How To Fix Your First Spring Boot API Without Facing White Label Error

Posted By : Mohit Jain | 29-Apr-2018

Java

Spring supports following types of stereotype annotations :-

1.) @Controller :- If our class instance(Object) is used as controller to get request from incoming url

2.)@Service :- If our class instance(Object) has bussiness logic(some calculation or filteration on data came as input from controller/as input from repository) to implement

3.)@Repository :-To persist or retrieve our data after or before our bussiness logic in @Service annotated class

4.)@Component :- If we want some utilities for our application(may be third party web service or our own created services)

For Our first Spring boot "Hello My First Spring Boot Application" as response "without getting white lablel error with status 404" ,we required only @Controller annotated class instance(Object)

/*******************************************************************************************************/

Main class

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringBootFirstApplication {

   public static void main(String[] args) {
       SpringApplication.run(SpringBootFirstApplication .class, args);
   }
}

/********************************************************************************************************/

Controller class

import org.springframework.stereotype.Controller;

import org.springframework.web.bind.annotation.RequestMapping;

@Controller

public class HelloController {

    @RequestMapping("/") 

    String home() {

        return "Hello My First Spring Boot Application";

}

}

Actually the instance of above controller is created and loaded in Application context(spring Ioc container) if and only if, above controller is scanned by spring container either of the following 3 ways (else you will encounter " white lablel error with status 404"):-

1.)If your controller class is within the same package of your main class

Example:- Main class and controller class is within " package com.mainpackage"

2. If you controller class is within child package of you main class package

Example:- Main class is in "package com.mainpackage" and controller class is within child of main package "package com.mainpackage. controller"

3.)If yourMain class and controller class package has no relation,then you must annotate or pass @ComponentScan(“com.someotherpackage.controller”) in your main class

Main class
import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication

@ComponentScan(“com.someotherpackage.controller”)
public class SpringBootFirstApplication {
    public static void main(String[] args) {

        SpringApplication.run(SpringBootFirstApplication .class, args);

    }

}