Using Logback and SLF4J for Logging in Spring Boot

Introduction – In modern Java projects a logging system is essential. Log4j and Logback are popular frameworks, both created by the same author, with Logback being the successor to Log4j. SLF4J serves as a façade that defines common logging APIs, while Logback and Log4j provide concrete implementations.

Why use Logback – Logback is more efficient, works in many environments, natively supports SLF4J, offers flexible customization, and is the default logging framework bundled with Spring Boot.

Adding dependencies

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-logging</artifactId>
</dependency>

Spring Boot already includes this starter, so you normally only need to add the web starter:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

Default configuration – By default Spring Boot logs to the console. To write to a file you can set logging.file or logging.path in application.properties . These two properties are mutually exclusive.

logging.file=my.log            # absolute or relative path
logging.path=/var/log          # creates spring.log in the directory
logging.level.com.example=DEBUG
logging.pattern.console=%d{HH:mm:ss.SSS} %-5level %logger{36} - %msg%n

logback‑spring.xml – Spring Boot recommends using a file whose name ends with -spring (e.g., logback-spring.xml ) and placing it under src/main/resources . This file allows full access to Spring’s profile support.

Core concepts

Logger – the entry point for logging calls.

Appender – the destination (console, file, etc.).

Layout/Encoder – defines the output format.

Basic XML example:

<configuration>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
    </encoder>
  </appender>
  <root level="DEBUG">
    <appender-ref ref="STDOUT"/>
  </root>
</configuration>

The logger element can inherit its level from its parent unless a specific level is set. Examples of level inheritance are shown in the original article.

Appender types

ConsoleAppender – writes to the console.

RollingFileAppender – writes to a file and rolls the file based on time, size, or both.

Rolling policies such as SizeAndTimeBasedRollingPolicy allow you to keep logs for a configurable period (e.g., 30 days) and limit total size (e.g., 20 GB).

<appender name="info_log" class="ch.qos.logback.core.rolling.RollingFileAppender">
  <File>logs/project_info.log</File>
  <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
    <fileNamePattern>logs/project_info.%d.%i.log</fileNamePattern>
    <maxHistory>30</maxHistory>
    <totalSizeCap>20GB</totalSizeCap>
    <maxFileSize>10MB</maxFileSize>
  </rollingPolicy>
  <encoder>
    <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} %-5level --- [%15.15(%thread)] %cyan(%-40.40(%logger{40})) : %msg%n</pattern>
    <charset>UTF-8</charset>
  </encoder>
</appender>

Filters – Two common filters are LevelFilter (matches a specific level) and ThresholdFilter (blocks events below a threshold). Example configurations are provided in the source.

<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
  <filter class="ch.qos.logback.classic.filter.LevelFilter">
    <level>INFO</level>
    <onMatch>ACCEPT</onMatch>
    <onMismatch>DENY</onMismatch>
  </filter>
  <encoder>
    <pattern>%-4relative [%thread] %-5level %logger{30} - %msg%n</pattern>
  </encoder>
</appender>

Best practices for logging statements

Avoid constructing log messages when the log level is disabled. Use if (logger.isDebugEnabled()) { … } or the placeholder syntax.

Prefer placeholders: logger.debug("The entry is {}", entry); . The message is formatted only when the level is enabled.

When logging exceptions, include the throwable as a separate argument to capture the stack trace: logger.error("Application error: {}", e.getMessage(), e); .

These techniques can reduce unnecessary string concatenation and improve performance, especially in high‑throughput services.

Finally, the article provides links to additional resources such as a 7701‑page interview question PDF and other technical tutorials.