Auto-Configuration — 스프링부트는 설정 없이 어떻게 동작할까

@Configuration
public class DataSourceConfig {
    @Bean
    public DataSource dataSource() {
        HikariDataSource ds = new HikariDataSource();
        ds.setJdbcUrl("jdbc:mysql://localhost:3306/mydb");
        ds.setUsername("root");
        ds.setPassword("password");
        ds.setMaximumPoolSize(10);
        return ds;
    }

    @Bean
    public JdbcTemplate jdbcTemplate(DataSource dataSource) {
        return new JdbcTemplate(dataSource);
    }

    @Bean
    public PlatformTransactionManager transactionManager(DataSource dataSource) {
        return new DataSourceTransactionManager(dataSource);
    }
}

spring:
  datasource:
    url: jdbc:mysql://localhost:3306/mydb
    username: root
    password: password

@ConditionalOnClass(DataSource.class)
// DataSource 클래스가 클래스패스에 있을 때 (라이브러리가 의존성에 포함)

@ConditionalOnMissingClass("com.example.SomeClass")
// 특정 클래스가 클래스패스에 없을 때

@ConditionalOnBean(DataSource.class)
// DataSource 타입의 빈이 이미 등록되어 있을 때

@ConditionalOnMissingBean(DataSource.class)
// DataSource 타입의 빈이 아직 없을 때

@ConditionalOnProperty(name = "feature.enabled", havingValue = "true")
// 특정 프로퍼티가 특정 값일 때

@ConditionalOnWebApplication(type = Type.SERVLET)
// 서블릿 웹 애플리케이션일 때

@ConditionalOnResource(resources = "classpath:schema.sql")
// 특정 리소스 파일이 존재할 때

// DataSourceAutoConfiguration (개념적 구조)
@AutoConfiguration
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@ConditionalOnMissingBean(type = "io.r2dbc.spi.ConnectionFactory")
@EnableConfigurationProperties(DataSourceProperties.class)
public class DataSourceAutoConfiguration {

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnMissingBean(DataSource.class) // 사용자가 직접 등록하면 적용 안 됨
    @ConditionalOnProperty(name = "spring.datasource.url")
    static class PooledDataSourceConfiguration {

        @Bean
        @ConditionalOnMissingBean
        DataSource dataSource(DataSourceProperties properties) {
            return properties.initializeDataSourceBuilder()
                .type(HikariDataSource.class)
                .build();
        }
    }
}

# 방법 1: 커맨드라인
java -jar app.jar --debug

# 방법 2: application.yml
debug: true

============================
CONDITIONS EVALUATION REPORT
============================

Positive matches:
-----------------
   DataSourceAutoConfiguration matched:
      - @ConditionalOnClass found required classes 'javax.sql.DataSource',
        'org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType'

Negative matches:
-----------------
   RabbitAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class
           'com.rabbitmq.client.Channel'

@Configuration
public class MyDataSourceConfig {

    @Bean
    public DataSource dataSource() {
        // 사용자가 직접 DataSource를 등록하면
        // DataSourceAutoConfiguration의 DataSource 빈은 생성되지 않음
        HikariDataSource ds = new HikariDataSource();
        ds.setJdbcUrl("jdbc:mysql://custom-server:3306/mydb");
        ds.setMaximumPoolSize(50); // 커스텀 설정
        return ds;
    }
}

// 방법 1: 어노테이션으로 제외
@SpringBootApplication(exclude = {
    DataSourceAutoConfiguration.class,
    SecurityAutoConfiguration.class
})
public class MyApplication { ... }

// 방법 2: 프로퍼티로 제외
// application.yml
spring:
  autoconfigure:
    exclude:
      - org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration
      - org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration

// 1. 자동 설정 클래스 작성
@AutoConfiguration
@ConditionalOnClass(MyLibrary.class)
@EnableConfigurationProperties(MyLibraryProperties.class)
public class MyLibraryAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public MyLibraryClient myLibraryClient(MyLibraryProperties properties) {
        return new MyLibraryClient(properties.getApiKey(), properties.getTimeout());
    }
}

// 2. 프로퍼티 클래스
@ConfigurationProperties(prefix = "mylib")
public class MyLibraryProperties {
    private String apiKey;
    private Duration timeout = Duration.ofSeconds(30);
    // getter, setter
}

// 3. META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports
com.example.MyLibraryAutoConfiguration

# 사용자가 설정만 넣으면 자동으로 MyLibraryClient 빈이 생성됨
mylib:
  api-key: my-api-key
  timeout: 10s

@AutoConfiguration(
    after = DataSourceAutoConfiguration.class,    // DataSource 이후에 실행
    before = JpaRepositoriesAutoConfiguration.class // JPA Repository 이전에 실행
)
public class MyAutoConfiguration { ... }

# application.yml
management:
  endpoints:
    web:
      exposure:
        include: conditions

// Boot 3.x — @Configuration도 동작했음
@Configuration
@ConditionalOnClass(MyLibrary.class)
public class MyAutoConfig { ... }

// Boot 4.0 — @AutoConfiguration 필수
@AutoConfiguration
@ConditionalOnClass(MyLibrary.class)
public class MyAutoConfig { ... }

@AutoConfiguration
@ConditionalOnThreading(Threading.VIRTUAL)
public class VirtualThreadTaskExecutorAutoConfiguration {
    // Virtual Threads가 활성화된 경우에만 적용되는 설정
}