EntityManagerFactoryBuilder cannot be used as advertised

[odrotbohm]

The reference documentation suggests to use EntityManagerFactoryBuilder to setup EntityManagerFactory instances in case one has to configure multiple ones. Unfortunately, the code shown doesn't work as the EMFB bean is created by JpaAutoConfiguration which expects a singular DataSource instance in the first place. Manually excluding that configuration then results in the EMFB not being present and the invocation of the factory method failing.

Maybe the auto-configuration for EMFB can be moved to a separate class that is not dependent on a single DataSource instance?

Related tickets: #5542, #5541.

[snicoll]

It isn't that easy unfortunately. The EMFB requires a JpaVendorAdapter and the latter uses the DataSource to determine the database in use. A bunch of other properties are also related to the platform itself.

It is weird to provide such service as a bean and then make it work on a totally unrelated DataSource. The only reason it is a bean in the first place as far as I can see is to be able to tune it if necessary.

Back to the doc, it is incomplete as it doesn't tell you how to obtain that EntityManagerFactoryBuilder. Just adding an injection point suggests indeed that Spring Boot is going to provide one but I don't think that's a good idea.

My suggestion at this point would be to clarify the doc. Thoughts?

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