What is the role of a repository in the architecture of a Spring application and why is it used?

In the architecture of a Spring application, the repository is responsible for interacting with the database or another data source. It encapsulates data access logic, providing a convenient interface for CRUD operations (create, read, update, delete). This allows separating business logic from storage details, improving code modularity and testability.

In Spring, the Repository pattern is often used together with Spring Data, where repository interfaces are automatically implemented by the framework.

Example of a repository interface for the User entity:

public interface UserRepository extends JpaRepository<User, Long> {
    List<User> findByLastName(String lastName);
}

Thus, the repository simplifies data handling and promotes a clean architecture of the application.