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.

<?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

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.

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.

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