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In this tutorial, we will discuss what is Spring Data JPA and we will understand how can we use Spring Data JPA to reduce the boilerplate code required to implement the data access object (DAO) layer.

What Spring Data JPA?

Let's first understand what is the problem and how Spring Data JPA provides a solution to solve the problem.

Problem

In typical three-layer Spring application architecture, we create three layers - Controller, Service, and DAO/Repository layer.

If we use JPA/Hibernate then write a lot of code while implementing DAO layer:

For example, we typically write the below code to implement DAO for a single Employee entity:

CustomerDAO

CustomerDAOImpl

What if we create DAO for other entities like Student, Product, Order, Employee, etc.

We are writing a lot of code and repeating the same code for other entities (only the difference is entity type and primary key fields, the rest of the code remains the same).

What will be the solution to avoid redundant code (reduce boilerplate code)?

Solution

Spring Data JPA provides a solution to reduce a lot of boilerplate code.

Spring Data JPA provides an out-of-the-box implementation for all the required CRUD operations for the JPA entity so we don't have to write the same boilerplate code again and again.

Spring Data JPA provides repositories so we just need to extend them to get full the out-of-the-box implementation for CRUD operations for an entity.

For example:

Therefore, we can use Spring Data JPA to reduce the amount of boilerplate code required to implement the data access object (DAO) layer.

Spring Data JPA is not a JPA provider. It is a library/framework that adds an extra layer of abstraction on the top of our JPA provider (like Hibernate).

Spring Data JPA uses Hibernate as a default JPA provider.

Spring Data Commons and Spring Data JPA Repositories/interfaces

Let's quickly discuss important repositories or interfaces of Spring Data Commons and Spring Data JPA.

The below diagram shows the main interfaces from Spring Data Commons and Spring Data JPA modules.

Spring Data Commons is part of the umbrella Spring Data project that provides shared infrastructure across the Spring Data projects. It contains technology-neutral repository interfaces as well as a metadata model for persisting Java classes.

Spring Data Commons project provides the following interfaces:

Repository<T, ID extends Serializable> interface

CrudRepository<T, ID extends Serializable> interface

PagingAndSortingRepository<T, ID extends Serializable> interface

QueryDslPredicateExecutor interface

Let's discuss the above interface in detail with its source code and methods.

The Repository<T, ID extends Serializable> interface

The Repository<T, ID extends Serializable> interface is a marker interface that has two purposes:

It captures the type of the managed entity and the type of the entity’s id.

It helps the Spring container to discover the “concrete” repository interfaces during classpath scanning.

Let's look at the source code of the Repository interface.

package org.springframework.data.repository;

import org.springframework.stereotype.Indexed;

@Indexed
public interface Repository<T, ID> {

}

The CrudRepository<T, ID extends Serializable> interface

The CrudRepository<T, ID extends Serializable> interface provides CRUD operations for the managed entity.

Let's look at the methods/APIs that the CrudRepository interface provides:

package org.springframework.data.repository;

import java.util.Optional;

@NoRepositoryBean
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();
}

Let's look at the usage of each method with a description.

long count() - Returns the number of entities available.
void delete(T entity) - Deletes a given entity.
void deleteAll() - Deletes all entities managed by the repository.
void deleteAll(Iterable<? extends T> entities) - Deletes the given entities.
void deleteById(ID id) - Deletes the entity with the given id.
boolean existsById(ID id) - Returns whether an entity with the given id exists.
Iterable findAll() - Returns all instances of the type.
Iterable findAllById(Iterable ids) - Returns all instances of the type with the given IDs.
Optional findById(ID id) - Retrieves an entity by its id.
save(S entity) - Saves a given entity.
Iterable saveAll(Iterable entities) - Saves all given entities.

The PagingAndSortingRepository<T, ID extends Serializable> interface

The PagingAndSortingRepository<T, ID extends Serializable> interface is an extension of CrudRepository to provide additional methods to retrieve entities using the pagination and sorting abstraction.

Let's look at the methods/APIs that the PagingAndSortingRepository interface provides:

package org.springframework.data.repository;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;

@NoRepositoryBean
public interface PagingAndSortingRepository < T, ID > extends CrudRepository < T, ID > {

    /**
     * Returns all entities sorted by the given options.
     * 
     * @param sort
     * @return all entities sorted by the given options
     */
    Iterable < T > findAll(Sort sort);

    /**
     * Returns a {@link Page} of entities meeting the paging restriction provided in the {@code Pageable} object.
     * 
     * @param pageable
     * @return a page of entities
     */
    Page < T > findAll(Pageable pageable);
}

The QueryDslPredicateExecutor interface

The QueryDslPredicateExecutor interface is not a “repository interface”. It declares the methods that are used to retrieve entities from the database by using QueryDsl Predicate objects.

