Spring Boot Starter 自动装配原理全解析

public String[] selectImports(AnnotationMetadata metadata) {
  // 1. 加载所有候选自动配置类
  List<String> configurations = getCandidateConfigurations();
  // 2. 去重、过滤、排序
  configurations = removeDuplicates(configurations);
  configurations = filter(configurations, autoConfigurationMetadata);
  return configurations.toArray(new String[0]);
}

hello-spring-boot-starter-parent（父POM）
├── hello-spring-boot-autoconfigure（自动配置模块）
└── hello-spring-boot-starter（Starter模块）

hello-spring-boot-starter/
├── hello-spring-boot-autoconfigure/
│   ├── src/
│   │   ├── main/
│   │   │   ├── java/com/example/autoconfigure/
│   │   │   │   ├── HelloAutoConfiguration.java
│   │   │   │   ├── HelloProperties.java
│   │   │   │   └── HelloService.java
│   │   │   └── resources/
│   │   │       └── META-INF/
│   │   │           └── spring.factories
│   │   └── test/
│   └── pom.xml
├── hello-spring-boot-starter/
│   └── pom.xml
└── pom.xml

<!-- pom.xml -->
<dependencies>
    <!-- Spring Boot 自动配置基础依赖 -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
        <version>3.1.5</version>
    </dependency>
    <!-- 可选：配置属性处理 -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-configuration-processor</artifactId>
        <version>3.1.5</version>
        <optional>true</optional>
    </dependency>
</dependencies>

public class HelloService {
    private String message = "Hello, World!";  // 默认消息
    public String sayHello() {
        return message;
    }
    // Getter和Setter用于通过配置修改message
    public String getMessage() { return message; }
    public void setMessage(String message) { this.message = message; }
}

@ConfigurationProperties(prefix = "hello")
public class HelloProperties {
    private String message = "Hello, World!";
    // Getter和Setter
    public String getMessage() { return message; }
    public void setMessage(String message) { this.message = message; }
}

@Configuration
@EnableConfigurationProperties(HelloProperties.class)  // 启用配置属性
@ConditionalOnClass(HelloService.class)  // 当HelloService在类路径时生效
public class HelloAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean  // 当用户未自定义HelloService时生效
    public HelloService helloService(HelloProperties properties) {
        HelloService service = new HelloService();
        service.setMessage(properties.getMessage());
        return service;
    }
}

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.example.autoconfigure.HelloAutoConfiguration

<!-- pom.xml -->
<dependencies>
    <!-- 引入自动配置模块 -->
    <dependency>
        <groupId>com.example</groupId>
        <artifactId>hello-spring-boot-autoconfigure</artifactId>
        <version>1.0.0</version>
    </dependency>
</dependencies>

<!-- 用户项目的pom.xml -->
<dependency>
    <groupId>com.example</groupId>
    <artifactId>hello-spring-boot-starter</artifactId>
    <version>1.0.0</version>
</dependency>

@RestController
public class HelloController {
    @Autowired
    private HelloService helloService;
    @GetMapping("/hello")
    public String hello() {
        return helloService.sayHello();
    }
}

hello.message=你好, Spring Boot!