I am studying Hibernate , and on the way I came across several doubts, one of my doubts, and what is important for my learning, is about the purpose of the persistence.xml that is my unit of persistence, I read about it, however, I can not understand its purpose.
I'd like to know what the purpose of this file is in relation to my application and how relevant is it to my project?
persistence.xml is a configuration file used by JPA, Hibernate is one of many implementations of JPA. The contents of this file have information such as the connection url , user , password . In addition to containing mapping of the classes that have become the tables, and this mapping is done through annotations.
Example of a persistence.xml file:
<?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="Hello" transaction-type="RESOURCE_LOCAL">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<properties>
<property name="hibernate.show_sql" value="true" />
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://127.0.0.1:3306/Hello"/>
<property name="javax.persistence.jdbc.user" value="root" />
<property name="javax.persistence.jdbc.password" value="root" />
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
<property name="hibernate.hbm2ddl.auto" value="update"/>
</properties>
</persistence-unit>
</persistence>
In <persistence-unit name=”Hello” transaction-type=”RESOURCE_LOCAL”> two important settings are defined here.
In unit name sets for the application the configuration that will be used. In the same file we can have several different types, like transaction-type that defines what our transaction type will be, in our case local . If it was a web application we would not need to control the transaction, the containner (JBoss, GlassFish,) would do that job.
In <provider>org.hibernate.ejb.HibernatePersistence</provider> indicates which will be the provider of our application. In our case, Hibernate.
The hibernate.show_sql option enables the display of generated SQL in the console.
In javax.persistence.jdbc.driver defines what the database connection agent we will be using, we will use MySQL, javax.persistence.jdbc.url the URL of our database together with the schema of our database.
In javax.persistence.jdbc.user and javax.persistence.jdbc.password user and database password. These attributes were defined in the MySQL installation. I used the root and root values.
In% with% of the dialect that Hibernate will use. We'll discuss more about this option in future posts, for example, how to automatically generate database script.
This hibernate.dialect option set to on, Hibernate will update its tables as needed. For example, if the table does not exist it will create ( hibernate.hbm2ddl.auto ), if you added a new column ( CREATE TABLE ).