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JPA

Quick intro

What is JPA?

The Java Persistence API is a specification for:

  • accessing
  • persisting
  • and managing

data between Java objects / classes and a relational database

Why JPA?

JPA is now considered the standard industry approach for Object to Relational Mapping (ORM) in the Java Industry.

But wait...

JPA itself is just a specification, not a product. It cannot perform persistence or anything else by itself. JPA is just a set of interfaces and requires an implementation

How am I going to use it?

A provider is needed. There are several:

  • Hibernate
  • EclipseLink
  • OpenJPA

Hibernate is used in more than 70% of projects

Use Gradle or Maven, it's that simple

All we need is a single file

persistence.xml

assuming you already have your database set up...

Persistence.xml

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<persistence ... version="2.1">
  <persistence-unit name="h2" transaction-type="RESOURCE_LOCAL">
    <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
      <properties>
        <property name="javax.persistence.jdbc.driver"
        value="org.h2.Driver"/>
        <property name="javax.persistence.jdbc.url"
        value="jdbc:h2:mem:testdb"/>
        <property name="hibernate.dialect"
        value="org.hibernate.dialect.H2Dialect"/>
        <property name="hibernate.hbm2ddl.auto" 
        value="update"/>
      </properties>
  </persistence-unit>
</persistence>

POJO

public class Author {

	private String firstName;
	private String lastName;

	// Constructors, getters and setters..

Entity

@Entity 
public class Author {

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

	private String firstName;
	private String lastName;

	public Author() {
	}
	// Constructors, getters and setters..

Persistence classes

EntityManagerFactory factory = 
	Persistence.createEntityManagerFactory("h2");
EntityManager manager = factory.createEntityManager();
EntityTransaction transaction = manager.getTransaction();

Persist entity

Author author = new Author("Tom", "Clancy");
transaction.begin();
manager.persist(author);
transaction.commit();

Find entity

Long authorId = 1L;
Author author = manager.find(Author.class, authorId);

but something's wrong...

author can be null!

Update entity using merge

transaction.begin();
Author author = new Author("Tom", "Clancy");
manager.persist(author);

Author authorToBeFound = new Author();
authorToBeFound.setId(1L);
authorToBeFound.setFirstName("Arnold");
authorToBeFound.setLastName("Schwarzenegger");

transaction.begin();
Author found = manager.merge(authorToBeFound);
transaction.commit();

Update entity using merge - generated SQL

Hibernate: 
insert into Author (id, first_name, last_name)
	values (null, ?, ?)

Author{id=1, firstName='Tom', lastName='Clancy'}

Hibernate:
update Author 
	set 
		first_name=?,
		last_name=?
where id=?

Author{id=1, firstName='Arnold', lastName='Schwarzenegger'}

Update entity using update

Long authorId = 1L;
author = manager.find(Author.class, authorId);

transaction.begin();
author.setId(1L);
author.setFirstName("Arnold");
author.setLastName("Schwarzenegger");
transaction.commit();

Update entity using update - generated SQL

Exaclty the same!

Remove entity

Optional.ofNullable(manager.find(Author.class, authorId))
	.ifPresent(found -> {
	transaction.begin();
	manager.remove(found);
	transaction.commit();
});

Remove entity using merge

Author toBeRemoved = new Author();
toBeRemoved.setId(1L);
transaction.begin();
manager.remove(manager.merge(toBeRemoved);
transaction.commit();