Logging

🔍 What Is It?

Spring Boot's logging stack:

• →SLF4J — the logging facade (API you code against)
• →Logback — the default implementation
• →Log4j2 — alternative implementation (better async performance)
• →Lombok @Slf4j — generates the logger field automatically

Log levels (in order of severity):

• →TRACE — extremely detailed (method entry/exit)
• →DEBUG — developer debugging
• →INFO — significant events (startup, key operations)
• →WARN — unexpected but recoverable situations
• →ERROR — errors that need attention

MDC (Mapped Diagnostic Context):

Thread-local key-value store that Logback includes in every log line from that thread. Use for: request ID, user ID, trace ID, tenant ID.

🎯 Why Do We Need It?

Good logging is essential for production operations:

• →Debugging production issues without attaching a debugger
• →Tracing a request across multiple services
• →Auditing user actions
• →Alerting on error patterns
• →Performance analysis

🏗️ What Happens in Real Projects?

Production logging setup:

• →JSON structured logs sent to ELK (Elasticsearch/Logstash/Kibana) or Splunk
• →MDC with requestId, userId, traceId on every log line
• →ERROR logs trigger PagerDuty alerts
• →Log levels changed at runtime via Actuator without restart

⚙️ How It Works

SLF4J is a facade — your code calls SLF4J API, which delegates to the actual implementation (Logback). This means you can switch from Logback to Log4j2 without changing any application code. Logback's PatternLayout or LogstashEncoder formats each log event into the output format.

💻 Example

Production Logging Setup — Structured JSON logging with MDC and request correlation.

@Service
@Slf4j  // Lombok generates: private static final Logger log = LoggerFactory.getLogger(OrderService.class);
public class OrderService {

    public OrderDetailDto placeOrder(CreateOrderRequest request, String userId) {
        log.info("Placing order for user {} with {} items",
            userId, request.getItems().size());

        try {
            Order order = orderRepository.save(Order.from(request));
            log.info("Order {} created successfully for user {}",
                order.getOrderNumber(), userId);

            paymentService.charge(order);
            log.debug("Payment processed for order {}", order.getOrderNumber());

            return OrderDetailDto.from(order);

        } catch (PaymentException e) {
            log.error("Payment failed for order request from user {}: {}",
                userId, e.getMessage(), e);  // include exception for stack trace
            throw e;
        } catch (Exception e) {
            log.error("Unexpected error placing order for user {}", userId, e);
            throw new RuntimeException("Order placement failed", e);
        }
    }
}

🤔 Common Confusions

🚨 Common Production Issues

✅ Best Practices

🏛️ Architect's Note

💬 Frequently Asked Questions

🎯 Key Takeaways

🔗 Related Topics

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↗ Where this topic fits