JPA

1.1. Overview

The process of mapping Java Objects to database tables and vice versa is called "Object-relational mapping" (ORM). The Java Persistence API (JPA) is one approach to ORM. Via JPA the developer can map, store, update and retrieve data from relational databases to Java Objects and vice versa, JPA permits the developer to work directly with objects rather then with SQL statements. JPA is a specification and several implementations are available. This tutorial will use EclipseLink as JPA implementation. The JPA implementation is typically called persistence provider. JPA can be used in Java-EE and Java-SE applications.

The mapping between Java objects and database tables is defined via persistence metadata. JPA defines the metadata via annotations in the Java class. Alternatively the metadata can be defined via XML or a combination of both. A XML configuration overwrites the annotations. The following description will be based on the usage of annotations. The JPA provider will use the persistence metadata information to perform the correct database operations.

JPA defines a SQL-like Query language for static and dynamic queries.

Most JPA persistence provider offer the option to create automatically the database schema based on the metadata.

1.2. Entity

A class which should be persisted in a database it must be annotated with "javax.persistence.Entity". Such a class is called entity. JPA will create a table for the entity in your database. Instances of the class will be a row in the table.

All entity classes must define a primary key, must have a non-arg constructor and or not allowed to be final. Keys can be a single field or a combination of fields. JPA allows to auto-generate the primary key in the database via the @GeneratedValue annotation.

By default, the table name corresponds to the class name. You can change this with the addition to the annotation @Table(name="NEWTABLENAME").

1.3. Persistence of fields

The fields of the Entity will be saved in the database. JPA can use either your instance variables (fields) or the corresponding getters and setters to access the fields. You are not allowed to mix both methods. If you want to use the setter and getter methods the Java class must follow the Java Bean naming conventions. JPA persists per default all fields of an Entity, if fields should not be saved they must be marked with @Transient.

By default each field is mapped to a column with the name of the field. You can change the default name via @Column(name="newColumnName").

The following annotations can be used.

Table 1. Annotations for fields / getter and setter

@Id	Identifies the unique ID of the database entry
@GeneratedValue	Together with ID defines that this value is generated automatically.
@Transient	Field will not be saved in database

1.4. Relationship Mapping

JPA allows to define relationships between classes, e.g. it can be defined that a class is part of another class (containment). Classes can have one to one, one to many, many to one, and many to many relationships with other classes.

A relationship can be bidirectional or unidirectional, e.g. in a bidirectional relationship both classes store a reference to each other while in an unidirectional case only one class has a reference to the other class. Within a bidirectional relationship you need to specify the owning side of this relationship in the other class with the attribute "mappedBy", e.g. @ManyToMany(mappedBy="attributeOfTheOwningClass".

Table 2. Relationship annotations

@OneToOne

@OneToMany

@ManyToOne

@ManyToMany

1.5. Entity Manager

The entity manager "javax.persistence.EntityManager" provides the operations from and to the database, e.g. find objects, persists them, remove objects from the database, etc. In a JavaEE application the entity manager is automatically inserted in the web application. Outside JavaEE you need to manage the entity manager yourself.

Entities which are managed by an Entity Manager will automatically propagate these changes to the database (if this happens within a commit statement). If the Entity Manager is closed (via close()) then the managed entities are in a detached state. If synchronize them again with the database a Entity Manager provides the merge() method.

The persistence context describes all Entities of one Entity manager.

1.6. Persistence Units

The EntityManager is created by the EntitiyManagerFactory which is configured by the persistence unit. The persistence unit is described via the file "persistence.xml" in the directory META-INF in the source folder. A set of entities which are logical connected will be grouped via a persistence unit. "persistence.xml" defines the connection data to the database, e.g. the driver, the user and the password,

Installation

2.1. EclipseLink

Download the EclipseLink implementation from http://www.eclipse.org/eclipselink/downloads/ . The download contains several jars. We need the following jars:

• eclipselink.jar

• javax.persistence_2.0.X.jar

2.2. Derby Database

The example later will be using Apache Derby as a database. Download Derby from http://db.apache.org/derby/ From this tutorial we will need the "derby.jar". For details on Derby (which is not required for this tutorial) please see Apache Derby .

Simple Example

3.1. Project and Entity

Create a Java Project "sawan.modi.jpa.simple". Create a folder "lib" and place the required JPA jars and derby.jar into this folder.

