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

[reda-alaoui]

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:

spring-hateoas/src/main/java/org/springframework/hateoas/config/HateoasConfiguration.java

Lines 136 to 154 in d6265fb

	private final AbstractMessageSource lookupMessageSource() {
	List<Resource> candidates = loadResourceBundleResources(I18N_DEFAULTS_BASE_NAME, false);
	if (candidates.isEmpty() && loadResourceBundleResources(I18N_BASE_NAME, true).isEmpty()) {
	return null;
	}
	ReloadableResourceBundleMessageSource messageSource = new ReloadableResourceBundleMessageSource();
	messageSource.setResourceLoader(context);
	messageSource.setBasename("classpath:".concat(I18N_BASE_NAME));
	messageSource.setDefaultEncoding(StandardCharsets.UTF_8.toString());
	if (!candidates.isEmpty()) {
	messageSource.setCommonMessages(loadProperties(candidates));
	}
	return messageSource;
	}

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.