spring - Spring Boot @Component 不会创建 Bean

Question

由于根据文档@Component为 Spring 容器注册 bean，我正在尝试使用以下代码创建一个简单的依赖注入示例：

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);
    }

    public void setBuildingSize(Long buildingSize) {
        this.buildingSize = buildingSize;
    }
}

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;
    }   
}

它构建并运行，但有一个NullPointerExceptionat building.setBuildingSize(12L);。然而，下面的工作没有任何问题：

//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();
    }
}

package pl.playground.facade;

//...

public class Building {

    private HeatingService service;
    private Long buildingSize;

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

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

    public void setBuildingSize(Long buildingSize) {
        this.buildingSize = buildingSize;
    }
}

package pl.playground.service;

public interface HeatingService {

    Double getMonthlyHeatingCost(Long size);
}

package pl.playground.service;

public class HeatingServiceImpl implements HeatingService {

    private final Double CUBIC_PRICE = 2.3;

    public HeatingServiceImpl() {}

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

• 为什么@Component不创建 Beans？

• 它以我认为应该在@ControllerWeb 应用程序中使用的方式工作，这有什么不同吗？究竟@Bean和@Component有什么不同？

• 我不明白什么？

编辑

考虑以下场景：

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;
    }

    //...
}

我不需要@Configuration在这里让它工作。这是我的担心。

标签： springspring-bootjavabeans

Answer 1

您的代码的问题是您正在尝试@Autowire使用静态字段。你根本无法做到这一点。看这里：你可以将@Autowired 与静态字段一起使用吗？

它无法工作，因为PlaygroundApplication该类不是由 spring 创建和管理的。注入仅在 spring 管理的实例中起作用。您可以将带有注释的@SpringBootApplication类视为配置类。Spring 创建这些类的实例，并且注入在它们内部工作，但仅在实例字段上工作。

第二个示例显示了从应用程序的方法访问 spring bean 的正确方法main。