조건부 어노테이션 — @ConditionalOnClass가 자동 설정을 제어하는 원리

@AutoConfiguration
@ConditionalOnClass(DataSource.class)  // DataSource 클래스가 있을 때만
public class DataSourceAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean  // 사용자가 직접 정의하지 않았을 때만
    public DataSource dataSource(DataSourceProperties properties) {
        return properties.initializeDataSourceBuilder().build();
    }
}

@AutoConfiguration
public class JacksonAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean  // ObjectMapper 빈이 없을 때만 생성
    public ObjectMapper objectMapper(Jackson2ObjectMapperBuilder builder) {
        return builder.build();
    }
}

// 사용자가 직접 정의하면 자동 설정은 건너뜀
@Configuration
public class MyJacksonConfig {
    @Bean
    public ObjectMapper objectMapper() {
        ObjectMapper mapper = new ObjectMapper();
        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        return mapper;  // 이 빈이 우선
    }
}

@Configuration
@ConditionalOnProperty(
    name = "app.feature.notification.enabled",
    havingValue = "true",
    matchIfMissing = true  // 프로퍼티가 없어도 기본 활성화
)
public class NotificationAutoConfiguration {

    @Bean
    public NotificationService notificationService() {
        return new EmailNotificationService();
    }
}

# 명시적으로 끄기
app:
  feature:
    notification:
      enabled: false

@Configuration
@ConditionalOnWebApplication(type = Type.SERVLET)
public class WebMvcConfiguration {
    // 서블릿 웹 환경에서만 적용
}

@Configuration
@ConditionalOnWebApplication(type = Type.REACTIVE)
public class WebFluxConfiguration {
    // 리액티브 웹 환경에서만 적용
}

@Configuration
@ConditionalOnExpression(
    "${app.cache.enabled:true} and '${app.cache.type}' == 'redis'"
)
public class RedisCacheConfig {
    // app.cache.enabled=true이고 app.cache.type=redis일 때만
}

1. @ConditionalOnClass / @ConditionalOnMissingClass  ← 가장 먼저 (빠른 필터링)
2. @ConditionalOnBean / @ConditionalOnMissingBean
3. @ConditionalOnProperty
4. @ConditionalOnResource
5. @ConditionalOnExpression
6. 기타 커스텀 Condition

@FunctionalInterface
public interface Condition {
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

// @ConditionalOnClass의 내부 구현 (간략화)
class OnClassCondition extends SpringBootCondition {

    @Override
    public ConditionOutcome getMatchOutcome(
            ConditionContext context, AnnotatedTypeMetadata metadata) {

        // ASM으로 바이트코드 수준에서 클래스 존재 여부 확인
        // Class.forName()을 쓰지 않으므로 클래스 로딩 없이 검사 가능
        List<String> missing = filter(candidates, ClassNameFilter.MISSING,
            context.getClassLoader());

        if (!missing.isEmpty()) {
            return ConditionOutcome.noMatch("클래스 없음: " + missing);
        }
        return ConditionOutcome.match();
    }
}

// 특정 OS에서만 동작하는 조건
public class OnLinuxCondition implements Condition {

    @Override
    public boolean matches(
            ConditionContext context, AnnotatedTypeMetadata metadata) {
        String os = context.getEnvironment()
            .getProperty("os.name", "").toLowerCase();
        return os.contains("linux");
    }
}

// 사용
@Configuration
@Conditional(OnLinuxCondition.class)
public class LinuxSpecificConfig {
    @Bean
    public FileWatcher linuxFileWatcher() {
        return new InotifyFileWatcher();
    }
}

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Conditional(OnLinuxCondition.class)
public @interface ConditionalOnLinux {
}

// 사용
@Configuration
@ConditionalOnLinux
public class LinuxSpecificConfig { }

java -jar myapp.jar --debug
# 또는 application.yml에서
# debug: true

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

Positive matches:  (적용된 자동 설정)
-----------------
   DataSourceAutoConfiguration matched:
      - @ConditionalOnClass found required classes
        'javax.sql.DataSource', 'org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType'
      - @ConditionalOnMissingBean (types: io.r2dbc.spi.ConnectionFactory) did not find any beans

   HikariDataSource matched:
      - @ConditionalOnClass found required class 'com.zaxxer.hikari.HikariDataSource'
      - @ConditionalOnMissingBean did not find any beans of type 'javax.sql.DataSource'

Negative matches:  (건너뛴 자동 설정)
-----------------
   MongoAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class
           'com.mongodb.client.MongoClient'

   RedisAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class
           'org.springframework.data.redis.core.RedisOperations'

@SpringBootApplication(exclude = {
    DataSourceAutoConfiguration.class,
    SecurityAutoConfiguration.class
})
public class MyApp { }

# 또는 application.yml
spring:
  autoconfigure:
    exclude:
      - org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

@Configuration
@ConditionalOnProperty(
    prefix = "feature",
    name = "new-payment-flow",
    havingValue = "true"
)
public class NewPaymentFlowConfig {
    @Bean
    public PaymentProcessor paymentProcessor() {
        return new NewPaymentProcessor();
    }
}

@Configuration
@ConditionalOnProperty(
    prefix = "feature",
    name = "new-payment-flow",
    havingValue = "false",
    matchIfMissing = true  // 기본: 이전 플로우
)
public class LegacyPaymentFlowConfig {
    @Bean
    public PaymentProcessor paymentProcessor() {
        return new LegacyPaymentProcessor();
    }
}

@Configuration
public class StorageConfig {

    @Bean
    @ConditionalOnProperty(name = "storage.type", havingValue = "s3")
    public StorageService s3Storage() {
        return new S3StorageService();
    }

    @Bean
    @ConditionalOnProperty(name = "storage.type", havingValue = "local",
        matchIfMissing = true)
    public StorageService localStorage() {
        return new LocalStorageService();
    }
}