This tutorial will walk you through the steps of mapping a JPA and Hibernate One To One bidirectional entity relationships example with Spring Boot, Spring Data JPA, Lombok, MySQL and Docker
What you will need
- JDK 8+ or OpenJDK 8+
- Maven 3+
- MySQL Server 5+ or Docker CE 18+
Init project structure and dependencies
Project structure
├── src
│ └── main
│ ├── java
│ │ └── com
│ │ └── hellokoding
│ │ ├── jpa
│ │ │ ├── book
│ │ │ │ ├── Book.java
│ │ │ │ ├── BookDetail.java
│ │ │ │ ├── BookDetailRepository.java
│ │ │ │ └── BookRepository.java
│ │ │ └── JpaApplication.java
│ │ └── springboot
│ └── resources
│ └── application.properties
├── Dockerfile
├── docker-compose.yml
└── pom.xml
Project dependencies
Define JPA Entities and Repositories
One-To-One Relationship
One to one relationship refers to the relationship between two entities/tables A and B in which one item/row of A may be linked with only one item/row of B, and vice versa.
In this example, library and address tables have a one-to-one relationship. A library has only one address, and an address is the address of only one library.
library.address_id is a foreign key references to address.id.
Try this example if the relationship owner uses its foreign key as primary key JPA/Hibernate One To One Shared Primary Key Bidirectional Relationship Mapping
Define 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.
@OneToOne defines a one-to-one relationship between 2 entities.
@JoinColumn defines foreign key column and indicates the owner of the relationship.
mappedBy indicates the inverse of the relationship.
unique = true enforces the unique constraint, 1 address belongs to only 1 library.
Spring Data JPA Repository
Spring Data JPA contains some built-in Repository abstracting common functions based on EntityManager to work with database such as findAll, findById, save, delete, deleteById. All we need for this example is extends JpaRepository.
Define properties and creating data
Application Properties
hk-mysql refers to Docker Compose service defined in the below docker-compose.yml file
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 Address will be saved at the same time with the owner Library 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
docker-compose up
Access to MySQL Server docker container by issuing below bash command and key in hellokoding on Enter password:
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
Update hk-mysql on application.properties to localhost and type the below command at the project root directory
mvn clean spring-boot:run