Mastering Spring Boot Logging: From Basics to Advanced Configuration

Logging Overview

Logging is familiar to us from Java SE where we use System.out.print to output logs. As projects become more complex, simple console prints are insufficient for monitoring, auditing, and data analysis, so dedicated logging frameworks are needed.

Uses of Logging

System Monitoring

Logs record system status, method response times, and can trigger alerts when thresholds are exceeded.

Data Collection

Statistics: Collect page views, unique visitors, clicks for analysis.

Recommendation Ranking: Record user behavior for recommendation algorithms.

Log Auditing

Logs provide evidence for security audits, illegal access detection, and tracing data modifications.

Logging in Spring Boot

Spring Boot outputs logs by default using the SLF4J facade backed by Logback. Unlike System.out.print, SLF4J provides richer information such as timestamps, thread names, and log levels.

Printing Logs

Obtain a logger via LoggerFactory.getLogger(YourClass.class) and use methods like info(), debug(), etc.

private static Logger logger = LoggerFactory.getLogger(LoggerController.class);

Example controller method:

@RestController
public class LoggerController {
    @RequestMapping("/logger")
    public String logger() {
        logger.info("--- log content ---");
        return "Print log";
    }
}

SLF4J Facade

SLF4J is a facade that abstracts underlying logging implementations (Log4j, Logback, etc.). It allows switching the concrete framework without changing application code.

Log Levels

FATAL

: System‑level fatal error. ERROR: High‑severity error. WARN: Warning. INFO: General information. DEBUG: Debug details. TRACE: Fine‑grained tracing.

Example of logging at all levels:

@RequestMapping("/printLog")
public String printLog() {
    logger.trace("=====trace=====");
    logger.debug("=====debug=====");
    logger.info("=====info=====");
    logger.warn("=====warn=====");
    logger.error("=====error=====");
    return "Print different level logs";
}

Configuration

Setting Log Levels

In application.properties: logging.level.root=debug Or in application.yml:

logging:
  level:
    root: debug

Log Persistence

Configure file name and path:

logging.file.name=logger/springboot.log

logging.file.path=D:/temp

Only one of the above takes effect; logging.file.name overrides logging.file.path.

File Rotation

Set rotation policy, e.g., split when size exceeds 1KB for demonstration:

logging.logback.rollingpolicy.file-name-pattern=${LOG_FILE}.%d{yyyy-MM-dd}.%i
logging.logback.rollingpolicy.max-file-size=1KB

Log Format

Console format ( logging.pattern.console) and file format ( logging.pattern.file) can be customized with placeholders such as %d for date, %5p for level, %t for thread, %c for logger name, and %m for message.

Simplified Logging with Lombok

Add Lombok dependency and annotate classes with @Slf4j to obtain a ready‑made log object.

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <optional>true</optional>
</dependency>

@Slf4j
@RestController
public class LogController {
    public void log() {
        log.info("===========log content===========");
    }
}

Summary

Logging is a crucial part of Java applications for problem detection, monitoring, auditing, and data analysis. Spring Boot provides a default SLF4J/Logback setup, and developers can control log levels, persistence, rotation, and formatting. Lombok further simplifies logger injection.