Introduction to Spring Data - Spring Data JPA, Spring Data REST and MongoDB

This article will teach you the fundamentals of Spring Data. We will look at the different Spring Data alternatives as well as two examples: Spring Data JPA and Spring Data Mongodb.

You will learn

Basics of Spring Data
Why is Spring Data needed?
What are the different interfaces provided by Spring Data?
How to get started with Spring Data?

What is Spring Data?

Consider the progress of databases in recent years.

When Spring Framework was built in the early 2000s, the only type of database available was relational databases such as Oracle, MS SQL Server, My SQL, and others. In recent years, a broad range of databases have gained popularity, the majority of which are not relational and do not use SQL. These databases are referred to using a wide range of words. For instance, consider NoSQL.

ORM frameworks (Hibernate) and standards (JPA) suited relational databases well. Nevertheless, modern databases have different requirements.

From http://projects.spring.io/spring-data/

The objective of Spring Data is to provide a familiar and consistent Spring-based programming model for data access while keeping the unique characteristics of the underlying data store. It makes data access technologies, relational and non-relational databases, map-reduce frameworks, and cloud-based data services simple to utilise.

To make things easier, Spring Data provides Abstractions (interfaces) that may be used regardless of the underlying data source.

Spring Data Commons

Spring Data Commons offers all of the common abstractions needed to connect to various data storage.

Crud Repository

CrudRepository is the main interface in Spring Data Commons. It offers general CRUD activities regardless of the underlying data storage. It extends Repository, which is the foundation class for all repositories that provide data store access.

The methods of the CrudRepository interface are all listed below.

public interface CrudRepository<T, ID> extends Repository<T, ID> {
	<S extends T> S save(S entity);

	<S extends T> Iterable<S> saveAll(Iterable<S> entities);

	Optional<T> findById(ID id);

	boolean existsById(ID id);

	Iterable<T> findAll();

	Iterable<T> findAllById(Iterable<ID> ids);

	long count();

	void deleteById(ID id);

	void delete(T entity);

	void deleteAll(Iterable<? extends T> entities);

	void deleteAll();
}

The CrudRepository’s methods are self-explanatory.

PagingAndSortingRepository

PagingAndSortingRepository is another key Spring Data interface. PagingAndSortingRepository allows you to

Sort your data using Sort interface
Paginate your data using Pageable interface, which provides methods for pagination - getPageNumber(), getPageSize(), next(), previousOrFirst() etc.

public abstract interface PagingAndSortingRepository extends CrudRepository {

public Iterable findAll(Sort sort);

public Page findAll(Pageable pageable);

}

Defining Custom Repositories

You may build your own repository by extending one of the repository classes: Repository, PagingAndSortingRepository, or CrudRepository.

An example is shown below

interface PersonRepository extends CrudRepository<User, Long> {

Defining custom queries

Spring Data also supports query generation from interface method names.

Consider the following example:

  List<Person> findByFirstNameAndLastname(String firstName, String lastname);

The approach described above allows you to search a data store by entering a person’s first and last name. This would construct the proper query for the data store, which would yield the person’s information.

You can find more details in the spring data documentation - https://docs.spring.io/spring-data/commons/docs/current/reference/html/#repositories.query-methods.query-creation

Auditing with Spring Data

Spring Data also supports auditing with simple annotations.

class Student {

  @CreatedBy
  private User createdUser;

  @CreatedDate
  private DateTime createdDate;

  // further properties omitted
}

There are corresponding annotations for updates as well

LastModifiedBy
LastModifiedDate

Spring Data Implementations

There are Spring Data Modules for any data store you want to utilise.

Spring Data JPA - ORM frameworks are used to connect to relational databases.
Spring Data MongoDB - Repositories for MongoDB.
Spring Data REST - HATEOAS RESTful resources are exposed around Spring Data repositories.
Spring Data Redis - Repositories for Redis.

Spring Data JPA

Spring Data JPA makes it possible to connect to relational databases through ORM frameworks.

The dependency is shown below:

<dependencies>
  <dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-jpa</artifactId>
  </dependency>
<dependencies>

Hibernate is the default JPA implementation.

The core interface is the JpaRepository.

public interface JpaRepository<T, ID> 
	extends PagingAndSortingRepository<T, ID>, 
	        QueryByExampleExecutor<T>

Some of the extra methods it provides (in comparison to PagingAndSortingRepository) are listed below. As you can see, all of these methods are JPA-specific.