Afterwards create the package "sawan.modi.jpa.simple.model" and create the following classes.

    
package sawan.modi.jpa.simple.model;

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

@Entity
public class Todo {
 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 private Long id;
 private String summary;
 private String description;

 public String getSummary() {
  return summary;
 }

 public void setSummary(String summary) {
  this.summary = summary;
 }

 public String getDescription() {
  return description;
 }

 public void setDescription(String description) {
  this.description = description;
 }

 @Override
 public String toString() {
  return "Todo [summary=" + summary + ", description=" + description
    + "]";
 }

}

   

3.2. Persistence Unit

Create a directory "META-INF" in your "src" folder and create the file "persistence.xml". This examples uses EclipseLink specific flags for example via the parameter "eclipselink.ddl-generation" you specify that the database scheme will be automatically dropped and created.

    

 xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
 version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
 
  sawan.modi.jpa.simple.model.Todo
  
   
       value="jdbc:derby:/home/vogella/databases/simpleDb;create=true" />
   
   

   
   
       value="database" />
  

 


   

The database specified via "javax.persistence.jdbc.url" will be automatically created by the Derby Driver. You may want to adjust the path, it currently is based on Linux notations and points to my home directory on my Linux system.

To see the SQL generated for the the databases set eclipselink.ddl-generation.output-mode value from "database" to "sql-script" or "both". Two files will get generated 'createDDL.jdbc' and 'dropDDL.jdbc'

3.3. Test your installation

Create the following Main class which will create a new entry every time it it called. After the first call you need to remove the property "eclipselink.ddl-generation" from persistence.xml otherwise you will receive an error as EclipseLink tries to create the database scheme again. Alternative you could set the property to "drop-and-create-tables" but this would drop your database schema at every run.

    
package sawan.modi.jpa.simple.main;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

import sawan.modi.jpa.simple.model.Todo;

public class Main {
 private static final String PERSISTENCE_UNIT_NAME = "todos";
 private static EntityManagerFactory factory;

 public static void main(String[] args) {
  factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
  EntityManager em = factory.createEntityManager();
  // Read the existing entries and write to console
  Query q = em.createQuery("select t from Todo t");
  List todoList = q.getResultList();
  for (Todo todo : todoList) {
   System.out.println(todo);
  }
  System.out.println("Size: " + todoList.size());

  // Create new todo
  em.getTransaction().begin();
  Todo todo = new Todo();
  todo.setSummary("This is a test");
  todo.setDescription("This is a test");
  em.persist(todo);
  em.getTransaction().commit();

  em.close();
 }
}

   

Run you program several times to see that the database is filled.

Relationship Example

Create a java project "sawan.modi.jpa.eclipselink", create again a folder "lib" and place the required JPA jars and derby.jar into this folder.

Create the package "sawan.modi.jpa.eclipselink.model" and the following classes.

   
package sawan.modi.jpa.eclipselink.model;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Family {
 @Id
 @GeneratedValue(strategy = GenerationType.TABLE)
 private int id;
 private String description;

 @OneToMany(mappedBy = "family")
 private final List members = new ArrayList();

 public int getId() {
  return id;
 }

 public void setId(int id) {
  this.id = id;
 }

 public String getDescription() {
  return description;
 }

 public void setDescription(String description) {
  this.description = description;
 }

 public List getMembers() {
  return members;
 }

}

  

   
package sawan.modi.jpa.eclipselink.model;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Transient;

@Entity
public class Person {
 @Id
 @GeneratedValue(strategy = GenerationType.TABLE)
 private String id;
 private String firstName;
 private String lastName;

 private Family family;

 private String nonsenseField = "";

 private List jobList = new ArrayList();

 public String getId() {
  return id;
 }

 public void setId(String Id) {
  this.id = Id;
 }

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 // Leave the standard column name of the table
 public String getLastName() {
  return lastName;
 }

 public void setLastName(String lastName) {
  this.lastName = lastName;
 }

 @ManyToOne
 public Family getFamily() {
  return family;
 }

 public void setFamily(Family family) {
  this.family = family;
 }

 @Transient
 public String getNonsenseField() {
  return nonsenseField;
 }

 public void setNonsenseField(String nonsenseField) {
  this.nonsenseField = nonsenseField;
 }

 @OneToMany
 public List getJobList() {
  return this.jobList;
 }

 public void setJobList(List nickName) {
  this.jobList = nickName;
 }

}
  

   
package sawan.modi.jpa.eclipselink.model;

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

@Entity
public class Job {
 @Id
 @GeneratedValue(strategy = GenerationType.TABLE)
 private int id;
 private double salery;
 private String jobDescr;

 public int getId() {
  return id;
 }

 public void setId(int id) {
  this.id = id;
 }

 public double getSalery() {
  return salery;
 }

 public void setSalery(double salery) {
  this.salery = salery;
 }

 public String getJobDescr() {
  return jobDescr;
 }

 public void setJobDescr(String jobDescr) {
  this.jobDescr = jobDescr;
 }

}

  

Create the file "persistence.xml" in "src/META-INF". Remember to change the path to the database.

