Hibernate

Question 1

Merge vs Update (hibernate)

Answer 1

update can fail if an instance of the object is already in the session.
Merge should be used in that case

Question 2

flush vs commit

Answer 2

session.flush() communicates a series of operations to the database (insert, update, delete).
The database maintains them as pending operations in a transaction.
The changes aren’t persisted permanently to disk, or visible to other transactions until the database receives a COMMIT

Question 3

Concept of JPA (entity manager,
persistence context, persistence
unit)

Answer 3

specification that defines the management of relational data in a Java application

Entity Manager: Manages entities and handles database operations in JPA.
Persistence Context: The cache that holds managed entities within a transaction.
Persistence Unit: A configuration unit that defines database connection settings and entity classes for JPA.

Question 4

get vs load (hibernate)

Answer 4

Get() returns the object by fetching it from database
or from hibernate cache whereas load() just returns
the reference of an object that might not actually exist

Question 5

entity states in Hibernate

Answer 5

New - no mapped to db row
Persistent - associated with db row
Detached - is in db, but not in session. any changed will not affect db
Removed - still in session, waiting for pysical remove

Question 6

Collection types in Hibernate

Answer 6

same + bag

Question 7

Caching levels Hibernate

Answer 7

session cache
Second Level Cache - when objects loaded by PK
query cache

Question 8

FetchMode

Answer 8

fetchmode - defines how hibernate will fetch data from db

SELECT - creates small queries
BatchSize - fixed data set
JOIN - produce duplicates, but 1 query