Wednesday, March 31, 2010

A middle-class man's conditional logging configuration

In an earlier post I described how one adapt a single configuration file for use in development as well as production environments. At those prehistoric times, that is January 2010, logback did not support conditional (if-then-else) configuration statements. So we made use of a default substitution value to do the the trick.

However, as of version 0.9.20 (to be released early April), logback supports conditional configuration statements. Thus, we can write:
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
<if condition='!isNull("catalina.home")'>
<then>
<File>${catalina.home}/logs/myApp.log</File>
</then>
<else>
<File>./logs/myApp.log</File>
</else>
</if>
...
</appender>
Thus, if the "catalina.home" system property is defined (non null), then the log file will be written to ${catalina.home}/logs/myApp.log, otherwise, we are in the development environment and the logs should be written to ./logs/myApp.log.

In this particular case, the if-then-else approach is more verbose than default variable substitution, i.e. simply writing ${catalina.home:-.}. However, it goes without saying that conditional configuration brings much needed flexibility to the table. Keep in mind that given the verbosity of XML, one could gradually end up with a configuration file containing many logical branches rendering the file ungrokable. In case you ask, nested if-then-else statements are supported. So please use conditionals with moderation so that complexity does not creep into your configuration files.

5 Comments:

Anonymous Juha said...

This is great news!

Btw, is 0.9.20 (and particularly this feature) going to be supported in JDK 1.3?

8:25 AM  
Anonymous Juha said...

Whoops, I just realized that I mixed slf4j and logback ;)

So, presumably this is going to required JDK1.5?

8:30 AM  
Blogger Ceki said...

Logback requires JDK 1.5 already, this feature is no different.

10:22 AM  
Blogger marry said...

Blogs are so informative where we get lots of information on any topic. Nice job keep it up!!
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9:17 AM  
Anonymous Anonymous said...

Thanks! Will this also work with RollingFileFileAppenders? In other words, I want a conditional around a fileNamePattern. How will this work?

10:07 PM  

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