Hibernate-Based-Migration

Hibernate Based Migrations are a ~~simple~~ method to do complex migrations of persisted data using Hibernate. Since Hibernate mappings and annotated entity classes always represent the current state of the persisted data, the Maven plugin offers goals to freeze files in a specific version and thaw them in a later version to use in a Migration class. Hibernate Migrations leverage Flyway's rarely used Java-based-Migrations feature.

Usage

1. Add the hibernate-based-migration-plugin to your project

<plugin>
    <groupId>dev.bodewig.hibernate-based-migration</groupId>
    <artifactId>hibernate-based-migration-plugin</artifactId>
    <version>1.0.2</version>
</plugin>

2. Add the thaw goal to your project lifecycle

<plugin>
    ...
    <executions>
        <execution>
            <id>thaw</id>
            <goals>
                <goal>thaw</goal>
            </goals>
        </execution>
    </executions>
</plugin>

3. Add and execute the freeze goal to copy persistence files

<plugin>
    ...
    <executions>
        <execution>
            <id>freeze</id>
            <goals>
                <goal>freeze</goal>
            </goals>
            <configuration>
                <persistenceClassesFileList>
                    <file>src/main/java/dev/bodewig/hibernate_based_migration/example/Fruit.java</file>
                </persistenceClassesFileList>
                <persistenceResourcesFileList>
                    <file>src/main/resources</file>
                </persistenceResourcesFileList>
            </configuration>
        </execution>
    </executions>
</plugin>

4. Change your persistence files

@Entity
public class Fruit {
	@Id
	public String name;
	public int weight;
	public String colorHex; // --> public String colorRgb;
}

5. Call the freeze goal again (you now have 2 frozen versions)

src
|- main
|- migration
   |- 1
   |  |- java/...
   |  |- resources/...
   |- 2
      |- java/...
      |- resources/...

6. Add a dependency to hibernate-based-migration

<dependency>
    <groupId>dev.bodewig.hibernate-based-migration</groupId>
    <artifactId>hibernate-based-migration</artifactId>
    <version>1.0.0</version>
</dependency>

7. Subclass HibernateMigration to create a migration between the frozen versions of your persistence files (read the Hibernate configuration from the frozen version directories)

public class V1__My_Migration extends HibernateMigration {

	private List<OldData> persistedData;

	@Override
	protected long getSerialVersionUID() { ... }

	@Override
	public Configuration configBefore() {
		return new Configuration().configure("/_1/hibernate.cfg.xml");
	}

	@Override
	public Configuration configAfter() {
		return new Configuration().configure("/_2/hibernate.cfg.xml");
	}

	@Override
	public void doBefore(StatelessSession session) {
		this.persistedData = loadDataWithHibernate(session);
	}

	@Override
	public void doSql(Context context) throws SQLException {
		try (Statement stmt = context.getConnection().createStatement()) {
			// change your database scheme
		}
	}

	@Override
	public void doAfter(StatelessSession session) {
		NewData data = migrateData(this.persistedData);
		saveDataWithHibernate(data);
	}
}

8. Add the flyway-maven-plugin to your project

<plugin>
    <groupId>org.flywaydb</groupId>
    <artifactId>flyway-maven-plugin</artifactId>
    <version>9.22.0</version>
</plugin>

9. Execute the migrate goal to run your migration class

<plugin>
    ...
    <executions>
        <execution>
            <id>migrate</id>
            <goals>
                <goal>migrate</goal>
            </goals>
        </execution>
    </executions>
</plugin>

10. Profit!

Your database scheme and your persisted data have been migrated - without doing complex value transformations in pure SQL.

A full example implementation is available in the hibernate-based-migration-example.