Mongo tries to connect automatically to port 27017(localhost)

Spring Boot has a feature called "auto configuration". In this case, as soon as the Mongo driver is detected on the classpath, the MongoAutoConfiguration is activated with default values, which point to localhost:27017. If you don't want that behaviour, you can now either configure the properties for MongoDB (see http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-mongodb for valid property keys) or disable the MongoAutoConfiguration:

@SpringBootApplication(exclude = {MongoAutoConfiguration.class, MongoDataAutoConfiguration.class})

Spring boot throws this exception when Mongo DB is not running. Please make sure that Mongodb is running. It got resolved for me after starting Mongo DB.

In some cases, if you are using reactive you also need to remove MongoReactiveAutoConfiguration

spring:
  autoconfigure:
    exclude:
      - org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration
      - org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration