Logback – Set log file name programmatically

In Logback, it is easy to set a log file name programmatically :

  1. In logback.xml, declares a variable like ${log.name}
  2. In Java, set the variable via System.setProperty("log.name", "abc")

1. Full example

1.1 A logback file, we will set the ${log.name} variable later.

src/main/resources/logback.xml

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

	<property name="PRO_HOME" value="/home/mkyong/ant/logs" />
	<property name="USER_HOME" value="${PRO_HOME}" />

	<timestamp key="bySecond" datePattern="yyyyMMdd.HHmmss" />

	<appender name="FILE-ENGINE-ERROR" class="ch.qos.logback.core.FileAppender">
		<file>${USER_HOME}/${log.name}.error</file>
		<encoder>
			<pattern>%d{yyyy-MM-dd HH:mm:ss} [%thread] %-5level %logger{36} - %msg%n</pattern>
		</encoder>
	</appender>
	
	<appender name="FILE-ENGINE" class="ch.qos.logback.core.FileAppender">
		<file>${USER_HOME}/${log.name}-${bySecond}.log</file>
		<encoder>
			<pattern>%d{yyyy-MM-dd HH:mm:ss} [%thread] %-5level %logger{35} - %msg%n</pattern>
		</encoder>
	</appender>

	<logger name="com.mkyong.core" level="debug" additivity="false">
		<appender-ref ref="FILE-ENGINE" />
	</logger>

	<logger name="org.springframework" level="error" additivity="false">
		<appender-ref ref="FILE-ENGINE-ERROR" />
	</logger>

	<root level="error">
		<appender-ref ref="FILE-ENGINE-ERROR" />
	</root>

</configuration>

1.2 In Java, just set the file name via System.setProperty

AntRunApp.java

package com.mkyong.core;

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

public class AntRunApp {

    private final Logger logger = LoggerFactory.getLogger(AntRunApp.class);

	public static void main(String[] args) {

		//Set this before the logger start.
		System.setProperty("log.name", "mkyong");

		AntRunApp obj = new AntRunApp();
		obj.start();

	}

	private void start() {

        logger.debug("------ Starting Ant------");

     	//...
	}

}

Output

Debug log file path
/home/mkyong/ant/logs/mkyong-20150323.221959.log

Error log file path
/home/mkyong/ant/logs/mkyong.error

2. log.name.rir_IS_UNDEFINED.log

2.1 A common error, normally it is caused by the static logger. For example.

AntRunApp.java

package com.mkyong.core;

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

public class AntRunApp {

	// static, this logger will be initialized before the program is run.
	// All logs will be redirected to log.name.rir_IS_UNDEFINED.log
    private static final Logger logger = LoggerFactory.getLogger(AntRunApp.class);

	private void start() {

		System.setProperty("log.name", "mkyong");
		
        logger.debug("------ Starting Ant------");

     	//...
	}

}

To fix it, just delete the static type.

2.2 If you logs before the System.setProperty, this will also cause the common Logback variable UNDEFINED error.

AntRunApp.java

package com.mkyong.core;

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

public class AntRunApp {

    private final Logger logger = LoggerFactory.getLogger(AntRunApp.class);

	private void start() {

		//Please set the log name first!
		logger.debug("------ Starting Ant------");
		
		System.setProperty("log.name", "mkyong");
		
	}

}

References

  1. Logback.xml examples
  2. Logback – different log file for each thread
  3. Logback – MDC documentation

About the Author

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mkyong
Founder of Mkyong.com, love Java and open source stuff. Follow him on Twitter. If you like my tutorials, consider make a donation to these charities.

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amateurTriaji Recent comment authors
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Triaji
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Triaji

if it’s happened in the Servlet Container (tomcat, jboss), using log4j2 usually it need to reconfigure the logger using LoggerContext.
What will happen if we’re doing that using Logback?

amateur
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amateur

Hello,
I am trying to implement your method but it’s now working for me, and I was wondering if that would be because I have log4j.properties instead of .xml? My code seems to replace the variable I created with an empty string.

Here is the content of my file

log4j.rootLogger=TRACE, STDOUT, file
log4j.appender.STDOUT=org.apache.log4j.ConsoleAppender
log4j.appender.STDOUT.layout=org.apache.log4j.PatternLayout
log4j.appender.STDOUT.layout.ConversionPattern=%c{1} – %m%n
log4j.appender.STDOUT.Threshold=INFO
log4j.appender.file=org.apache.log4j.DailyRollingFileAppender
log4j.appender.file.File=${basedir}/${log.file}.log
log4j.appender.file.DatePattern=’-‘yyyy-MM-dd-HH’.log’
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%-5p %d{yyyy-MM-dd HH:mm:ss} %C:%L – %m%n