernate

Ans. This configuration specifies if hibernate should creates the Schema / Table on its own if the respective table is not found.

"update" doesn't create the table if it's not found whereas configuration set as "create" creates the schema automatically.

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Q33. What is the difference between these 2 annotations ?

@Entity

@Entity ( name="EMPLOYEES" )

Hibernate

Ans. The first annotation will try to map the Class with the Table as of same name as Class whereas the second annotation will specify the Entity name as "EMPLOYEES" and hence will try to map with Table Name "EMPLOYEES".

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Q34. What is the difference between these 2 annotations ?

@Entity ( name ="EMPLOYEES")
@Entity @Table ( name=""EMPLOYEES"" )

@Entity ( name="EMP")
@Table ( name="EMPLPYEES" )

Hibernate

Ans. First Annotation will set the Entity name as EMPLOYEES and hence will try to map with the same Table name.

The second annotation will make the Entity mapped to table EMPLOYEES irrespective of the Entity Name ( which is class name in this case ).

Third Annotations will set the different names for Enitity and Table and will explicitly map them.

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Q35. What are the different ID generating strategies using @GeneratedValue annotation ?

Hibernate

Ans. Auto , Identity , Sequence and Table.

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Q36. How to do Eager loading in Hibernate ?

Hibernate

Ans. Using

lazy = false in hibernate config file

or

@Basic(fetch=FetchType.EAGER) at the mapping

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Q37. What is cascade ?

Hibernate

Ans. Instead of Saving Parent as well as Child Entities individually , Hibernate provides the option to persist / delete the related entities when the Parent is persisted.

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Q38. What are the different Cascade types ?

Hibernate

Ans. Detach, Merge , Persist , Remove , Refresh

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Q39. Which type of associated Entities are Eagerly loaded by Default ?

Hibernate

Ans. OneToOne

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Q40. After which Hibernate version , related Entities are initialized lazily ?

Hibernate

Ans. After Hibernate 3.0

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Q41. Can we declare Entity class as final ?

Hibernate

Ans. Yes but as Hibernate creates the Proxy Classes inherited from the Entity Classes to communicate with Database for lazy initialization. Declaring entity classes as final will prohibit communication with database lazily and hence will be a performance hit.

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Q42. What is the difference between JPA and Hibernate ?

Hibernate

Ans. JPA or Java Persistence API is a standard specification for ORM implementations whereas Hibernate is the actual ORM implementation or framework.

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Q43. What is the advantage of JPA ?

Database

Ans. Its a specification that guides the implementation of ORM frameworks. Implementations abiding by the specification would mean that one can be replaced with other in an application without much hassle. Only the Features that are added over the specification needs to be taken care of if any such change is made.

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Q44. What is Criteria in Hibernate ?

Hibernate

Ans. Criteria is a simplified API for retrieving entities by composing Criterion objects.

For example - session.createCriteria(Employee.class).add( Restrictions.like("name", "A%") ).list();

will return all employee objects having name starting with A.

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Q45. How do we specify the criteria if it involves mapping between two entities or join between tables ?

Hibernate

Ans. The following code returns the list of Employee objects having employee name starting with A and Dept Name ( Department , Employee Mapped ).

session.createCriteria(Employee.class,"emp")
.createAlias("emp.department", "dept",Criteria.INNER_JOIN)
.add( Restrictions.like("name", "A%") )
.add(Restrictions.eq("dept.name","Finance")
.list();

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Q46. Name few Restriction Methods ?

Hibernate

Ans. eq, ge, gt , between, in , isNull, isEmpty, isNotnull, ne , like, lt , or , not

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Q47. In Hibernate, While defining Criteria, Have you ever faced any problem while adding restrictions with user defined types or Enums ?

Hibernate

Ans. Yes, with Enum as was getting an exception while doing equality check for enum field.

got it fixed by adding @Enumerated(EnumType.STRING) to the field in entity.

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Q48. What is your choice while writing queries within Hibernate - criteria or HQL ?

Hibernate

Ans. I prefer using Criteria for Dynamic queries and HQL for static queries.

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Q49. How to specify the or/and combination restrictions within Criteria in Hibernate ?

Hibernate

Ans. session.createCriteria(Employee.class).add( Restrictions.or(Restrictions.like("name", "A%"),Restrictions.like("name", "B%"))).list();

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Q50. Difference between JDBC and Hibernate ?

Database

Ans. JDBC is a standard Java Api for Database communication whereas Hibernate is an ORM framework that uses JDBC to connect with Database.

Hibernate is another layer of object table mapping over JDBC that provide a database independent, object oriented and an efficient way of database communication.

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Q51. Can we have an entity in hibernate without primary Key ?

Hibernate

Ans. No, Every entity in hibernate needs to have a key, either primary or composite.

If we dont have a primary key on table, there are various ways this problem can be countered.

1. By using composite key on entity ( make sure that the appropriate unique constraint in defined on columns in Database )

2. By mapping Id in entity to ROWID of table.

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Q52. How are DB transactions handled in Hibernate ?

Hibernate

Ans. // Non-managed environment idiom
Session sess = factory.openSession();
Transaction tx = null;
try {
tx = sess.beginTransaction();

// do some work
...

tx.commit();
}
catch (RuntimeException e) {
if (tx != null) tx.rollback();
throw e; // or display error message
}
finally {
sess.close();
}

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Q53. What are the points to be considered if we move from Eager initialization to Lazy Initialization in Hibernate ?

Hibernate

Ans. Make sure that the properties of dependent Hibernate entities are not accessed and if yes, better wrap the whole code within single transaction.

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Q54. How does Lazy Initialization helps improving performance of an application ?

Hibernate

Ans. Lazy Initialization means , Load Dependencies when required. Which means less load on application resources as only required data is loaded upfront. It's not only good for better performance but for better resource utilization too.

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Q55. Difference between save and flush in Hibernate ?

Hibernate

Ans. Save is used to save the entity to the session whereas flush is to save the session content to DB.

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Q56. What is a fetch mode in Hibernate ?

Hibernate

Ans. It specifies an association fetching strategy. It could be DEFAULT, JOIN or SELECT as following

@Fetch(FetchMode.SELECT)

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Q57. What are the methods to connect to database in Java ?

Database

Ans. JDBC ( Java Database Connectivity ),ODBC (Open Database Connectivity), Hibernate, JPA ( Java Persistence API ), JOOQ,MyBatis

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Q58. Can we have 2 entity fields mapped to the same column in Hibernate ?

Hibernate

Ans. No, it's not legal in hibernate. 1 column should be mapped to only one field in entity.

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Q59. If we would like to load just the mapping id for the mapped entity, how can we accomplish that.

For example -

Employee entity is mapped to department entity through department id. We want that when we load employee.getDepartment().getDepartmentId(), it shouldn't load the complete department object but just the Id.

Hibernate

Ans. We can use

@AccessType("property") for getDepartmentId() method within Department class in that case.

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Q60. If we would like to load just the mapping id for the mapped entity in some cases and whole entity in some cases, because we would like to lazily initialize the dependent entity in some case and not load the entity at all in some case, how can we accomplish that.

For example -

Employee entity is mapped to department entity through department id. We want that when we load employee.getDepartment().getDepartmentId(), it shouldn't load the complete department object but just the Id.

Hibernate

Ans. a

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