[FIXED] Spring Boot @Component doesn't create Beans

Issue

Since according to the docs @Component registers beans for the Spring container I'm trying to create a simple example of dependency injection using the following code:

package pl.playground;

//...

@SpringBootApplication
public class PlaygroundApplication {

    @Autowired
    private static Building building;

    public static void main(String[] args) {
        building.setBuildingSize(12L);
        System.out.println(building.monthlyHeatingCost());
    }
}

package pl.playground.facade;

//...

@Component
public class Building {

    private HeatingService service;
    private Long buildingSize;

    @Autowired
    public Building(HeatingService service) {
        this.service = service;
    }

    public Double monthlyHeatingCost() {
        return service.getMonthlyHeatingCost(buildingSize);
    }

    // getters & setters...
}

package pl.playground.service;

public interface HeatingService {
    Double getMonthlyHeatingCost(Long size);
}

package pl.playground.service;

//...

@Component
public class HeatingServiceImpl implements HeatingService {

    private final Double CUBIC_PRICE = 2.3;

    public HeatingServiceImpl() {}

    @Override
    public Double getMonthlyHeatingCost(Long size) {
        return size * CUBIC_PRICE;
    }   
}

It builds and runs, but there is a NullPointerException at building.setBuildingSize(12L);. However the one below works without any issues:

//PlaygroundApplication.java
package pl.playground;

//...

@SpringBootApplication
public class PlaygroundApplication {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Config.class);

        Building building = context.getBean(Building.class);

        building.setBuildingSize(12L);
        System.out.println(building.monthlyHeatingCost());
    }
}

package pl.playground.config;

//...

@Configuration
public class Config {

    @Bean
    public Building building(HeatingService service) {
        return new Building(service);
    }

    @Bean
    public HeatingServiceImpl heatingServiceImpl() {
        return new HeatingServiceImpl();
    }
}

The rest is the same as before.

• Why is @Component not creating Beans?

• It is working the way I think it should when used inside a @Controller of a web app, does that make a difference? How does exactly @Bean and @Component differ?

• What am I failing to understand?

EDIT

Consider the following scenario:

package pl.playground;

//...

@SpringBootApplication
public class ExampleApplication {

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

package pl.playground.controller;

//...

@Controller
public class Controller {

    private Facade facade;

    @Autowired
    public Controller(Facade facade) {
        this.facade = facade;
    }

    @GetMapping("/")
    public String getIndexPage(Model model) {
        return "index";
    }
}

package pl.playground.facade;

//...

@Component
public class Facade {

    private PostsService postService;
    private UserService userService;
    private TagService tagService;

    @Autowired
    public Facade(PostsService retrieve, UserService user, TagService tag) {
        this.postService = retrieve;
        this.userService = user;
        this.tagService = tag;
    }

    //...
}

I don't need @Configuration here for it to work. That's my concern.

Solution

Well. I used your original question and is working without any issues. @cezary-butler pointed out in the comments you can autowire into PlaygroundApplication but you can get hold of it easily in the static main method using context.getBean(Building.class)

    @SpringBootApplication
    public class PlaygroundApplication {

      public static void main(String[] args) {
        ConfigurableApplicationContext context = 
              SpringApplication.run(PlaygroundApplication.class);
        Building building = context.getBean(Building.class);
        building.setBuildingSize(12L);
        System.out.println(building.monthlyHeatingCost());
      }
    }

• Here is the sample repo https://github.com/kavi-kanap/stackoverflow-63072236

Answered By - Kavithakaran Kanapathippillai
Answer Checked By - Timothy Miller (JavaFixing Admin)