Java

References

• [Book1] - "Pro JPA 2: Mastering the Java™ Persistence API (Expert's Voice in Java Technology)".
  A great book if you are already comfortable with Java technologies (but even if you don't, it includes an overview of some Java technologies that are commonly used with JPA like EJB 3.0);

Introduction

Entities

• Annotate the class with @Entity (javax.persistence.Entity);
• The class must have a field annotated with @Id (javax.persistence.Id);
• The class must be serializable, i.e. it must implement Serializable (java.io.Serializable);
• The class must have a default no-args constructor;

Entity Manager

• create and remove persistent entity instances;
• find entities by their primary key;
• query over entities;

Employee emp = new Employee(158);
em.persist(emp);

Employee emp = em.find(Employee.class, 158);

public void removeEmployee(int id) {
    Employee emp = em.find(Employee.class, id);
    if (emp != null) {
        em.remove(emp);
    }
}

Employee emp = em.find(Employee.class, id);
if (emp != null) {
    emp.setSalary(emp.getSalary() + raise);
}

• [Book1, pag. 23] - "Entity Manager"

Persistence Unit (PU)

• the set of entities that can be managed by a given EntityManager instance, i.e. the set of all classes that are related or grouped by the application;
• the connection to the database (server name, port and database name);
• the database user account to be used to connect to the database (user name and pass);
• the JPA provider;
• the JDBC driver to connect to the database;
• the type of transaction API to be used;

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
  <persistence-unit name="JPAJavaSEPU" transaction-type="RESOURCE_LOCAL">
    <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
    <class>myentities.Employee</class>
    <properties>
      <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpa_javase"/>
      <property name="javax.persistence.jdbc.password" value="1234"/>
      <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
      <property name="javax.persistence.jdbc.user" value="root"/>
    </properties>
  </persistence-unit>
</persistence> 

• PU name is "JPAJavaSEPU";
• JPA provider is EclipseLink;
• Transaction type is "RESOURCE_LOCAL";
• Connecting to a MySQL database:
   - database name is "jpa_javase";
   - database server name is "localhost" listening on port  "3306";
   - JDBC driver is Connector/J ("com.mysql.jdbc.Driver");
   - database connection user/pass is "root" "1234";

Transaction management in JPA

• Java EE:
  The typical situation when running inside the Java EE container environment is that the standard Java Transaction API (JTA) is used.
  
  The transaction model when running in the container is to assume the application will ensure that a transactional context is present when one is required. If a transaction is not present, then either the modifying operation will throw an exception or the change will simply never be persisted to the data store.
  
   
• Java SE:
  When executing in Java SE, we need to begin and commit the transaction before and after calling an operational method.
  
  In a Java SE environment the transaction service that should be used is the EntityTransaction service.
  You get the EntityTransaction service by calling:
  em.getTransaction();
  and then use it to start and commit the transactions.
  
  
  Example using EntityTransaction service:
  

  em.getTransaction().begin();
  createEmployee(158, "John Doe", 45000);
  em.getTransaction().commit();

• [Book1, pag. 26] - "Transactions";
• [Book1, Chapter 3] - more info about Java EE Transactions;

Queries in JPA

Using JPA in JavaSE