Invalid inputs are the root of all evil! Or would you run your coffee maker on rocks? How to start ? We need the dependency: <dependency> <groupId> org.springframework.boot </groupId> <artifactId> spring-boot-starter-validation </artifactId> </dependency> When do we validate? As a rule of thumb for REST APIs: As soon as a controller method accepts a @RequestBody , it must be preceded by a @Valid ! ...