Add JPA components only on single DataSource

[kazuki43zoo]

Update auto-configuration logic so that JPA components
are only added when there is a single candidate DataSource bean.
Related with gh-6449

Please review.

[snicoll]

That's a bold move but I think I like it. I have a feeling it's going to break user's app, wdyt @wilkinsona ?

[wilkinsona]

I think it might too. I don't think such a change belongs in 1.4.1.

[zachmarshall]

Coming in via #9394 so apologies if I've missed something, but should DataSourceAutoConfiguration also be included?

[snicoll]

@zachmarshall if there is a DataSource (be it one or more) it's pointless to execute DataSourceAutoConfiguration. The PR needs to be reviewed but essentially the idea is to stop requiring a single DataSource and just backoff if there are more than one.

I really don't think that's a good way to fix your issue though. Let's keep the conversation on #9394 please.

[zachmarshall]

@snicoll what if you wanted DataSourceInitializer to run on the DataSource you create? Then DataSourceAutoConfiguration isn't useless right (since it creates the DataSourceInitializer)? Moreover, I'd really like to use a @Primary bean along with the autoconfigured DataSource that DataSourceAutoConfiguration creates (e.g. in the delegate pattern), but I understand if that is not supported and/or you think it shouldn't be done.

[snicoll]

There is somewhat related issue for datasource initialization #9528

[snicoll]

Duplicate of #5541

Thanks @kazuki43zoo for the PR, I ended up taking a slightly different route.