/**
 * Saves an entity and flushes changes instantly.
 *
 * @param entity
 * @return the saved entity
 */
<S extends T> S saveAndFlush(S entity);

	/**
 * Deletes the given entities in a batch which means it will create a single {@link Query}. Assume that we will clear
 * the {@link javax.persistence.EntityManager} after the call.
 *
 * @param entities
 */
void deleteInBatch(Iterable<T> entities);

/**
 * Deletes all entities in a batch call.
 */
void deleteAllInBatch();

Recommended Reading for Spring Data JPA - http://www.springboottutorial.com/introduction-to-jpa-with-spring-boot-data-jpa

Spring Data REST

Spring Data REST may be used to deliver HATEOAS RESTful resources that are nested within Spring Data repositories.

An example using JPA is shown below

@RepositoryRestResource(collectionResourceRel = "todos", path = "todos")
public interface TodoRepository
		extends PagingAndSortingRepository<Todo, Long> {

A few example REST Services are shown below:

POST

URL : http://localhost:8080/todos
Use Header : Content-Type:application/json

Request Content

{
  "user": "Jill",
  "desc": "Learn Hibernate",
  "done": false
}

Response Content

{
  "user": "Jill",
  "desc": "Learn Hibernate",
  "done": false,
  "_links": {
    "self": {
      "href": "http://localhost:8080/todos/1"
    },
    "todo": {
      "href": "http://localhost:8080/todos/1"
    }
  }
}

The response contains the href of the newly created resource.

GET

URI - http://localhost:8080/todos

Response

{
  "_embedded" : {
    "todos" : [ {
      "user" : "Jill",
      "desc" : "Learn Hibernate",
      "done" : false,
      "_links" : {
        "self" : {
          "href" : "http://localhost:8080/todos/1"
        },
        "todo" : {
          "href" : "http://localhost:8080/todos/1"
        }
      }
    } ]
  },
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/todos"
    },
    "profile" : {
      "href" : "http://localhost:8080/profile/todos"
    },
    "search" : {
      "href" : "http://localhost:8080/todos/search"
    }
  },
  "page" : {
    "size" : 20,
    "totalElements" : 1,
    "totalPages" : 1,
    "number" : 0
  }
}

GET to http://localhost:8080/todos/1

{
  "user" : "Jill",
  "desc" : "Learn Hibernate",
  "done" : false,
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/todos/1"
    },
    "todo" : {
      "href" : "http://localhost:8080/todos/1"
    }
  }
}

Spring Data REST also allows you to search by column name.

Example - http://localhost:8080/todos?user=Jill

Spring Data Rest can be extended by defining custom methods in the repositories. http://localhost:8080/todos/search/findByUser?user=Jill can be used expose specific search method defined below.

@RepositoryRestResource(collectionResourceRel = "todos", path = "todos")
public interface TodoRepository
		extends PagingAndSortingRepository<Todo, Long> {

	List<Todo> findByUser(@Param("user") String user);

}

Spring Data REST supports

Spring Data JPA
Spring Data MongoDB
Spring Data Neo4j
Spring Data GemFire
Spring Data Cassandra

Recommended Reading for Spring Data Rest - http://www.springboottutorial.com/introduction-to-spring-data-rest-using-spring-boot

Spring Data MongoDB

Spring Data MongoDB supports the use of MongoDB as a data storage.

The key interface is MongoRepository.

public interface MongoRepository<T, ID> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> 

Some of the most significant methods (in addition to PagingAndSortingRepository) are listed below. Search samples may be found here.

	/* 
	 * (non-Javadoc)
	 * @see org.springframework.data.repository.query.QueryByExampleExecutor#findAll(org.springframework.data.domain.Example)
	 */
	@Override
	<S extends T> List<S> findAll(Example<S> example);

	/* 
	 * (non-Javadoc)
	 * @see org.springframework.data.repository.query.QueryByExampleExecutor#findAll(org.springframework.data.domain.Example, org.springframework.data.domain.Sort)
	 */
	@Override
	<S extends T> List<S> findAll(Example<S> example, Sort sort);

Introduction to Spring Data - Spring Data JPA, Spring Data REST and MongoDB

You will learn

What is Spring Data?

Spring Data Commons

Crud Repository

PagingAndSortingRepository

Defining Custom Repositories

Defining custom queries

Auditing with Spring Data

Spring Data Implementations

Spring Data JPA

Spring Data REST

POST

GET

Spring Data MongoDB

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