Configure MessageSource if no "messageSource" bean defined

[cac03]

Hello.

This PR fixes an issue with MessageSourceAutoConfiguration when a MessageSource bean with non standard name defined.

In particular ApplicationContext expects a MessageSource bean defined only with "messageSource" name, but MessageSourceAutoConfiguration does not convey the same policy.

So when a MessageSource is defined with non-standard name MessageSourceAutoConfiguration does not expose a MessageSource named "messageSource" and at the same time ApplicationContext does not use defined MessageSource and message resolution process fails.

See AbstractApplicationContext.initMessageSource().
It is also stated in the reference documentation:

When an ApplicationContext is loaded, it automatically searches for a MessageSource bean defined in the context. The bean must have the name messageSource. If such a bean is found, all calls to the preceding methods are delegated to the message source. If no message source is found, the ApplicationContext attempts to find a parent containing a bean with the same name. If it does, it uses that bean as the MessageSource. If the ApplicationContext cannot find any source for messages, an empty DelegatingMessageSource is instantiated in order to be able to accept calls to the methods defined above.

Project reproducing the issue.
See created tests. In particular:

• MessageSourceWithNonStandardNameTests demonstrates expected behaviour.
• MessageSourceWithStandardNameTests demonstrates behaviour when MessageSource is defined with "messagesSource" name.
• NotMessageSourceMessageSourceTests demonstrates that a bean named "messageSource" must actually be MessageSource

[philwebb]

Thanks for the PR. We think that this change makes sense but we'll target it to 2.2.x just in case someone has overridden AbstractApplicationContext.initMessageSource() to use different logic and is relying on the existing behavior.

[cac03]

@philwebb thank you.
I'm curious to know if you discussed the idea to drop the requirement about MessageSource bean name in AbstractApplicationContext itself.
If yes, then did you decide to leave it as is because of other applications relying on this requirement?

[philwebb]

@cac03 We didn't discuss removing that requirement. The AbstractApplicationContext is part of Spring Framework and I don't think they'd be able to make such a change because of back-compatibility concerns.

[snicoll]

@cac03 thank you for making your first contribution to Spring Boot. This is now merged with a polish commit.