This examples shows how to apply DAO pattern in JPA based Spring application. We are going to use Hibernate as a JPA provider. We are also going to use LocalEntityManagerFactoryBean to load EntityManagerFactory (see last example).

Example

A Generic Dao Interface

public interface Dao<T> {
  void save(T t);
  T load(long id);
  void delete(long id);
  void update(T t);
  List<T> loadAll();
}

A JPA Entity

@Entity
public class Person {
  @Id
  @GeneratedValue
  private long id;
  private String firstName;
  private String lastName;
  private String address;
    .............
}

The DAO implementation

package com.logicbig.example;

import org.springframework.stereotype.Repository;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.PersistenceUnit;
import java.util.List;

@Repository
public class PersonDaoJpaImpl implements Dao<Person> {

  @PersistenceUnit
  private EntityManagerFactory emf;

  @Override
  public void save(Person person) {
      EntityManager em = emf.createEntityManager();
      em.getTransaction().begin();
      em.persist(person);
      em.getTransaction().commit();
      em.close();
  }

  @Override
  public Person load(long id) {
      EntityManager em = emf.createEntityManager();
      Person person = em.find(Person.class, id);
      em.close();
      return person;
  }

  @Override
  public void delete(long id) {
      EntityManager em = emf.createEntityManager();
      Person employee = em.find(Person.class, id);

      em.getTransaction().begin();
      em.remove(employee);
      em.getTransaction().commit();
      em.close();
  }

  @Override
  public void update(Person person) {

      EntityManager em = emf.createEntityManager();
      em.getTransaction().begin();
      em.merge(person);
      em.getTransaction().commit();
      em.close();
  }

  @Override
  public List<Person> loadAll() {
      EntityManager em = emf.createEntityManager();
      List<Person> persons = em.createQuery("Select t from Person t")
                               .getResultList();
      em.close();
      return persons;
  }
}

Note that, we used JPA's @PersistenceUnit annotation on EntityManagerFactory. In Spring application PersistenceAnnotationBeanPostProcessor (implicitly activated with Spring's Java Config) is responsible to process @PersistenceContext and @PersistenceUnit annotations.

The Service Layer

@Service
public class PersonService {
  @Autowired
  private Dao<Person> dao;

  public void savePerson(Person person) {
      try {
          dao.save(person);
      } catch (DataAccessException e) {
          e.printStackTrace();
      }
  }

  public List<Person> getAllPersons() {
      try {
          return dao.loadAll();
      } catch (DataAccessException e) {
          e.printStackTrace();
      }
      return null;
  }

  public Person getPersonById(long id) {
      try {
          return dao.load(id);
      } catch (DataAccessException e) {
          e.printStackTrace();
      }
      return null;
  }

  public void deletePerson(long id) {
      try {
          dao.delete(id);
      } catch (DataAccessException e) {
          e.printStackTrace();
      }
  }

  public void updatePerson(Person person) {
      try {
          dao.update(person);
      } catch (DataAccessException e) {
          e.printStackTrace();
      }
  }
}

The Example Client

@Component
public class ExampleClientBean {

  @Autowired
  PersonService personService;

  public void run() {
      System.out.println("'Persisting persons'");
      Person person = Person.create("Dana", "Whitley", "464 Gorsuch Drive");
      personService.savePerson(person);

      person = Person.create("Robin", "Cash", "64 Zella Park");
      personService.savePerson(person);

      List<Person> allPersons = personService.getAllPersons();
      System.out.println("Persons loaded: " + allPersons);

      person = personService.getPersonById(2);
      System.out.println("Person loaded by id : " + person);

      System.out.println("'updating person address with id 2'");
      person.setAddress("111 Yellow Hill");
      personService.updatePerson(person);

      System.out.println("'Deleting person by id 1'");
      personService.deletePerson(1);

      allPersons = personService.getAllPersons();
      System.out.println("Persons loaded: " + allPersons);
  }
}

Java Config and main class

package com.logicbig.example;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.orm.jpa.LocalEntityManagerFactoryBean;

@Configuration
@ComponentScan
public class AppConfig {

  @Bean
  public LocalEntityManagerFactoryBean entityManagerFactoryBean() {
      LocalEntityManagerFactoryBean factory = new LocalEntityManagerFactoryBean();
      factory.setPersistenceUnitName("example-unit");
      return factory;
  }

  public static void main(String[] args) {
      AnnotationConfigApplicationContext context =
              new AnnotationConfigApplicationContext(AppConfig.class);
      context.getBean(ExampleClientBean.class).run();

      context.getBean(LocalEntityManagerFactoryBean.class).destroy();
  }
}

Output

'Persisting persons'
 Persons loaded: [Person{id=1, firstName='Dana', lastName='Whitley', address='464 Gorsuch Drive'}, Person{id=2, firstName='Robin', lastName='Cash', address='64 Zella Park'}]
 Person loaded by id : Person{id=2, firstName='Robin', lastName='Cash', address='64 Zella Park'}
'updating person address with id 2'
'Deleting person by id 1'
 Persons loaded: [Person{id=2, firstName='Robin', lastName='Cash', address='111 Yellow Hill'}]

Example Project

Dependencies and Technologies Used:

• spring-context 5.0.1.RELEASE: Spring Context.
• spring-orm 5.0.1.RELEASE: Spring Object/Relational Mapping.
• hibernate-core 5.2.12.Final: The core O/RM functionality as provided by Hibernate.
  Implements javax.persistence:javax.persistence-api version 2.1
• h2 1.4.196: H2 Database Engine.
• JDK 1.8
• Maven 3.3.9