JPA, Hibernate and Spring Data JPA

JPA Lifecycle Callbacks

• @PrePersist — fires before INSERT
• @PostPersist — fires after INSERT
• @PreUpdate — fires before UPDATE
• @PostUpdate — fires after UPDATE
• @PreRemove — fires before DELETE

They are hooks, when a persist method is called, JPA fires the chosen method We can use these hooks to decide when certain actions should happen

for example:

@PrePersist
public void prePersist() {
    createdAt = LocalDateTime.now();
}

With @PrePersist, Before INSERT is fired, the hook is called and we set the variable createdAt = LocalDateTime.now()

persist → saving data to a permanent storage
transient → object is just stored in memory

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JPA vs Hibernate vs Spring Data JPA

JPA is the the standard specification - a set of rules and interfaces that define how Java objects should map to database dables

Hibernate is the implementation of JPA - it’s the engine doing the work:

• Generating SQL
• Managing Connections
• Handling Transactions, etc

When we use @Entity, or @Column or other JPA annotations - Hibernate reads them and uses them as instructions in talking with the database

Spring Data JPA is a layer on top of Hibernate that reduces the need for boilerplate code even more

• Works by extending JpaRepository and Spring handles it for us
  • With JpaRepository we can interact with DB without even touching sql

Basic CRUD:

function	description
save(entity)	INSERT or UPDATE
findById(id)	SELECT by primary key
findAll()	SELECT all rows
delete(entity)	DELETE
deleteById(id)	DELETE by primary key
count()	COUNT(*)
existsById(id) —	exists check

Derived query methods

// you write this:
List<ConversationHistory> findBySessionIdOrderByCreatedAtAsc(String sessionId);

// Spring generates this SQL:
SELECT * FROM conversation_history 
WHERE session_id = ? 
ORDER BY created_at ASC

For more complex queries we just use @Query

@Query("SELECT c FROM ConversationHistory c WHERE c.sessionId = :sessionId AND c.createdAt > :since")
List<ConversationHistory> findRecentBySession(String sessionId, LocalDateTime since);