How to configure Log4j to log to console

Okay, you wrote a lot of Log4j debug statements all over your codebase but do not want them to appear in QA and/or Production environment. How to do that? You need to configure Log4j by modifying log4j.xml. You could also use log4j.properties in place of log4j.xml but some advanced settings can only be set using the latter. Might as well use something that offers greater benefits.

In log4j, there are levels of logging. Each level represents the severity or type of log is being made. From ascending order of level, these are TRACE, DEBUG, INFO, WARN, ERROR, and FATAL. FATAL has the highest level; whereas TRACE sits on the lowest level.

Software Environment

Windows 7 Professional SP1
Eclipse – Kepler Release
Java 1.7 (1.7.0_67 – Windows x86)
Apache Log4j
- Reference only log4j-1.2.16.jar

Your log4j.xml

This file should be included in the classpath.

<?xml version="1.0" encoding="UTF-8" ?>
<log4j:configuration debug="true" xmlns:log4j="http://jakarta.apache.org/log4j/">
    <appender class="org.apache.log4j.ConsoleAppender" name="console">
        <layout class="org.apache.log4j.PatternLayout"><param name="ConversionPattern" value="%d{yyyy/MM/dd HH:mm:ss} %-5p %c{1}:%L - %m%n">
        </layout>
    </appender> 
    <root>
        <priority value="error"></priority>
        <!--  or 
            <level value="error"></level>
        -->
        <appender-ref ref="console">
    </appender-ref></root> 
</log4j:configuration>

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

<log4j:configuration debug="true" xmlns:log4j="http://jakarta.apache.org/log4j/">

</layout>

</appender>

<root>

<!-- or

-->

<appender-ref ref="console">

</appender-ref></root>

</log4j:configuration>

Notice particularly the level or priority tags, they allow you to specify the minimum level of the log. If INFO is specified, only INFO, WARN, ERROR, and FATAL logs are displayed. In this configuration, only ERROR and FATAL logs will be shown.

Sample Usage

The first example uses a logger.error() method to log an error. This logs the message in the ERROR level.

import org.apache.log4j.Logger;
public class LoggerTest {
    private static Logger logger = Logger.getLogger(LoggerTest.class.getName());
        public static void main(String[] args) {
            logger.error("turreta.com");
        }
    }
}

import org.apache.log4j.Logger;

public class LoggerTest {

private static Logger logger = Logger.getLogger(LoggerTest.class.getName());

public static void main(String[] args) {

logger.error("turreta.com");

}

Sample output

log4j: reset attribute= "false".
log4j: Threshold ="null".
log4j: Level value for root is  [error].
log4j: root level set to ERROR
log4j: Class name: [org.apache.log4j.ConsoleAppender]
log4j: Parsing layout of class: "org.apache.log4j.PatternLayout"
log4j: Setting property [conversionPattern] to [%d{yyyy/MM/dd HH:mm:ss} %-5p %c{1}:%L - %m%n].
log4j: Adding appender named [console] to category [root].
2014/11/10 23:53:43 ERROR LoggerTest:5 - turreta.com

log4j: reset attribute= "false".

log4j: Threshold ="null".

log4j: Level value for root is [error].

log4j: root level set to ERROR

log4j: Class name: [org.apache.log4j.ConsoleAppender]

log4j: Parsing layout of class: "org.apache.log4j.PatternLayout"

log4j: Setting property [conversionPattern] to [%d{yyyy/MM/dd HH:mm:ss} %-5p %c{1}:%L - %m%n].

log4j: Adding appender named [console] to category [root].

2014/11/10 23:53:43 ERROR LoggerTest:5 - turreta.com

Another example tries to log a WARN message but that message is not displayed.

import org.apache.log4j.Logger;
public class LoggerTest {
    private static Logger logger = Logger.getLogger(LoggerTest.class.getName());
    public static void main(String[] args) {
        logger.debug("projecting knowledge");
        logger.error("turreta.com");
    }
}

import org.apache.log4j.Logger;

public class LoggerTest {

private static Logger logger = Logger.getLogger(LoggerTest.class.getName());

public static void main(String[] args) {

logger.debug("projecting knowledge");

logger.error("turreta.com");

}

Outputs:

log4j: reset attribute= "false".
log4j: Threshold ="null".
log4j: Level value for root is  [error].
log4j: root level set to ERROR
log4j: Class name: [org.apache.log4j.ConsoleAppender]
log4j: Parsing layout of class: "org.apache.log4j.PatternLayout"
log4j: Setting property [conversionPattern] to [%d{yyyy/MM/dd HH:mm:ss} %-5p %c{1}:%L - %m%n].
log4j: Adding appender named [console] to category [root].
2014/11/11 00:06:53 ERROR LoggerTest:6 - turreta.com