Accessing Data with JPA

JPA is a specification which specifies how to access, manage and persist information/data between java objects and relational databases. It provides a standard approach for ORM, Object Relational Mapping. Spring Boot provides a seemless integration with JPA.

Spring Data

Spring Data is a Spring-based programming model for data access. It reduces the amount of code needed for working with databases and datastores. It consists of several modules. The Spring Data JPA simplifies the development of Spring applications that use JPA technology.

With Spring Data, we define a repository interface for each domain entity in the application. A repository contains methods for performing CRUD operations, sorting, and paginating data. A repository is created by extending specific repository interfaces, such as CrudRepository, PagingAndSortingRepository, or JpaRepository.

JpaRepository

JpaRepository is JPA specific extension of Repository. It contains the full API of CrudRepository and PagingAndSortingRepository. So it has API for basic CRUD operations and also API for pagination and sorting.

Spring Boot JpaRepository example

Maven Dependencies

To enable JPA in a Spring Boot application, we need the spring-boot-starter and spring-boot-starter-data-jpa dependencies:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <version>2.2.6.RELEASE</version>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
    <version>2.2.6.RELEASE</version>
</dependency>

The spring-boot-starter contains the necessary auto-configuration for Spring JPA. Also, the spring-boot-starter-jpa project references all the necessary dependencies such as hibernate-core.

Define a Simple Entity

In this example, you store Customer objects, each annotated as a JPA entity. The following listing shows the Customer class :

package com.example.accessingdatajpa;

import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;

@Entity
public class Customer {

  @Id
  @GeneratedValue(strategy=GenerationType.AUTO)
  private Long id;
  private String firstName;
  private String lastName;

  protected Customer() {}

  public Customer(String firstName, String lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  @Override
  public String toString() {
    return String.format(
        "Customer[id=%d, firstName='%s', lastName='%s']",
        id, firstName, lastName);
  }

  public Long getId() {
    return id;
  }

  public String getFirstName() {
    return firstName;
  }

  public String getLastName() {
    return lastName;
  }
}

The Customer class is annotated with @Entity, indicating that it is a JPA entity. (Because no @Table annotation exists, it is assumed that this entity is mapped to a table named Customer.

The Customer object’s id property is annotated with @Id so that JPA recognizes it as the object’s ID. The id property is also annotated with @GeneratedValue to indicate that the ID should be generated automatically.

The other two properties, firstName and lastName, are left unannotated. It is assumed that they are mapped to columns that share the same names as the properties themselves.

Create Simple Queries

Spring Data JPA focuses on using JPA to store data in a relational database. Its most compelling feature is the ability to create repository implementations automatically, at runtime, from a repository interface.

To see how this works, create a repository interface that works with Customer entities as the following listing shows:

package com.example.accessingdatajpa;

import java.util.List;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface CustomerRepository extends JpaRepository<Customer, Long> {

  List<Customer> findByLastName(String lastName);

  Customer findById(long id);
}

@Repository is a marker annotation, which indicates that the underlying interface is a repository. CustomerRepository extends the JpaRepository interface. The type of entity and ID that it works with, Customer and Long, are specified in the generic parameters on JpaRepository. By extending JpaRepository, CustomerRepository inherits several methods for working with Customer persistence, including methods for saving, deleting, and finding Customer entities.

Spring Data JPA also lets you define other query methods by declaring their method signature. For example, CustomerRepository includes the findByLastName() method.

In a typical Java application, you might expect to write a class that implements CustomerRepository. However, that is what makes Spring Data JPA so powerful: You need not write an implementation of the repository interface. Spring Data JPA creates an implementation when you run the application.

Now you can wire up this example and see what it looks like!

Create an Application Class

The following listing shows the finished AccessingDataJpaApplication class:

package com.example.accessingdatajpa;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class AccessingDataJpaApplication {

  private static final Logger log = LoggerFactory.getLogger(AccessingDataJpaApplication.class);

  public static void main(String[] args) {
    SpringApplication.run(AccessingDataJpaApplication.class);
  }

  @Bean
  public CommandLineRunner demo(CustomerRepository repository) {
    return (args) -> {
      // save a few customers
      repository.save(new Customer("Jack", "Bauer"));
      repository.save(new Customer("Chloe", "O'Brian"));
      repository.save(new Customer("Kim", "Bauer"));
      repository.save(new Customer("David", "Palmer"));
      repository.save(new Customer("Michelle", "Dessler"));

      // fetch all customers
      log.info("Customers found with findAll():");
      log.info("-------------------------------");
      for (Customer customer : repository.findAll()) {
        log.info(customer.toString());
      }
      log.info("");

      // fetch an individual customer by ID
      Customer customer = repository.findById(1L);
      log.info("Customer found with findById(1L):");
      log.info("--------------------------------");
      log.info(customer.toString());
      log.info("");

      // fetch customers by last name
      log.info("Customer found with findByLastName('Bauer'):");
      log.info("--------------------------------------------");
      repository.findByLastName("Bauer").forEach(bauer -> {
        log.info(bauer.toString());
      });
      // for (Customer bauer : repository.findByLastName("Bauer")) {
      //  log.info(bauer.toString());
      // }
      log.info("");
    };
  }

}

The AccessingDataJpaApplication class includes a demo() method that puts the CustomerRepository through a few tests. First, it fetches the CustomerRepository from the Spring application context. Then it saves a handful of Customer objects, demonstrating the save() method and setting up some data to work with. Next, it calls findAll() to fetch all Customer objects from the database. Then it calls findById() to fetch a single Customer by its ID. Finally, it calls findByLastName() to find all customers whose last name is “Bauer”. The demo() method returns a CommandLineRunner bean that automatically runs the code when the application launches.

When you run your application, you should see output similar to the following:

== Customers found with findAll():
Customer[id=1, firstName='Jack', lastName='Bauer']
Customer[id=2, firstName='Chloe', lastName='O'Brian']
Customer[id=3, firstName='Kim', lastName='Bauer']
Customer[id=4, firstName='David', lastName='Palmer']
Customer[id=5, firstName='Michelle', lastName='Dessler']

== Customer found with findById(1L):
Customer[id=1, firstName='Jack', lastName='Bauer']

== Customer found with findByLastName('Bauer'):
Customer[id=1, firstName='Jack', lastName='Bauer']
Customer[id=3, firstName='Kim', lastName='Bauer']

By default, Spring Boot enables JPA repository support and looks in the package (and its subpackages) where @SpringBootApplication is located. If your configuration has JPA repository interface definitions located in a package that is not visible, you can point out alternate packages by using @EnableJpaRepositories and its type-safe basePackageClasses=MyRepository.class parameter.

I hope you have enjoyed this post and it helped you with basics of accessing the data using JPA interface provided by Spring boot . Please like and share and feel free to comment if you have any suggestions or feedback.