How to define separate log files for different log levels

In our previous post, we learned how to add a variable to the log file name. This time, we are going to separate the logs based on their levels so that a log file has only one log level (or more if you want).

In a typical log4j properties file, you can define a threshold, which allows you to record logs whose levels are in the defined level or above.

For example,
FATAL: shows messages at a FATAL level only
ERROR: Shows messages classified as ERROR and FATAL
INFO: Shows messages classified as INFO, WARNING, ERROR, and FATAL etc...

In order to filter out a range of levels to record, we can use Apache's LevelRangeFilter in which we can define LevelMin and LevelMax parameters to filter the logs to record. As of now, I could find only the xml implementation of this feature.

So if you want a file to have only WARN level messages, all you got to do is to add the following entry in your log4j.xml inside the <appender>.

<filter class="org.apache.log4j.varia.LevelRangeFilter">
    <param name="LevelMin" value="WARN"/>
    <param name="LevelMax" value="WARN"/>
</filter>

A complete log4j.xml file that has defined a file to log warnings and errors will look like the following.

<?xml version="1.0" encoding="UTF-8" ?>
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/" debug="false">

    <appender name="console" class="org.apache.log4j.ConsoleAppender">
        <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>

    <!--Warn logs to be printed-->
    <appender name="warn-file" class="org.apache.log4j.RollingFileAppender">
        <param name="append" value="false"/>
        <param name="maxFileSize" value="10MB"/>
        <param name="maxBackupIndex" value="10"/>
        <param name="file" value="warn_log.log"/>
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern"
                   value="%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n"/>
        </layout>
        <filter class="org.apache.log4j.varia.LevelRangeFilter">
            <param name="LevelMin" value="WARN"/>
            <param name="LevelMax" value="WARN"/>
        </filter>
    </appender>

    <!--Error logs to be printed-->
    <appender name="error-file" class="org.apache.log4j.RollingFileAppender">
        <param name="append" value="false"/>
        <param name="maxFileSize" value="10MB"/>
        <param name="maxBackupIndex" value="10"/>
        <param name="file" value="error_log.log"/>
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern"
                   value="%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n"/>
        </layout>
        <filter class="org.apache.log4j.varia.LevelRangeFilter">
            <param name="LevelMin" value="ERROR"/>
            <param name="LevelMax" value="ERROR"/>
        </filter>
    </appender>


    <root>
        <level value="INFO"/>
        <appender-ref ref="console"/> 
        <appender-ref ref="warn-file"/>
        <appender-ref ref="error-file"/>
    </root>

</log4j:configuration>

Cheers!