Logback - Quick Reference

When to Use This Skill

Configure logging in Spring Boot/Java applications
File appender with rotation
High-performance async logging

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: logback for comprehensive documentation.

Configuration

logback-spring.xml (Spring Boot)

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <include resource="org/springframework/boot/logging/logback/defaults.xml"/>

    <property name="LOG_PATH" value="${LOG_PATH:-logs}"/>
    <property name="LOG_FILE" value="${LOG_FILE:-app}"/>

    <!-- Console Appender -->
    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){faint} %clr(${LOG_LEVEL_PATTERN:-%5p}) %clr(${PID:- }){magenta} %clr(---){faint} %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} %clr(:){faint} %m%n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}</pattern>
        </encoder>
    </appender>

    <!-- Rolling File Appender -->
    <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>${LOG_PATH}/${LOG_FILE}.log</file>
        <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
            <fileNamePattern>${LOG_PATH}/${LOG_FILE}.%d{yyyy-MM-dd}.%i.log.gz</fileNamePattern>
            <maxFileSize>100MB</maxFileSize>
            <maxHistory>30</maxHistory>
            <totalSizeCap>3GB</totalSizeCap>
        </rollingPolicy>
        <encoder>
            <pattern>%d{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>

    <!-- Async Appender -->
    <appender name="ASYNC" class="ch.qos.logback.classic.AsyncAppender">
        <appender-ref ref="FILE"/>
        <queueSize>512</queueSize>
        <discardingThreshold>0</discardingThreshold>
    </appender>

    <!-- Logger Configuration -->
    <logger name="com.myapp" level="DEBUG"/>
    <logger name="org.springframework" level="INFO"/>
    <logger name="org.hibernate.SQL" level="DEBUG"/>

    <root level="INFO">
        <appender-ref ref="CONSOLE"/>
        <appender-ref ref="ASYNC"/>
    </root>

    <!-- Profile-specific -->
    <springProfile name="prod">
        <root level="WARN">
            <appender-ref ref="ASYNC"/>
        </root>
    </springProfile>
</configuration>

JSON Format (ELK Stack)

<appender name="JSON" class="ch.qos.logback.core.ConsoleAppender">
    <encoder class="net.logstash.logback.encoder.LogstashEncoder">
        <includeMdcKeyName>requestId</includeMdcKeyName>
        <includeMdcKeyName>userId</includeMdcKeyName>
    </encoder>
</appender>

MDC Usage

import org.slf4j.MDC;

MDC.put("requestId", UUID.randomUUID().toString());
MDC.put("userId", user.getId());
try {
    // Business logic
} finally {
    MDC.clear();
}

When NOT to Use This Skill

SLF4J API questions: Focus on API usage, not Logback implementation details
Log4j2 configuration: Different XML structure and features
Application code logging: Use SLF4J API, Logback is just the implementation
Non-Java projects: Use language-appropriate logging frameworks
Simple console output: System.out may be sufficient for basic scripts

Anti-Patterns

Anti-Pattern	Why It's Bad	Solution
Synchronous file appender in high-traffic	Blocks application threads	Use `AsyncAppender` wrapper
No rolling policy	Logs fill disk space	Use `RollingFileAppender` with size/time policies
Logging to console in production	Performance overhead, lost logs	Use file appenders, ship to centralized logging
DEBUG level in production	Performance impact, disk usage	Use INFO or WARN in production profiles
Not clearing MDC	Memory leaks, wrong context in threads	Always clear MDC in finally block
Hardcoded log paths	Breaks across environments	Use properties: `${LOG_PATH}`

Quick Troubleshooting

Issue	Cause	Solution
Configuration not loaded	Wrong file name/location	Use `logback-spring.xml` in src/main/resources
Logs not rotating	Missing rolling policy	Add `SizeAndTimeBasedRollingPolicy`
Performance degradation	Synchronous appenders	Wrap with `AsyncAppender`
MDC values not appearing	Pattern missing MDC placeholders	Add `%X{key}` to pattern
Duplicate log entries	Logger additivity enabled	Set `additivity="false"` on logger
Profile-specific config ignored	Using logback.xml instead	Rename to `logback-spring.xml` for Spring profiles