Some usage of @ConditionalOnMissingBean does not allow a bean defined as an interface to cause a concrete implementation to back off

[wilkinsona]

See DATAMONGO-2355 and this question on Stack Overflow for some background.

We have some places in our auto-configuration where we define a bean using a concrete type where it's typically injected as an interface. The definition that prompted this issue is the following:

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/data/mongo/MongoReactiveDataAutoConfiguration.java

Lines 74 to 79 in 54126f6

	@Bean
	@ConditionalOnMissingBean
	public ReactiveMongoTemplate reactiveMongoTemplate(ReactiveMongoDatabaseFactory reactiveMongoDatabaseFactory,
	MongoConverter converter) {
	return new ReactiveMongoTemplate(reactiveMongoDatabaseFactory, converter);
	}

A ReactiveMongoOperations bean won't cause the reactiveMongoTemplate bean to back off when it probably should. We're inconsistent about this as another bean in the same class will back off in the presence of a bean defined using the interface:

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/data/mongo/MongoReactiveDataAutoConfiguration.java

Lines 81 to 88 in 54126f6

	@Bean
	@ConditionalOnMissingBean(MongoConverter.class)
	public MappingMongoConverter mappingMongoConverter(MongoMappingContext context,
	MongoCustomConversions conversions) {
	MappingMongoConverter mappingConverter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, context);
	mappingConverter.setCustomConversions(conversions);
	return mappingConverter;
	}

While beans should generally be defined using their concrete type so that the bean factory has as much type information as possible, mocking a bean using an interface that it implements is a use case for defining a bean using an interface. With the current arrangement, a test that mocks MongoOperations and attempts to autowire that mock will fail:

@Autowired
MongoOperations mongoOperations;

@TestConfiguration
static class MockOperationsConfiguration {

	@Bean
	MongoOperations mockedMongoOperations(MongoConverter converter) {
		MongoOperations mongoOperations = mock(MongoOperations.class);
		given(mongoOperations.getConverter()).willReturn(converter);
		return mongoOperations;
	}

}

The failure occurs because there are two MongoOperations beans: mockedMongoOperations from the test configuration and mongoTemplate from Boot's auto-configuration.

[wilkinsona]

The necessary changes are not very widespread, touching 13 files with 30 additions and 17 deletions.

[philwebb]

Let's try in 2.1.x then

[wilkinsona]

I messed up a bit and this went into 2.2 early and was actually part of 2.2.0.M6: 80b8f19