4.5. Logging

4.5.1. Logging Framework

• SLF4J + Logback must be used. (Provided by default in Spring Boot)
• System.out.println() must not be used for logging purposes.

4.5.2. Log Level Policy

Level	Usage Criteria
ERROR	System failures requiring immediate response
WARN	Potential issues; abnormal but service remains operational
INFO	Major business events, application state changes
DEBUG	Debugging information during development (disabled in production)
TRACE	Detailed flow tracing (disabled in production)

4.5.3. Log Writing Rules

• Declare a logger per class.

private static final Logger log = LoggerFactory.getLogger(UserService.class);

• Alternatively, use the Lombok @Slf4j annotation.
• Use parameter binding in log messages.

// Bad
log.info("User created: " + userId);

// Good
log.info("User created: userId={}", userId);

• Include the stack trace when logging exceptions.

log.error("User lookup failed: userId={}", userId, exception);

4.5.4. Production Log Format

<pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>