Introduce EntityManagerFactoryBuilder types [SPR-8128]

[spring-projects-issues]

Chris Beams opened SPR-8128 and commented

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This issue is a sub-task of #12076

Issue Links:

• Provide @Configuration support for enabling load-time weaving (@EnableLoadTimeWeaving) [SPR-8317] #12965 Provide @Configuration support for enabling load-time weaving (@EnableLoadTimeWeaving)

[spring-projects-issues]

Chris Beams commented

Deferring this issue indefinitely. After quite a bit of work on this front, it is simply not straightforward to map the current EntityManagerFactoryBeans onto the same kind of builder style approach that (Annotation)SessionFactoryBuilder follows in the Hibernate world.

The implementation I've worked up is fully functional, but suffers from the need to inject the bean ClassLoader, LoadTimeWeaver and ResourceLoader into the Builder before it can be properly used. There's really no way around this, and it's just not a great usage story.

Fortunately, in the JPA world, it's not common to actually directly inject the EntityManagerFactory due to the way that @PersistenceContext works.

For this reason, it's actually reasonable for users to configure and return a Local(Container)EntityManagerFactoryBean from a @Bean method - it has no real downside, because that @Bean method will probably never be referenced for injection purposes.

This is a minor concession - it would be nice to tell users that it's never necessary to return a FactoryBean from a @Bean method, but considering the alternative, this is probably an acceptable exception. As mentioned above, #12965 will round this support out with the introduction of @EnableLoadTimeWeaving, for JPA implementations (EclipseLink) that depend on it.

For future reference, I've committed the fully functioning branch of work to https://github.com/cbeams/spring-framework/tree/SPR-8128.