Spring Boot @Component doesn't create Beans

Question

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.

Answer 1

TLDR;

A Spring context needs to be created before any bean can be injected. In the first scenario, just the fact of having a @SpringBootApplication decorator does not ensure a context in the scope of the class it decorates.

• SpringApplication.run(ExampleApplication.class, args); instantiates a context (and e.g. a web server among other things)
• var context = new AnnotationConfigApplicationContext(Config.class); instantiates a scoped context

Thus the first example had null inside of Building as there was no context with the bean to inject.

Answer 2

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

Answer 3

The problem with your code is that you are trying to @Autowire on a static field. You simply cannot do that. Look here: Can you use @Autowired with static fields?

It fails to work because the PlaygroundApplication class is not being created and managed by spring. The injection works only inside instances managed by spring. You can treat class annotated with @SpringBootApplication as configuration classes. Spring creates instances of those classes and injection works inside them but only on instance fields.

The second example shows the correct way to access spring beans from main method of the application.