Hibernate

 Hibernate Dev Process - To do List

1. Add Hibernate Configuration file
2. Annotate Java Class
3. Develop Java code to perform database operations

Entity Class: Java class that is mapped to a database table.

Two options for Mapping

1. XML config file (legacy)
2. Java Annotations (modern, preferred)

Java Annotations

1. Map class to database table
2. Map fields to database columns

SessionFactory : 

    Reads the hibernate config file. 
    Creates Session Objects
    Heavy-weight Object
    Only create once in your app

Session

     Wraps a JDBC connection
     Main Object used to save/retrieve objects
     Short-lived object
     Retrieved from SessionFactory

Primary key ID Generation Strategies 

GenerationType.AUTO   :  Pick an appropriate strategy for the particular database
GenerationType.IDENTITY : Assign primary keys using database identity column
GenerationType.SEQUENCE : Assign primary keys using a database sequence
GenerationType.TABLE : Assign primary keys using an underlying database table to ensure uniqueness 

custom ID generation

We can define your own CUSTOM generation strategy
Create implementation of org.hibernate.id.IdentifierGenerator
Override the method public Serializable generate()

https://www.baeldung.com/querydsl-with-jpa-tutorial

hibernate dialect 

https://www.javatpoint.com/dialects-in-hibernate

In hibernate we will begintranscation and and commit the transaction for select as well whereas in sql we will do that only for DML script

Hibernate Query Language (HQL)

Query language for retrieving objects
similar in nature to SQL

   where, like, order by, join, in, etc ...

Advance Mapping

One - to - One 
One - to - Many, Many - to - One
Many - to - Many

Important Database concepts

Primary key and Foreign key
Cascade 

Cascade:

You can cascade operations
Apply the same operations to related entities 

Uni-Directional

Bi-Directional

Referential Integrity

Entity LifeCycle

Detach : If entity is detached, it is not associated with a Hibernate session

Merge: If instance is detached from session, then merge will reattach to session

Persist: Transitions new instance to managed state. Next flush/commit will save in db

Remove: Transitions managed entity to be removed. Next flush/commit will delete from  db

Refresh: Reload/synch object with data from db. Prevents stale data

Cascade Types

PERSIST  : If entity is persisted/saved , related entity will also be persisted

REMOVE: If entity is removed/deleted, related entity will also be deleted

REFRESH: If entity is refreshed, related entity will also be refreshed

DETACH  : If entity is detached(not associated w/ session), then related entity will also be detached.

MERGE    : If entity is merged, then related entity will also be merged

ALL           : All of above cascade types

mappedBy

Look at the 

Annotations 

@Entity

@Table

@Id

@Column

@GeneratedValue

@OneToOne

@ManyToOne

@OneToMany

@ManytoMany

@JoinColumn