   

 xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
 version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
 
  
  
  sawan.modi.jpa.eclipselink.model.Person
  sawan.modi.jpa.eclipselink.model.Family
  sawan.modi.jpa.eclipselink.model.Job
  
  
   
        value="jdbc:derby:/home/vogella/databases/relationsshipDb;create=true" />
   
   

   
   
       value="database" />
  

 



  

The following check is implemented as a JUnit Test. For details please see See JUnit Tutorial . The setup() method will create a few test entries. After the test entries are created, they will be read and the one field of the entries is changed and saved to the database.

   
package sawan.modi.jpa.eclipselink.main;

import static org.junit.Assert.assertTrue;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

import org.junit.Before;
import org.junit.Test;

import sawan.modi.jpa.eclipselink.model.Family;
import sawan.modi.jpa.eclipselink.model.Person;

public class JpaTest {

 private static final String PERSISTENCE_UNIT_NAME = "people";
 private EntityManagerFactory factory;

 @Before
 public void setUp() throws Exception {
  factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
  EntityManager em = factory.createEntityManager();

  // Begin a new local transaction so that we can persist a new entity
  em.getTransaction().begin();

  // Read the existing entries
  Query q = em.createQuery("select m from Person m");
  // Persons should be empty

  // Do we have entries?
  boolean createNewEntries = (q.getResultList().size() == 0);

  // No, so lets create new entries
  if (createNewEntries) {
   assertTrue(q.getResultList().size() == 0);
   Family family = new Family();
   family.setDescription("Family for the Knopfs");
   em.persist(family);
   for (int i = 0; i < 40; i++) {
    Person person = new Person();
    person.setFirstName("Jim_" + i);
    person.setLastName("Knopf_" + i);
    em.persist(person);
    // First we have to persists the job
    // Now persists the new person
    family.getMembers().add(person);
    em.persist(person);
    em.persist(family);
   }
  }

  // Commit the transaction, which will cause the entity to
  // be stored in the database
  em.getTransaction().commit();

  // It is always good practice to close the EntityManager so that
  // resources are conserved.
  em.close();

 }

 @Test
 public void checkAvailablePeople() {

  // Now lets check the database and see if the created entries are there
  // Create a fresh, new EntityManager
  EntityManager em = factory.createEntityManager();

  // Perform a simple query for all the Message entities
  Query q = em.createQuery("select m from Person m");

  // We should have 40 Persons in the database
  assertTrue(q.getResultList().size() == 40);

  em.close();
 }

 @Test
 public void checkFamily() {
  EntityManager em = factory.createEntityManager();
  // Go through each of the entities and print out each of their
  // messages, as well as the date on which it was created
  Query q = em.createQuery("select f from Family f");

  // We should have one family with 40 persons
  assertTrue(q.getResultList().size() == 1);
  assertTrue(((Family) q.getSingleResult()).getMembers().size() == 40);
  em.close();
 }

 @Test(expected = javax.persistence.NoResultException.class)
 public void deletePerson() {
  EntityManager em = factory.createEntityManager();
  // Begin a new local transaction so that we can persist a new entity
  em.getTransaction().begin();
  Query q = em
    .createQuery("SELECT p FROM Person p WHERE p.firstName = :firstName AND p.lastName = :lastName");
  q.setParameter("firstName", "Jim_1");
  q.setParameter("lastName", "Knopf_!");
  Person user = (Person) q.getSingleResult();
  em.remove(user);
  em.getTransaction().commit();
  Person person = (Person) q.getSingleResult();
  // Begin a new local transaction so that we can persist a new entity

  em.close();
 }
}

  

The project should now look like the following.

You should be able to run the JUnit tests successfully .

Thank you

Thank you for practicing with this tutorial.

I maintain this tutorial in my private time.