A Repository is another DDD concept that is used to describe an abstraction that mediates the domain model from data mapping layers.

With Spring Data, we use an @Repository annotation to wrap common persistence operations for a specific Entity type and, hopefully, keep the underlying storage details out of the domain.

To create a Patient Repository, first create a package called repositories in the base package and then create a PatientRepository Java interface. Finally, add the following code to the interface:

@Repository
public interface PatientRepository extends JpaRepository<Patient, Long> {
}

Surprisingly, that’s not much code. But what does it actually do?

By extending the JpaRepository class we are creating a custom Spring Data Repository for the Patient Entity that will be initialized with a JPA entity manager. It is a powerful abstraction in Spring Data that also provides several default CRUD operations and a custom query system that we will discuss later in more detail.

When Spring manages the Repository, it can generate JPA queries based on the Entity definition that it is typed to and we will rarely have to deal with JPA directly.

// TODO: talk about why I’m not using @RestRepositories. In short, I find Spring Data REST to be a little too opinionated and harder to customize.

Before we get too far ahead of ourselves, lets enable some basic configuration options that will help with future debugging:

Edit the default application.properties file in the src/main/resources folder and add:

spring.jpa.show-sql=true
spring.jpa.properties.hibernate.format_sql=true

I do STRONGLY recommend adding these properties and keeping them set for the duration of your local development. You will frequently need to review the SQL that is being created and executed by Hibernate.