You can initialize the log4j logger object in your
class and then add it to the ApplicationData. Now you can use the
logger from your .drl-file.

Here the example java class:

public class MyClass
{
private static Logger _logger = Logger.getLogger( MyClass.class );

public MyClass()
{
initRuleEngine();
}

public void initRuleEngine( String ruleFile )
{
_logger.info( "Initialization of Drools rule engine..." );
URL ruleURL = this.getClass().getClassLoader().getResource(ruleFile);
try
{
URL ruleURL =
this.getClass().getClassLoader().getResource( ruleFile );
if ( ruleURL != null )
{
ruleBase = RuleBaseLoader.loadFromUrl( ruleURL );

if ( ruleBase != null )
{
_logger.info( "Rule set loaded from drools config
file: " + ruleFile );
workingMemory = ruleBase.newWorkingMemory();
workingMemory.addEventListener( wmListener );

//put logger reference to application data
workingMemory.setApplicationData("logger", _logger);
}
else
{
_logger.fatal( "Rule set couldn't be loaded!" );
}
}
}
catch ( DroolsException e )
{
_logger.fatal( "Drools exception occured", e );
// _logger.fatal( e.getCause() );
// _logger.fatal( e.getMessage() );
// _logger.fatal( e.getStackTrace() );
}
catch ( IOException e )
{
_logger.fatal( "I/O Exception occured", e );
// _logger.fatal( e.getCause() );
// _logger.fatal( e.getMessage() );
// _logger.fatal( e.getStackTrace() );
}
catch ( SAXParseException e )
{
_logger.fatal( "Drools rule definition file seems to be corrupt",
e );
}
catch ( Exception e )
{
_logger.fatal( "Exception occured", e );
// _logger.fatal( e.getCause() );
// _logger.fatal( e.getMessage() );
// _logger.fatal( e.getStackTrace() );
}
}
}

Now the fragment of a .drl-file:

<?xml version="1.0" encoding="UTF-8"?>
<rule-set name="MyRules" xmlns="http://drools.org/rules"
xmlns:java="http://drools.org/semantics/java"
xmlns:xs="http://www.w3.org/2001/XMLSchema-instance"
xs:schemaLocation="http://drools.org/rules rules.xsd
http://drools.org/semantics/java java.xsd">

<!--Required imports-->
org.drools.examples.state.State
...

<!--Application data variables-->
<application-data
identifier="logger">org.apache.log4j.Logger</application-data>
...

<!--check BarBaz attribute-->


Foo

<java:condition>foo != null</java:condition>
<!--Check the barBaz field-->
<java:condition>foo.getBarBaz()== 0 </java:condition>
<java:consequence>
logger.error("BarBaz is null!" );
...
</java:consequence>

</rule-set>

code with "Juri Albert"