Using Logback with SLF4J in Spring Boot: Configuration, Customization, and Best Practices

Logback is the successor of Log4j, offering higher efficiency and native SLF4J support; it is the default logging framework for Spring Boot.

Why Use Logback

Logback provides better performance, flexible configuration, and seamless integration with Spring Boot.

Getting Started

1. Add Dependency

Maven dependency:

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

In practice you only need spring-boot-starter , which already includes spring-boot-starter-logging (Logback+SLF4J). Adding spring-boot-starter-web brings in the web starter.

2. Default Configuration

Spring Boot logs to the console by default. To write to a file, set logging.file or logging.path in application.properties . Note that both cannot be used simultaneously; logging.file takes precedence.

logging.file=my.log
logging.path=/var/log
logging.level.com.example=DEBUG
logging.pattern.console=%d{HH:mm:ss.SSS} %-5level %logger - %msg%n

logback‑spring.xml Detailed Explanation

Spring Boot recommends using a configuration file named logback-spring.xml (instead of logback.xml ) placed under src/main/resources . You can also use a custom name and specify it with logging.config=classpath:logback-config.xml .

The core elements are <configuration> , <appender> , <logger> , and <root> . An example basic configuration:

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

Elements:

<logger> : defines a logger with name , optional level , and additivity .

<root> : the root logger, only supports the level attribute.

<appender> : writes logs to a destination (e.g., ConsoleAppender , RollingFileAppender ).

<encoder> : controls the log output format via the pattern attribute.

<filter> : can be LevelFilter or ThresholdFilter to restrict log levels.

Appender Types

Two commonly used appenders are ConsoleAppender (outputs to console) and RollingFileAppender (writes to a file with rolling policies).

Rolling Policies

TimeBasedRollingPolicy rolls logs based on time (daily, monthly, etc.). SizeAndTimeBasedRollingPolicy adds size‑based rolling to the time policy.

Example of a Complete logback‑spring.xml

<?xml version="1.0" encoding="UTF-8"?>
<configuration debug="true">
  <!-- Console appender -->
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} %highlight(%-5level) --- [%15.15(%thread)] %cyan(%-40.40(%logger{40})) : %msg%n</pattern>
      <charset>UTF-8</charset>
    </encoder>
  </appender>

  <!-- Info file appender with size‑and‑time rolling -->
  <appender name="info_log" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <File>logs/project_info.log</File>
    <append>true</append>
    <filter class="ch.qos.logback.classic.filter.LevelFilter">
      <level>ERROR</level>
      <onMatch>DENY</onMatch>
      <onMismatch>ACCEPT</onMismatch>
    </filter>
    <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)] %-40.40(%logger{40}) : %msg%n</pattern>
      <charset>UTF-8</charset>
    </encoder>
  </appender>

  <!-- Error file appender (threshold filter) -->
  <appender name="error_log" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <File>logs/project_error.log</File>
    <append>true</append>
    <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
      <level>ERROR</level>
    </filter>
    <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
      <fileNamePattern>logs/project_error.%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)] %-40.40(%logger{40}) : %msg%n</pattern>
      <charset>UTF-8</charset>
    </encoder>
  </appender>

  <root level="INFO">
    <appender-ref ref="STDOUT"/>
  </root>

  <logger name="com.sailing.springbootmybatis" level="INFO">
    <appender-ref ref="info_log"/>
    <appender-ref ref="error_log"/>
  </logger>

  <logger name="com.sailing.springbootmybatis.mapper" level="DEBUG" additivity="false">
    <appender-ref ref="info_log"/>
    <appender-ref ref="error_log"/>
  </logger>
</configuration>

Logging Code Tips

Use parameterized logging to avoid unnecessary string concatenation:

logger.debug("The entry is {}.", entry);

For multiple parameters, use additional placeholders or an Object[] array. To log exceptions with stack traces, include the throwable as a separate argument:

logger.error("Program exception, details: {}", e.getLocalizedMessage(), e);

These practices improve performance, especially when the log level is disabled.