Spring Boot使用注解重构IoC容器的实战示例

// 没有管家的苦日子
Employee developer = new Developer();
Employee manager = new Manager();
Project project = new Project(developer, manager);

// 有管家的幸福生活
@Autowired
private Project project; // 管家，帮我组建一个项目团队！

// 启动Spring容器（聘请管家）
ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
// 向管家索取需要的服务
ProjectService projectService = context.getBean(ProjectService.class);

@SpringBootApplication // ← 这就是聘书，包含三个重要指令
public class CompanyApplication {
    public static void main(String[] args) {
        SpringApplication.run(CompanyApplication.class, args); // 正式聘请管家
    }
}

@SpringBootConfiguration  // ← 指令1：这是家正规公司（配置类）
@EnableAutoConfiguration  // ← 指令2：管家可以自主决策（自动配置）
@ComponentScan            // ← 指令3：在公司内寻找人才（组件扫描）
public class CompanyApplication {
    // ...
}

@Component         // ← 普通员工工牌
public class Developer {
    public void code() {
        System.out.println("编写代码...");
    }
}
@Service           // ← 管理层工牌（特殊待遇）
public class ProjectService {
    private final Developer developer;
    public ProjectService(Developer developer) {
        this.developer = developer;
    }
}
@Repository        // ← 仓库管理员工牌（有错误转换特权）
public class ProjectRepository {
    // 数据访问逻辑
}
@Controller        // ← 前台接待员工牌（处理外部请求）
public class ProjectController {
    private final ProjectService projectService;
    public ProjectController(ProjectService projectService) {
        this.projectService = projectService;
    }
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component // 所有注解都是@Component的特殊化
public @interface Service {
    // ...
}

@Service
public class ProjectService {
    // 方式1：字段注入（直接配助手）- 简单但不推荐
    @Autowired
    private Developer developer;
    // 方式2：构造器注入（招聘时指定助手）- 推荐方式
    private final Designer designer;
    @Autowired
    public ProjectService(Designer designer) {
        this.designer = designer; // 一旦配备，不可更改
    }
    // 方式3：Setter注入（后期配置设备）- 可选依赖
    private Printer printer;
    @Autowired
    public void setPrinter(Printer printer) {
        this.printer = printer; // 可以后期更换
    }
}

@Configuration // ← 人力资源部
public class CompanyConfig {
    @Bean // ← 手动签发聘书
    public SpecialExpert specialExpert() {
        return new SpecialExpert(); // 来自第三方库的专家
    }
    @Bean
    public ProjectTeam projectTeam(Developer developer, SpecialExpert expert) {
        // 管家会自动提供需要的依赖
        return new ProjectTeam(developer, expert);
    }
}

@Configuration
public class SmartConfiguration {
    // 只有当公司有数据库时，才招聘DBA
    @Bean
    @ConditionalOnClass(DataSource.class)
    public DatabaseAdmin databaseAdmin() {
        return new DatabaseAdmin();
    }
    // 只有当需要处理Web请求时，才配备Web团队
    @Bean
    @ConditionalOnWebApplication
    public WebTeam webTeam() {
        return new WebTeam();
    }
    // 只有当配置了邮件服务器时，才设立邮件部门
    @Bean
    @ConditionalOnProperty(name = "mail.host")
    public EmailService emailService() {
        return new EmailService();
    }
}

# spring.factories文件内容
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaAutoConfiguration

@Configuration
@ConditionalOnClass(DataSource.class) // 有数据库驱动才生效
@ConditionalOnProperty(prefix = "spring.datasource", name = "url") // 配置了URL才生效
@EnableConfigurationProperties(DataSourceProperties.class) // 绑定配置属性
public class DataSourceAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean // 如果没有自定义DataSource才创建
    public DataSource dataSource(DataSourceProperties properties) {
        return properties.initializeDataSourceBuilder().build();
    }
}

// 1. 启动类（公司总部）
@SpringBootApplication
public class CompanyApplication {
    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(CompanyApplication.class, args);
        // 从容器中获取bean
        Project project = context.getBean(Project.class);
        project.start();
    }
}
// 2. 服务层（管理部门）
@Service
public class ProjectService {
    private final Developer developer;
    private final Designer designer;
    // 构造器注入
    public ProjectService(Developer developer, Designer designer) {
        this.developer = developer;
        this.designer = designer;
    }
    public void startProject() {
        developer.code();
        designer.design();
    }
}
// 3. 组件类（员工）
@Component
public class Developer {
    public void code() {
        System.out.println("程序员编写代码...");
    }
}
@Component
public class Designer {
    public void design() {
        System.out.println("设计师设计界面...");
    }
}
// 4. 配置类（人力资源部）
@Configuration
public class CompanyConfig {
    @Bean
    public Project project(ProjectService projectService) {
        return new Project(projectService);
    }
}
// 5. 项目类
public class Project {
    private final ProjectService projectService;
    public Project(ProjectService projectService) {
        this.projectService = projectService;
    }
    public void start() {
        System.out.println("项目启动！");
        projectService.startProject();
    }
}