#EnableAutoConfiguration(exclude =...) on tests failed in Spring Boot 2.6.0 - spring

Question

I tried to upgrade my data-mongo example project to Spring Boot 2.6.0. There is a test designed to run against Testcontainers, I also included the embedded mongo dep for other tests, so I have to exclude the AutoConfiguration for embedded mongo to make sure this test working on Docker/testcontainers.
The following configuration worked well with Spring Boot 2.5.6.
#DataMongoTest
#ContextConfiguration(initializers = {MongodbContainerInitializer.class})
#EnableAutoConfiguration(exclude = EmbeddedMongoAutoConfiguration.class)
#Slf4j
#ActiveProfiles("test")
public class PostRepositoryTest {}
But after upgrading to Spring Boot 2.6.0 and running the application, I got the exception like this.
[ main] s.c.a.AnnotationConfigApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: o
rg.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'embeddedMongoServer' defined in class path resource [org/springframework/boot/autoconfig
ure/mongo/embedded/EmbeddedMongoAutoConfiguration.class]: Unsatisfied dependency expressed through method 'embeddedMongoServer' parameter 0; nested exception is org.springframework.bea
ns.factory.BeanCreationException: Error creating bean with name 'embeddedMongoConfiguration' defined in class path resource [org/springframework/boot/autoconfigure/mongo/embedded/Embed
dedMongoAutoConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [de.flap
doodle.embed.mongo.config.MongodConfig]: Factory method 'embeddedMongoConfiguration' threw exception; nested exception is java.lang.IllegalStateException: Set the spring.mongodb.embedd
ed.version property or define your own MongodConfig bean to use embedded MongoDB
Obviously, #EnableAutoConfiguration(exclude =...) did not affect the context in tests when upgrading to Spring Boot 2.6.0.
Update: Temporarily resolved it, see my answer below.

Answer 1

Just add:
#TestPropertySource(properties = "spring.mongodb.embedded.version=3.5.5")
annotation before your Unit Test and it will start working.
#Henning's answer has a good explanation of why you need this.

Answer 2

As of Spring Boot 2.6, the property spring.mongodb.embedded.version must be set to use the auto-configured embedded MongoDB. It's mentioned in the release notes: https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.6-Release-Notes#embedded-mongo
This is also what the error message you posted, advises to do: Set the spring.mongodb.embedd ed.version property or define your own MongodConfig bean to use embedded MongoDB
The annotation #DataMongoTest is meta-annotated with #ImportAutoConfiguration and #AutoConfigureDataMongo, and is designed to trigger auto-configuration of MongoDB unless explicitly disabled as you do in the working configuration examples.
In your first configuration example, the annotation #EnableAutoConfiguration(exclude = EmbeddedMongoAutoConfiguration.class) does not override this effect of #DataMongoTest.
With Spring Boot 2.5.6, the auto-configured MongodConfig bean is most likely also part of the application context but not effectively used. But this depends on the rest of the code and in particular on the MongodbContainerInitializer.

Answer 3

Use #ImportAutoConfiguration(exclude = ...) or #DataMongoTest(excludeAutoConfiguration = ...) on test classes to overcome this barrier when upgrading to Spring Boot 2.6.0.
#DataMongoTest
#ImportAutoConfiguration(exclude = EmbeddedMongoAutoConfiguration.class)
//other config are ommitted
public class PostRepositoryTest {}
//or
#DataMongoTest(excludeAutoConfiguration = EmbeddedMongoAutoConfiguration.class)
public class PostRepositoryTest {}