Support for schema migrations with JPA?

I've been briefly looking at JPA recently, and I was wondering what the deal 开发者_如何转开发is with database schema migrations and staying lined up with the classes you've created.

Is there support in JPA for this stuff? Utilities? Best Practises?

Cheers!

I won't rely on JPA providers to update the database schema. Check Liquibase for one of the good approaches.

The short answer is no.

If you change your beans, then you will have to migrate the existing schema by hand. So for Rails style database migrations you will have to look elsewhere.

You can however generate the initial ddl from your Java beans easily. The example below illustrates schema creation with EclipseLink version 2.0:

<?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="JPATestPU" transaction-type="RESOURCE_LOCAL">
        <provider>
            org.eclipse.persistence.jpa.PersistenceProvider
        </provider>
        <class>org.randompage.MyEntity</class>
        <properties>
            <property name="javax.persistence.jdbc.user" value="johndoe"/>
            <property name="javax.persistence.jdbc.password" value="secret"/>
            <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:h2:~/.h2/testdb;FILE_LOCK=NO"/>
            <property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
            <property name="eclipselink.logging.level" value="INFO"/>
        </properties>
    </persistence-unit>
</persistence>

The key element here is

 <property name="eclipselink.ddl-generation" value="drop-and-create-tables"/> 

This tells EclipseLink to drop existing tables and generate new once from your JPA mapping. This procedure is highly vendor specific so for other JPA vendors (Hibernate, OpenJPA...) you will have to consult their specific documentation.

If you set the generateDdl to Hibernate (if it is the underlying implementation), then it generates the database schema according to your current dialect. So after changing the dialect it will automatically generate the database.

Other JPA providers may have different properties for this.