Logback – Duplicate log messages

Review a simple Java application and log a message via Logback.

App.java

package com.favtuts.test;

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

public class App {

	private static final Logger log = LoggerFactory.getLogger(App.class);

	public static void main(String[] args) {
		log.debug("Testing");
	}
}

P.S Tested with Logback 1.1.2

1. Problem

A simple logback.xml to log a message to console.

logback.xml

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

	<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
		<layout class="ch.qos.logback.classic.PatternLayout">
			<Pattern>
				%d{yyyy-MM-dd HH:mm:ss} [%thread] %-5level %logger{36} - %msg%n
			</Pattern>
		</layout>
	</appender>

	<logger name="com.favtuts.test" level="debug">
		<appender-ref ref="STDOUT" />
	</logger>

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

</configuration>

Run above program, the Logback will logs a message twice?

2015-01-25 13:55:21 [main] DEBUG com.favtuts.test.App - Testing
2015-01-25 13:55:21 [main] DEBUG com.favtuts.test.App - Testing

2. Solution

This is caused by the Appenders accumulate. To fix it, add a additivity="false" to the application logger

logback.xml

	<logger name="com.mkyong.test" level="debug" additivity="false">
		<appender-ref ref="STDOUT" />
	</logger>

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

Output

2015-01-25 13:58:24 [main] DEBUG com.favtuts.test.App - Testing

References

1. Logback Configuration