Spring Boot 自动装配：@EnableAutoConfiguration 原理

@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}
// 没有 web.xml，没有 DispatcherServlet 配置，没有内嵌 Tomcat 配置
// 但启动后：Tomcat 跑在 8080，DispatcherServlet 已注册，JSON 序列化就绪
// 这一切是怎么发生的？

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration      // ① 本质就是 @Configuration，标记当前类是配置类
@EnableAutoConfiguration      // ② 开启自动装配 —— 今天的核心主角
@ComponentScan(               // ③ 扫描当前包及其子包下的 @Component
    excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class)
    }
)
public @interface SpringBootApplication {
    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};  // 排除指定的自动配置类

    @AliasFor(annotation = EnableAutoConfiguration.class)
    String[] excludeName() default {};
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage    // 记录自动配置的基础包路径
@Import(AutoConfigurationImportSelector.class)  // ← 关键！导入选择器
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    Class<?>[] exclude() default {};
    String[] excludeName() default {};
}

public class AutoConfigurationImportSelector
        implements DeferredImportSelector {

    @Override
    public String[] selectImports(AnnotationMetadata metadata) {
        // 1. 获取所有候选配置类的全限定名
        List<String> configurations =
            getCandidateConfigurations(metadata, attributes);
        //    └→ 内部调用 SpringFactoriesLoader.loadFactoryNames()
        //       读取 META-INF/spring.factories 文件
        //       key = EnableAutoConfiguration.class.getName()

        // 2. 去重
        configurations = removeDuplicates(configurations);

        // 3. 排除用户 exclude 的类
        Set<String> exclusions = getExclusions(metadata, attributes);
        configurations.removeAll(exclusions);

        // 4. 排序（按 @AutoConfigureOrder / @AutoConfigureBefore / After）
        sort(configurations);

        return configurations.toArray(new String[0]);
    }
}

# Spring Boot 2.x 的注册方式
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
  org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
  org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
  # ... 共 130+ 个自动配置类

# 新格式：每行一个全限定名，更简洁
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

@Configuration
@ConditionalOnClass(ObjectMapper.class)   // classpath 下有 Jackson 才生效
public class JacksonAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean             // 用户没自己定义 ObjectMapper 时才装配
    public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
        return builder.createXmlMapper(false).build();
    }
}

@Configuration
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@EnableConfigurationProperties(DataSourceProperties.class)
public class DataSourceAutoConfiguration {

    @Configuration
    @ConditionalOnMissingBean(DataSource.class)  // 用户没配 DataSource 才走这里
    @Conditional(EmbeddedDatabaseCondition.class) // classpath 下有 H2/HSQL/Derby
    static class EmbeddedDatabaseConfiguration {
        @Bean
        public DataSource dataSource() {
            return new EmbeddedDatabaseBuilder()
                .setType(EmbeddedDatabaseType.H2).build();
        }
    }

    @Configuration
    @ConditionalOnProperty(prefix = "spring.datasource", name = "url")
    static class PooledDataSourceConfiguration {
        // 用户配了 spring.datasource.url 才走 HikariCP 连接池
    }
}

<!-- 自动配置模块的 pom -->
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
    </dependency>
</dependencies>

@ConfigurationProperties(prefix = "sms")
public class SmsProperties {
    private String accessKey;       // sms.access-key
    private String secretKey;       // sms.secret-key
    private String signName = "默认签名"; // sms.sign-name

    // getter / setter 省略
}

@Configuration
@ConditionalOnClass(SmsClient.class)     // classpath 下有 SmsClient 才生效
@ConditionalOnProperty(                   // 配置了 sms.access-key 才生效
    prefix = "sms",
    name = "access-key",
    matchIfMissing = false
)
@EnableConfigurationProperties(SmsProperties.class)
public class SmsAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean               // 用户没自定义时才装配默认实现
    public SmsClient smsClient(SmsProperties props) {
        return new DefaultSmsClient(
            props.getAccessKey(),
            props.getSecretKey(),
            props.getSignName()
        );
    }

    @Bean
    @ConditionalOnMissingBean
    public SmsTemplate smsTemplate(SmsClient client) {
        return new SmsTemplate(client);    // 封装发送短信的便捷方法
    }
}

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.example.sms.autoconfigure.SmsAutoConfiguration

com.example.sms.autoconfigure.SmsAutoConfiguration

<!-- Starter 模块：一个 Bean 都没有，只做依赖引入 -->
<dependencies>
    <dependency>
        <groupId>com.example</groupId>
        <artifactId>sms-spring-boot-autoconfigure</artifactId>
        <version>${project.version}</version>
    </dependency>
    <dependency>
        <groupId>com.example</groupId>
        <artifactId>sms-sdk</artifactId>
        <version>1.0.0</version>   <!-- 引入底层 SDK -->
    </dependency>
</dependencies>

sms:
  access-key: AKID2024XXXX
  secret-key: SK2024YYYY
  sign-name:  "我的公司"

@Service
public class OrderService {

    @Autowired
    private SmsTemplate smsTemplate;  // 自动注入，零配置

    public void notifyShipped(String phone, String orderNo) {
        smsTemplate.send(phone, "您的订单 " + orderNo + " 已发货");
    }
}