Let's look at the methods/APIs that the QueryDslPredicateExecutor interface provides:

package org.springframework.data.querydsl;

import java.util.Optional;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;

import com.querydsl.core.types.OrderSpecifier;
import com.querydsl.core.types.Predicate;

public interface QuerydslPredicateExecutor < T > {

    Optional < T > findOne(Predicate predicate);

    Iterable < T > findAll(Predicate predicate);

    Iterable < T > findAll(Predicate predicate, Sort sort);

    Iterable < T > findAll(Predicate predicate, OrderSpecifier << ? > ...orders);

    Iterable < T > findAll(OrderSpecifier << ? > ...orders);

    Page < T > findAll(Predicate predicate, Pageable pageable);

    long count(Predicate predicate);

    boolean exists(Predicate predicate);
}

Spring Data JPA Repository Interfaces

Spring Data JPA module deals with enhanced support for JPA-based data access layers.

Spring Data JPA project provides the following interfaces:

JpaRepository<T, ID extends Serializable> interface
JpaSpecificationExecutor interface

JpaRepository<T, ID extends Serializable> interface

The JpaRepository<T, ID extends Serializable> interface is a JPA-specific repository interface that combines the methods declared by the common repository interfaces behind a single interface.

Let's look at the methods/APIs that the JpaRepository interface provides:

package org.springframework.data.jpa.repository;

import java.util.List;

import javax.persistence.EntityManager;

import org.springframework.data.domain.Example;
import org.springframework.data.domain.Sort;
import org.springframework.data.repository.NoRepositoryBean;
import org.springframework.data.repository.PagingAndSortingRepository;
import org.springframework.data.repository.query.QueryByExampleExecutor;

@NoRepositoryBean
public interface JpaRepository < T, ID > extends PagingAndSortingRepository < T, ID > , QueryByExampleExecutor < T > {

    List < T > findAll();

    List < T > findAll(Sort sort);

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

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

    void flush();

    <S extends T > S saveAndFlush(S entity);

    void deleteInBatch(Iterable < T > entities);

    void deleteAllInBatch();

    T getOne(ID id);

    @Override <
    S extends T > List < S > findAll(Example < S > example);

    @Override <
    S extends T > List < S > findAll(Example < S > example, Sort sort);
}

JpaSpecificationExecutor interface

The JpaSpecificationExecutor interface is not a “repository interface”. It declares the methods that are used to retrieve entities from the database by using Specification objects that use the JPA criteria API.

Let's look at the methods/APIs that the JpaSpecificationExecutor interface provides:

package org.springframework.data.jpa.repository;

import java.util.List;
import java.util.Optional;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;
import org.springframework.data.jpa.domain.Specification;
import org.springframework.lang.Nullable;

public interface JpaSpecificationExecutor<T> {

 Optional<T> findOne(@Nullable Specification<T> spec);

 List<T> findAll(@Nullable Specification<T> spec);

 Page<T> findAll(@Nullable Specification<T> spec, Pageable pageable);

 List<T> findAll(@Nullable Specification<T> spec, Sort sort);

 long count(@Nullable Specification<T> spec);
}

Conclusion

In this tutorial, we have seen what is Spring Data JPA and how to reduce a boilerplate code to develop a DAO layer. We have also seen Spring Data and Spring Data JPA repository interfaces.

What's next

Well, once you understood what is Spring Data JPA then you can check the below tutorial to learn how to use Spring Data JPA in the Spring boot project:

How to use Spring Data JPA in Spring Boot Project

Spring Data JPA Tutorial - Getting Started

Spring Data JPA Tutorial - Java Based Configuration

Spring Data JPA - Query Creation from Method Names

Spring Data JPA - Creating Database Queries With Named Queries

Spring Data JPA @NamedNativeQuery and @NamedNativeQueries Example

Spring Data JPA - Creating Database Queries using @Query Annotation

Spring Data JPA - Auditing with Spring Boot 2 and MySQL Example

Spring Data JPA - Working with Multiple Datasources using Spring Boot

Spring Data JPA Repository Testing using Spring Boot @DataJpaTest

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