Spring 애플리케이션에서 로그를 잘 남기는 방법

User 123 logged in to system

{
  "timestamp": "2025-05-16T21:15:00+09:00",
  "id": "123",
  "level": "info",
  "event": "logged in"
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'net.logstash.logback:logstash-logback-encoder'
}

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="net.logstash.logback.encoder.LogstashEncoder">
            <includeMdc>true</includeMdc>
            <includeContext>false</includeContext>
            <customFields>{"application":"my-spring-app"}</customFields>
        </encoder>
    </appender>
 
    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>logs/application.log</file>
        <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
            <fileNamePattern>logs/application-%d{yyyy-MM-dd}.log</fileNamePattern>
            <maxHistory>30</maxHistory>
        </rollingPolicy>
        <encoder class="net.logstash.logback.encoder.LogstashEncoder">
            <includeMdc>true</includeMdc>
            <includeContext>false</includeContext>
            <customFields>{"application":"my-spring-app"}</customFields>
        </encoder>
    </appender>
 
    <root level="INFO">
        <appender-ref ref="CONSOLE" />
        <appender-ref ref="FILE" />
    </root>
</configuration>

@Component
public class LoggingFilter extends OncePerRequestFilter {
 
    @Autowired
    private RequestMappingHandlerMapping handlerMapping;
 
    @Override
    protected void doFilterInternal(
\\t\\t\\tHttpServletRequest request, HttpServletResponse response, 
            FilterChain filterChain) throws ServletException, IOException {
        try {
            // timestamp - logback에서 기본 제공하지만 MDC에도 추가
            String timestamp = LocalDateTime.now()
\\t\\t\\t\\t\\t\\t.format(DateTimeFormatter.ISO_DATE_TIME);
            MDC.put("timestamp", timestamp);
 
            // thread 정보 추가
            MDC.put("thread", Thread.currentThread().getName());
 
            // 요청 ID 생성 (단일 요청 식별용)
            String requestId = UUID.randomUUID().toString();
            MDC.put("requestId", requestId);
 
            // 클라이언트 정보 MDC에 추가
            MDC.put("clientIp", request.getRemoteAddr());
            MDC.put("userAgent", request.getHeader("User-Agent"));
            MDC.put("requestUrl", request.getRequestURI());
            MDC.put("requestMethod", request.getMethod());
 
           filterChain.doFilter(request, response);
        } finally {
            // 필터 체인 완료 후 MDC 정리
            MDC.clear();
        }
    }
}

@RestController
public class DemoController {
    private static final Logger logger = LoggerFactory.getLogger(DemoController.class);
 
    @GetMapping("/hello")
    public String hello() {
        logger.info("Hello endpoint called");
 
        try {
            // 비즈니스 로직
            logger.debug("Processing business logic");
 
            // 구조화된 로깅을 위해 중요 데이터 포함
            logger.info("Operation completed successfully", 
                        "operation", "hello", 
                        "processingTime", "10ms");
 
            return "Hello, World!";
        } catch (Exception e) {
            logger.error("Error in hello endpoint", e);
            throw e;
        }
    }
}