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})
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
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.