Java Specialization Period

Before we start...

Assignment check: have you met any trouble in making the previous exercise?

JPA : the Java Persistence API

JPA provides a natural way of persisting Java objects in a database
JPA is a specification, meaning that you can find several implementations of that standard
The most spread implementation of JPA is Hibernate, which has the most complete support of the JPA standard

JPA : Before JPA

The features brought by JPA were defined by simple observation of what is generally costly while persisting objects

Manage the JDBC connection is hard for unexperimented developers
Mix SQL and Java code can lead to serious maintenance issues
Written SQL queries can be dependant of the database provider
Transform result sets to Java Objects is sometimes very long and can lead to consistency errors
There is often a redundancy between your Java class organization and their equivalent tables in the database

JPA : the Java Persistence API, Concepts

JPA helps to solve those troubles

It provides abstraction from the real JDBC connection used internally
You can better separate queries and Java code
The query language is abstracted from SQL (JPQL)
Queries natively return Java Objects, thanks to mapping annotations
You can drive your database schema directly from your Java code

JPA : Schema with Hibernate Implementation

JPA Annotations

To place a class under the control of the JPA provider, you must map this class thanks to annotations

Say the JPA implementation that a class is associated to a table

@Entity
@Table(name="IDENTITIES")
public class Identity { /*...*/ }

Say what field of that class is mapped to what column of the table
```
Column(name="displayName")
private String displayName;
```

JPA Hibernate implementation

Besides of the classes annotation, hibernate takes a configuration file named hibernate.cfg.xml

This configuration looks like hereafter


<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration
    PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">
            org.apache.derby.jdbc.EmbeddedDriver
        </property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">root</property>
        <property name="hibernate.connection.url">jdbc:derby:/test</property>
        <property name="hibernate.dialect">org.hibernate.dialect.DerbyDialect</property>
        <property name="hibernate.show_sql">true</property>
        <property name="hibernate.hbm2ddl.auto">update</property>
        <mapping class="fr.tbr.iam.identity.Identity" />
</hibernate-configuration>

Derby presentation

To test your program without having setting a full environment, it exits some lightweight databases

Derby is one of them: it is very convenient to perform quick tests
Derby is supported by the hibernate connectors
This database can be embedded, so it runs only during the run time of the main program

Java Specialization period - Lecture 6