Logging in Spring Boot

  • Process of records events, errors, messages, and execution flow of an application.
  • Spring Boot provides built-in logging support using SLF4J and Logback by default.
  • All logs can be configured via application.properties/application.yml.

Key Components

Component Description
SLF4J Simple Logging Facade for Java — abstraction layer for logging frameworks
Logback Default logging implementation
Logger for logging messages
Log Levels Severity of logs (TRACE < DEBUG < INFO < WARN < ERROR)

Enabling Log Levels

By default, only INFO and WARN logs are active .To enable others: In application.properties

logging.level.root=INFO
logging.level.com.example=DEBUG

Logging Level Examples 

#applies to all packages (java.*, org.*, com.*, etc.)
logging.level.root=DEBUG 

#applies to only org.framework package
logging.level.org.framework=DEBUG  

#applies to only com.alamgir package
logging.level.com.alamgir=DEBUG

Log display hirarchey

Level Logs Displayed
TRACE TRACE, DEBUG, INFO, WARN, ERROR
DEBUG DEBUG, INFO, WARN, ERROR
INFO INFO, WARN, ERROR
WARN WARN, ERROR
ERROR ERROR only
OFF None

Key Point

  • TRACE : very detailed, fine-grained info
  • DEBUG : debugging info, app flow
  • INFO : general runtime info, milestones
  • WARN : potential issues, recoverable problems
  • ERROR : serious problems, app failures

Example :

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

@Component
public class MyService {
    private static final Logger logger = LoggerFactory.getLogger(MyService.class);

    public void doSomething() {
        logger.trace("Trace log message");
        logger.debug("Debug log message");
        logger.info("Info log message");
        logger.warn("Warn log message");
        logger.error("Error log message");
    }
}

Log Output

Logs are printed in console by default.Can also write to files: 

logging.file.name=app.log
logging.file.path=/logs

Logging Best Practices (Logback)

  • Don’t log sensitive data (passwords, tokens).
  • Avoid excessive logs : too many logs increase I/O load, disk usage, and reduce performance.
  • Use appropriate log levels ( DEBUG for development, INFO for production)


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