This tutorial will walk you through the steps of mapping a JPA and Hibernate One To Many bidirectional entity relationships example with with Spring Boot, Spring Data JPA, Lombok, MySQL and Docker
What you'll need
- JDK 8+ or OpenJDK 8+
- Maven 3+
- MySQL Server 5+ or Docker CE 18+
Init project structure and dependencies
├── src │ └── main │ ├── java │ │ └── com │ │ └── hellokoding │ │ ├── jpa │ │ │ ├── book │ │ │ │ ├── Book.java │ │ │ │ ├── BookCategory.java │ │ │ │ ├── BookCategoryRepository.java │ │ │ │ └── BookRepository.java │ │ │ └── JpaApplication.java │ │ └── springboot │ └── resources │ └── application.properties ├── Dockerfile ├── docker-compose.yml └── pom.xml
Define JPA Entities and Repositories
One-to-many relationship refers to the relationship between two entities/tables A and B in which one element/row of A may be linked with many elements of B, but a member of B is linked to only one element of A.
In this example, the
book tables have a one-to-many relationship. One category may be linked with many books but one book is linked to only one category.
book.book_category_id is a foreign key references to
JPA and Hibernate Entities
JPA Entity is defined with
@Entity annotation, represent a table in your database.
@Id declares the entity identifier.
@Column maps the entity's field with the table's column. If
@Column is omitted, the field name of the entity will be used as column name by default.
@ManyToOne defines a one-to-many and many-to-one relationship between 2 entities.
@JoinColumn indicates the entity is the owner of the relationship and the corresponding table has a column with a foreign key to the referenced table.
mappedBy indicates the entity is the inverse of the relationship.
Spring Data JPA Repository
Spring Data JPA contains some built-in
Repository abstracting common functions based on
EntityManager to work with database such as
deleteById. All we need for this example is
Define properties and creating data
hk-mysql refers to Docker Compose service defined in the below
spring.jpa.hibernate.ddl-auto=create allows JPA/Hibernate auto create database and table schema for you.
On production environment, you may like to disable the DDL Auto feature by using spring.jpa.hibernate.ddl-auto=validate or spring.jpa.hibernate.ddl-auto=none (default). Check out this example as one of the approaches Database Migration/Evolution Example with Flyway and JPA/Hibernate
Creating data with JPA and Hibernate
Thanks to CascadeType.ALL, associated entity
Book will be saved at the same time with the owner
BookCategory without the need of calling its save function explicitly
Run the example
Run with Docker
Prepare Dockerfile for Java/Spring Boot application and docker-compose.yml for MySQL Server
Type the below command at the project root directory, make sure your local Docker is running
Access to MySQL Server docker container by issuing below bash command and key in
docker exec -it hk-mysql mysql -p
Query schema and data created by JPA/Hibernate based on your mapping
Run with JDK/OpenJDK, Maven and MySQL Server local
localhost and type the below command at the project root directory
mvn clean spring-boot:run