Hibernate

Front 1

What is hibernate?

 

Back 1

Hibernate is an open-source and lightweight ORM tool that is used to store, manipulate and retrieve data from the database.

Front 2

What is ORM?

Back 2

ORM is an acronym for Object/Relational mapping. It is a programming strategy to map object with the data stored in the database. It simplifies data creation, data manipulation and data access.

Front 3

What are the core interfaces of Hibernate?

Back 3

The core interfaces of Hibernate framework are:

* Configuration
* SessionFactory
* Session
* Query
* Criteria
* Transaction

Front 4

What is SessionFactory?

Back 4

SessionFactory provides the instance of Session. It is a factory of Session. It holds the data of second level cache that is not enabled by default.

Front 5

Is SessionFactory a thread-safe object?

Back 5

Yes, SessionFactory is a thread-safe object, many threads can access it simultaneously.

Front 6

What is Session?

Back 6

It maintains a connection between hibernate application and database.

It provides methods to store, update, delete or fetch data from the database such as persist(), update(), delete(), load(), get() etc.

It is a factory of Query, Criteria and Transaction i.e. it provides factory methods to return these instances.

Front 7

Is Session a thread-safe object?

Back 7

No, Session is not a thread-safe object, many threads can't access it simultaneously. In other words, you cannot share it between threads.

Front 8

What is the difference between session.save() and session.persist() method?

Back 8

save()

1)returns the identifier (Serializable) of the instance.

2)Syn: public Serializable save(Object o)

persist()

1)return nothing because its return type is void

2)Syn: public void persist(Object o)

Front 9

What is the difference between get and load method?

Back 9

get()

Returns null if object is not found;get() method always hit the database; It returns real object not proxy; It should be used if you are not sure about the existence of instance.

load()

Throws ObjectNotFoundException if object is not found;load() method doesn't hit the database;It returns proxy object; It should be used if you are sure that instance exists.

 

 

 

 

Front 10

What is the difference between update and merge method?

Back 10

update() should be used if session doesn't contain an already persistent state with same id. It means update should be used inside the session only. After closing the session it will throw error.

merge() should be used if you don't know the state of the session, means you want to make modification at any time.

Front 11

What are the states of object in hibernate?

Back 11

1. Transient: The object is in transient state if it is just created but has no primary key (identifier) and not associated with session.
2. Persistent: The object is in persistent state if session is open, and you just saved the instance in the database or retrieved the instance from the database.
3. Detached: The object is in detached state if session is closed. After detached state, object comes to persistent state if you call lock() or update() method.

Front 12

What are the inheritance mapping strategies?

Back 12

There are 3 ways of inheritance mapping in hibernate.

1. Table per hierarchy
2. Table per concrete class
3. Table per subclass

Front 13

How to make a immutable class in hibernate?

Back 13

If you mark a class as mutable="false", class will be treated as an immutable class. By default, it is mutable="true".

Front 14

Is it possible to perform collection mapping with One-to-One and Many-to-One?

Back 14

No, collection mapping can only be performed with One-to-Many and Many-to-Many

Front 15

What is lazy loading in hibernate?

Back 15

Lazy loading in hibernate improves the performance. It loads the child objects on demand.

Since Hibernate 3, lazy loading is enabled by default, you don't need to do lazy="true". It means not to load the child objects when parent is loaded.

Front 16

What is HQL (Hibernate Query Language)?

Back 16

Hibernate Query Language is known as an object oriented query language. It is like structured query language (SQL).

The main advantage of HQL over SQL is:

1. You don't need to learn SQL
2. Database independent
3. Simple to write query

Front 17

What is the difference between first level cache and second level cache?

Back 17

First Level Cache

1)First Level Cache is associated with Session.

2)It is enabled by default.

Second Level Cache

1)Second Level Cache is associated with SessionFactory.

2)It is not enabled by default.