Benefits and usage of spring data JPA

What are the benefits of using spring data JPA?

Spring data JPA addresses the following situations:

• unclear how the persistence layer will develop
• the first prototype starts because of time and focus with a map, later it probably it will be replaced with a longterm persistence
• persistence layer might change from relational to NoSQL or vice versa 
• for the sake of a set of fast running unit-tests the persistence layer might be configured to use light-weight persistence like a simple map
• …

To be really open regarding the persistence layer, the domain layer should be separated from the data access layer. For this an approach like the repository pattern from Martin Fowler is common praxis.

The repository enforces to treat objects of a type as a "conceptual set" like a collection. 

With a simple DAO approach you see the DAO as a gateway for accessing the database. 

This DAO tend to grow extensively as new querying or update functionality is needed.

This leads to poor responsibility. With the repository you treat all the objects as a conceptual set. 

For querying and update extensions the repository will make usage of DAO(s). 

So the DAO are well-focussed and have a single responsibility for gathering / changing data.

The set objects of a type are handled in the repository.

In the beginning of your development you can just have a simple in-memory storage as a map 

in order to focus on the domain logic etc. Later you can delegate the storage and access to sophisticated DAO(s).

So the domain objects used by the business logic in the domain layer are developed against the interfaces that are used by the repository interfaces.

On the top of the jpa entities the repository layer is placed.

Next to the domain objects the repository interfaces are placed. They always present to the outside the interfaces that are used by the domain layer which are exposed by the repository interfaces. These repository interfaces provide basic CRUD functionalities.

Example:

domain layer: Customer implements ICustomer

repository layer: CustomerRepository delivers ICustomer

persistence layer: CustomerRepositoryImpl implements the CustomerRepository

The CustomerRepositoryImpl also could make further usage of DAOs to access the objects.

The CustomerRepositoryImpl will make usage of the EntityManager of JPA and will define the transactional context:

public class CustomerRepositoryImpl implements CustomerRepository {

    private EntityManager entityManager;

    @Transactional

    public ICustomer save(ICustomer customer) {

      Customer c = new Customer(customer);

       entityManager.persist©;

       return c;

    } 

}

Usage of spring data jpa

The spring data jpa has the objective to simply the development of the repository layer, mentioned above as this code is boilerplate-code. With spring data jpa you only have to define the interface, an implementation for delegation and provide the corresponding spring configuration.

The rest will be instantiated and delivered by Spring.

So first of all we have to define the repository interface:

public interface CustomerJpaRepository extends JpaRepository<Customer, Long> {

    Customer save(Customer customer);

}

Second we need the spring configuration:

<jpa:repositories base-package="…">

   <jpa:repository id="customerJpaRepository" />

</jpa:repositories>

Unfortunately spring data jpa can't operate directly on the interface defined as the CustomerJpaRepository. It always needs the specific jpa entity. 

Therefore we need to implement the interface of CustomerJpaRepository.

But we will inject an instance of the CustomerJpaRepository and all operations will delegate to this instance:

public class CustomerJpaRepositoryImpl implements CustomerJpaRepository {

       private CustomerJpaRepository repo;

      public Customer save(Customer customer) {

           return repos.save(customer);

      }  

}

In the background spring data jpa will dynamically provide an instance of the interface and make it available under the id customerJpaRepository.

Advantages of spring data jpa

Spring data jpa provides finder methods out of the box. 

So based on naming conventions findByX will be provided by spring data jpa dynamically and will result to an entity result where all the entities will have for their field X the corresponding parameter value.

Besides there are other useful features like paging including sorting and others.