Category: Spring

bookmark_borderSpring: Accessing Data with JPA

Posted on by Jay Jo
  1. Prerequisite
  2. What is Spring Data?
  3. Started with Spring Initializr
  4. Add Dependencies
  5. Define an Entity
  6. Create Simple Queries
  7. Create an Application Class
  8. Execute Application and Check Result

Prerequisite

  • IDE (Integrated development environment)
  • JDK 8 or later
  • Maven 3.2+

What is Spring Data JPA?

Spring Data JPA provides repository support for the Java Persistence API (JPA). It eases development of applications that need to access JPA data sources.

Started with Spring Initializr

Add Dependencies

  • Add JPA and H2 dependencies
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

<dependency>
  <groupId>com.h2database</groupId>
  <artifactId>h2</artifactId>
  <scope>runtime</scope>
</dependency>
  • H2 is a relational database management system

Define a Entity

Create Customer.java in src/main/java/com/example/jpa

  • annotated as a JPA entity
package com.example.jpa;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.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;
  }
}

Customer class have 3 attributes: id, firstName, and lastName.

@Entity: JPA entity

Create Simple Queries

Spring Data JPA: By JPA, save data to Relational DataBase

Create CustomerRepository.java in src/main/java/com/example/jpa/CustomerRepository.java

package com.example.jpa;

import java.util.List;

import org.springframework.data.repository.CrudRepository;

public interface CustomerRepository extends CrudRepository<Customer, Long> {

  List<Customer> findByLastName(String lastName);

  Customer findById(long id);
}

By extending CustomerRepository and CrudRepository Customer, you will get methods (saving, deleting, and finding).

And Spring Data JPA: you can define query method. For example, CustomerRepository includes findByLastName() method.

General Java Application do not have to write an implementation of repository interface. JPA will create when Application is running.

Create an Application Class

Edit JpaApplication.java in src/main/java/com/example/jpa

package com.example.jpa;

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 JpaApplication {

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

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

Main() method: Using SpringApplication.run() method, execute the application.

Execute Application and Check Result

In root directory.

$ mvn package
$ java -jar target/board-0.0.1-SNAPSHOT.jar

Check result

== 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']
ANOTE.DEV