In some situations, `HateoasConfiguration#lookupMessageSource` wrongly returns null instead of `ReloadableResourceBundleMessageSource`

While trying to define rest-messages.properties files to generate HAL Forms property prompts, I discovered the following - IMHO buggy - behavior.

Steps to reproduce

Given a maven project, suppose you have the following src/main/resources/rest-message.properties

foo._prompt=Foo

The compilation copies rest-message.properties to target/classes/rest-message.properties.

Now you run a Spring Boot test. The context initialization triggers the following method:

https://github.com/spring-projects/spring-hateoas/blob/d6265fb1c360040c049830bb784b3e487a1542a8/src/main/java/org/springframework/hateoas/config/HateoasConfiguration.java#L136-L154

Actual result

The method returns null because loadResourceBundleResources(I18N_BASE_NAME, true) returns an empty collection. Consequently, the prompts are never rendered in the test context.

Expected result

The method should return a non null AbstractMessageSource to allow prompt rendering in the test context.

Digging

In this case, if we ignore line 138 to 142 and build a ReloadableResourceBundleMessageSource anyway, the latter is able to resolve any message from the main file:

@Test
void test() {
   ReloadableResourceBundleMessageSource messageSource =
        new ReloadableResourceBundleMessageSource();
    messageSource.setResourceLoader(context);
    messageSource.setBasename("classpath:".concat("rest-messages"));
    messageSource.setDefaultEncoding(StandardCharsets.UTF_8.toString());
    String message = messageSource.getMessage("foo._prompt", new Object[0], Locale.getDefault());
    assertThat(message).isEqualTo("Foo");
}

Therefore, I think the preliminary checks (138 to 142) are not "in sync" with the way ReloadableResourceBundleMessageSource actually works.

Start in HateoasConfiguration.java around lines 136–154 and reproduce the Spring Boot test with src/main/resources/rest-message.properties containing foo._prompt=Foo. Check why loadResourceBundleResources(I18N_BASE_NAME, true) is empty, then verify the resulting message source is non-null and resolves foo._prompt from the main resource.