Spring Data JPA & Hibernate

Map Java objects to tables, model relationships, write repository queries and avoid the common performance traps.

This section is being written. There are no lessons in it yet: below is the plan for what it will cover. It opens in the syllabus as soon as its first lessons are ready.

What you will be able to do

Explain what ORM, JPA and Hibernate are and when to use them
Map classes to tables, including one-to-many and many-to-many relationships
Write repository queries with derived methods, JPQL and native SQL
Use pagination, sorting and transactions in a Spring application
Find and fix the N+1 problem and other common JPA performance traps
Manage schema changes with Flyway migrations

What it will cover

16.1ORM, JPA & Hibernate
Why we map objects to tables and how JPA, Hibernate and Spring Data relate.
- The object-relational mismatch
- JPA (the specification) vs Hibernate (the implementation)
- Spring Data JPA on top of both
- Adding the starter and configuring PostgreSQL
- Seeing the generated SQL in the log
16.2Entities & Mapping
Turn Java classes into database tables with annotations.
- @Entity, @Table, @Id and @GeneratedValue
- @Column, lengths, nullability and uniqueness
- Enums, dates and BigDecimal fields
- Embedded value objects with @Embeddable
- Auditing: created and updated timestamps
- equals and hashCode for entities
16.3Repositories & Queries
Read and write data with Spring Data repositories and custom queries.
- JpaRepository and the built-in CRUD methods
- Derived query methods from method names
- @Query with JPQL
- Native SQL queries
- Projections and DTO queries
- @Modifying update and delete queries
16.4Relationships
Map one-to-one, one-to-many and many-to-many links between entities.
- @ManyToOne and @OneToMany
- Owning side, mappedBy and join columns
- Bidirectional helpers to keep both sides in sync
- @ManyToMany and when to use a join entity instead
- @OneToOne
- Cascade types and orphanRemoval
16.5The Persistence Context & Entity Lifecycle
Understand how Hibernate tracks objects so its behaviour stops being a surprise.
- Transient, managed, detached and removed states
- Dirty checking: updates without save()
- Flush and when SQL is actually sent
- save vs saveAndFlush, persist vs merge
- LazyInitializationException and why it happens
16.6Transactions in Spring
Use @Transactional correctly so business operations are all-or-nothing.
- @Transactional on service methods
- Rollback rules for checked and unchecked exceptions
- Read-only transactions
- Propagation basics: REQUIRED and REQUIRES_NEW
- Self-invocation and other reasons @Transactional is ignored
- Optimistic locking with @Version vs pessimistic locks
16.7Pagination, Sorting & Searching
Return large result sets in pages and support filters.
- Pageable, Page and Slice
- Sorting by one or more fields
- Exposing page and sort in REST endpoints
- Optional filters with Specifications
- Keyset pagination for large tables (the idea)
16.8Performance Traps
Spot and fix the problems that make JPA applications slow.
- Lazy vs eager fetching
- The N+1 query problem
- Fixes: JOIN FETCH, @EntityGraph and batch fetching
- Returning DTOs instead of entities
- Open Session in View and why to turn it off
- Counting queries in tests and logs
16.9Schema Migrations with Flyway
Version your database schema like code instead of letting Hibernate change it.
- Why ddl-auto=update is unsafe
- Adding Flyway and writing V1 migration scripts
- Changing a live schema with new migrations
- Seed data and repeatable migrations
- Validating entities against the schema
16.10JPA Mini Project: ShopLite on JPA
Move ShopLite from JdbcTemplate to Spring Data JPA with relationships, paging and migrations.
- Entities for products, categories, customers, orders and order items
- Flyway scripts from the original SQL schema
- Paged and filtered product search
- Order placement with @Transactional and stock locking
- Fixing N+1 in the order history endpoint

Start with what’s live

These sections are ready to study now.

See the whole roadmap

What you will be able to do

Explain what ORM, JPA and Hibernate are and when to use them

Map classes to tables, including one-to-many and many-to-many relationships

Write repository queries with derived methods, JPQL and native SQL

Use pagination, sorting and transactions in a Spring application

Find and fix the N+1 problem and other common JPA performance traps

Manage schema changes with Flyway migrations

What it will cover

16.1ORM, JPA & Hibernate

Why we map objects to tables and how JPA, Hibernate and Spring Data relate.

The object-relational mismatch
JPA (the specification) vs Hibernate (the implementation)
Spring Data JPA on top of both
Adding the starter and configuring PostgreSQL
Seeing the generated SQL in the log

16.2Entities & Mapping

Turn Java classes into database tables with annotations.

@Entity, @Table, @Id and @GeneratedValue
@Column, lengths, nullability and uniqueness
Enums, dates and BigDecimal fields
Embedded value objects with @Embeddable
Auditing: created and updated timestamps
equals and hashCode for entities

16.3Repositories & Queries

Read and write data with Spring Data repositories and custom queries.

JpaRepository and the built-in CRUD methods
Derived query methods from method names
@Query with JPQL
Native SQL queries
Projections and DTO queries
@Modifying update and delete queries

16.4Relationships

Map one-to-one, one-to-many and many-to-many links between entities.

@ManyToOne and @OneToMany
Owning side, mappedBy and join columns
Bidirectional helpers to keep both sides in sync
@ManyToMany and when to use a join entity instead
@OneToOne
Cascade types and orphanRemoval

16.5The Persistence Context & Entity Lifecycle

Understand how Hibernate tracks objects so its behaviour stops being a surprise.

Transient, managed, detached and removed states
Dirty checking: updates without save()
Flush and when SQL is actually sent
save vs saveAndFlush, persist vs merge
LazyInitializationException and why it happens

16.6Transactions in Spring

Use @Transactional correctly so business operations are all-or-nothing.

@Transactional on service methods
Rollback rules for checked and unchecked exceptions
Read-only transactions
Propagation basics: REQUIRED and REQUIRES_NEW
Self-invocation and other reasons @Transactional is ignored
Optimistic locking with @Version vs pessimistic locks

16.7Pagination, Sorting & Searching

Return large result sets in pages and support filters.

Pageable, Page and Slice
Sorting by one or more fields
Exposing page and sort in REST endpoints
Optional filters with Specifications
Keyset pagination for large tables (the idea)

16.8Performance Traps

Spot and fix the problems that make JPA applications slow.

Lazy vs eager fetching
The N+1 query problem
Fixes: JOIN FETCH, @EntityGraph and batch fetching
Returning DTOs instead of entities
Open Session in View and why to turn it off
Counting queries in tests and logs

16.9Schema Migrations with Flyway

Version your database schema like code instead of letting Hibernate change it.

Why ddl-auto=update is unsafe
Adding Flyway and writing V1 migration scripts
Changing a live schema with new migrations
Seed data and repeatable migrations
Validating entities against the schema

16.10JPA Mini Project: ShopLite on JPA

Move ShopLite from JdbcTemplate to Spring Data JPA with relationships, paging and migrations.

Entities for products, categories, customers, orders and order items
Flyway scripts from the original SQL schema
Paged and filtered product search
Order placement with @Transactional and stock locking
Fixing N+1 in the order history endpoint