How can I configure logback conditionally based on context name?

Question

I have three separate projects, each with their own embedded logback.xml files. Each of these files includes a common logging config file in the user's home durectory:

<include file="${user_home}/loggingConfig.xml"/>

After the include, I have this specification:

<root level="error">
    <appender-ref ref="${appender:-console}" />
</root>

This allows the user to configure their log levels and appenders, and have them applied by the core logging config file.

For example, in ~/loggingConfig.xml I have this line:

<property name="appender" value="file" />

But co-workers who prefer console logging leave that line out.

The problem is I would like to use different appenders for each log file. In otherwords, I would like to conditionally set a different appender based on which project is reading the customized config file.

I realize I could configure each project to read differently named config files, but I would like to eliminate the clutter and allow for shared configuration as well.

score 20 · Accepted Answer · answered Apr 09 '13 at 19:55

The documentation is a bit spare for advanced configuration, but I found that you can use the logback context name as a variable with conditional logging. So for each project I define a custom context name in the projects logback.xml file:

<contextName>project1</contextName>

etc...

Then in my ~/loggingConfig.xml file I can do this:

<property name="appender" value="file" />

<!--if condition='property("CONTEXT_NAME").equalsIgnoreCase("project1")'>
    <then>
        <property name="appender" value="file" />
    </then>
</if-->
<if condition='property("CONTEXT_NAME").equalsIgnoreCase("project2")'>
    <then>
        <property name="appender" value="console" />
    </then>
</if>
<if condition='property("CONTEXT_NAME").equalsIgnoreCase("project3")'>
    <then>
        <property name="appender" value="file" />
    </then>
</if>

This can get a bit clunky, but in reality I am using this solution to configure properties used by a single appender for different projects, while still having a graceful fallback to a default value for projects that don't have their own conditional block.

To complete this solution: conditions are possible with the required Janino library. See http://logback.qos.ch/manual/configuration.html#conditional — rudi, Jan 19 '21 at 10:28

score 18 · Answer 2 · answered Jan 06 '16 at 18:23

In case this helps anyone else, this is how I set up conditional logback configuration with a property that can contain multiple appenders:

<root level="${logback.loglevel}">
    <if condition='isDefined("logback.appenders")'>
        <then>
            <if condition='property("logback.appenders").contains("CONSOLE")'>
                <then>
                    <appender-ref ref="CONSOLE"/>
                </then>
            </if>
            <if condition='property("logback.appenders").contains("FILE")'>
                <then>
                    <appender-ref ref="FILE"/>
                </then>
            </if>
            <if condition='property("logback.appenders").contains("GELF")'>
                <then>
                    <appender-ref ref="GELF"/>
                </then>
            </if>
        </then>
        <else>
            <appender-ref ref="CONSOLE"/>
        </else>
    </if>
</root>

This may have been possible in earlier versions, however in more recent versions you will get the following warning if you try to nest inside of root: `WARN in IfNestedWithinSecondPhaseElementSC - elements cannot be nested within an , or element` — wrestang, Feb 23 '23 at 17:21

How can I configure logback conditionally based on context name?

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