SLF4J, Logback & Log Levels

Logging in production enterprise applications is not simply about printing messages to a terminal; it is about emitting structured, filterable, thread-safe operational telemetry. Using System.out.println or System.err.println in enterprise Java is considered a critical code-quality defect for multiple reasons.

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package com.example.order;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Service;

@Service
public class OrderService {
    // Canonical logger declaration:
    private static final Logger log = LoggerFactory.getLogger(OrderService.class);

    public void processOrder(String orderId, long amountCents) {
        log.info("Processing order id={} amountCents={}", orderId, amountCents);
        try {
            // business logic
        } catch (Exception ex) {
            log.error("Failed to process order id={}: {}", orderId, ex.getMessage(), ex);
        }
    }
}

SLF4J defines five standard log levels ordered by increasing severity: TRACE < DEBUG < INFO < WARN < ERROR. Setting a log level instructs the logger to emit messages at that severity and above while discarding lower-severity entries.

Spring Boot provides seamless logging configuration directly inside application.yml for basic level overrides, file logging, and console formatting. For advanced enterprise setups (rolling file policies, custom appenders, syslog), use logback-spring.xml.

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logging:
  level:
    root: INFO
    org.springframework.web: WARN
    org.hibernate.SQL: DEBUG # Show SQL queries
    org.hibernate.orm.jdbc.bind: TRACE # Show SQL bound parameters
    com.example.order: DEBUG
  pattern:
    console: "%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){faint} %clr(%5p) %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} : %m%n"
  file:
    name: "/var/log/order-service/application.log"

Why name the file logback-spring.xml instead of logback.xml? If you use logback.xml, Logback initializes before Spring Boot loads its Environment. By using logback-spring.xml, Spring Boot controls Logback initialization, allowing you to use <springProfile name="prod"> tags and read properties via <springProperty scope="context" name="appName" source="spring.application.name"/>.

A common developer panic in staging or production is: "I added logging, but nothing shows up in the console or log file!" Check the logger level, the effective root and package configuration, the active appender, and the deployment sink before changing application code.

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@GetMapping("/pay")
public String pay() {
    log.debug("Initiating payment calculation"); // If root is INFO and package level is unset, this is silently discarded!
    return "ok";
}