What is white label error in springboot?

As a beginner to SpringBoot, this error can be happened to anyone at least once, due to some reasons.

1. The Application class (eg: SpringAppApplication.java) should be at the root level, and the controller classes can be placed under the same level or maybe in a subdirectory. This application class should be denoted using @SpringBootApplication or extends from SpringBootApplication
2. You may miss the @RestController annotation on top of the controller class.
3. Can be missed the mapping annotation.( @PostMapping("/save") or @GetMapping("/id") etc.)
4. finally check whether you are entering the correct request mapping address(URL), because you may miss a slash or annotation. else double-check the spelling. eg:@RequestMapping("/customer")