Logback – different log file for each thread

In this tutorial, we will show you how to use Logback Mapped Diagnostic Context (MDC) and SiftingAppender to create a separate log file for each thread.

P.S Tested with Logback 1.1.2, should work in earlier version.

Note

More info, refer to this Logback MDC documentation

1. logback.xml example

A logback.xml file to show you how to declare and configure SiftingAppender and MDC.

logback.xml

<?xml version="1.0" encoding="UTF-8"?>
<configuration>

	<property name="USER_HOME" value="C:\\logs\\analyzer" />

	<appender name="FILE-THREAD" class="ch.qos.logback.classic.sift.SiftingAppender">

		<!-- This is MDC value -->
		<!-- We will assign a value to 'logFileName' via Java code -->
		<discriminator>
			<key>logFileName</key>
			<defaultValue>head0</defaultValue>
		</discriminator>

		<sift>

		  <!-- A standard RollingFileAppender, the log file is based on 'logFileName' at runtime  -->
		  <appender name="FILE-${logFileName}"
			class="ch.qos.logback.core.rolling.RollingFileAppender">
			<file>${USER_HOME}/${logFileName}.log</file>

			<encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
				<Pattern>
					%d{yyyy-MM-dd HH:mm:ss} %mdc [%thread] %level %logger{35} - %msg%n
				</Pattern>
			</encoder>

			<rollingPolicy
				class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
				<FileNamePattern>${USER_HOME}/${logFileName}.%i.log.zip
				</FileNamePattern>
				<MinIndex>1</MinIndex>
				<MaxIndex>10</MaxIndex>
			</rollingPolicy>

			<triggeringPolicy
				class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
				<MaxFileSize>10MB</MaxFileSize>
			</triggeringPolicy>

		  </appender>

		</sift>
	</appender>

	<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
		<layout class="ch.qos.logback.classic.PatternLayout">
			<Pattern>
				%-5level %logger{36} - %msg%n
			</Pattern>
		</layout>
	</appender>

	<logger name="com.favtuts.analyzer.core" level="debug"
		additivity="false">
		<appender-ref ref="FILE-THREAD" />
		<appender-ref ref="STDOUT" />
	</logger>

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

</configuration>

2. Java Threads Example

A simple thread example, declared the ‘logFileName’ value via MDC.put

Head.java

package com.favtuts.analyzer.core;

import java.util.List;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;

public class Head implements Runnable {

	static Logger logger = LoggerFactory.getLogger(Head.class);

	private String name;
	
	@Override
	public void run() {

		MDC.put('logFileName', getName());

		logger.debug("hello");

		//remember remove this
		MDC.remove('logFileName');

	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

}

Start 10 threads

	int count = 1;
	while(count<=10){
		Head head = new Head();
		head.setName("head-" + count);
		threadPools.execute(head);
		count++;
	}

Output : 10 separate log files for 10 threads.

Note

With this MDC feature, you can even log output to a separate log file for each login user, each URI request, each remote host and etc.

References

1. Logback MDC documentation