A good thing in Grails is that in Grails artifacts like controllers and services we have a log property to add log statements in our code. If we want to have the output of these log statements we must use a special naming convention for the log names. Each logger is prefixed with grails.app followed by the Grails artifact. Valid artifact values are controllers, services, domain, filters, conf and taglib. This is followed by the actual class name. So for example we have a controller SampleController in the package mrhaki.grails then the complete logger name isgrails.app.controllers.mrhaki.grails.SampleContoller.

The following sample configuration is for pre-Grails 3:

// File: grails-app/conf/Config.groovy
...
log4j = {
    ...
    info 'grails.app.controllers'
    debug 'grails.app.controllers.mrhaki.grails.SampleController'
    info 'grails.app.services'
    ...
}
...

In Grails 3 we can use a common Logback configuration file. In the following part of the configuration we set the log levels:

// File: grails-app/conf/logback.groovy
...
logger 'grails.app.controllers', INFO, ['STDOUT']
logger 'grails.app.controllers.mrhaki.grails.SampleController', DEBUG, ['STDOUT']
logger 'grails.app.services', INFO, ['STDOUT']
...