For the moment, you can only use predefined patternLayout here

http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PatternLayout.html

An evolution could be to add a system to define our own layout

basically a custom layout could be

---------------------------------------------------------
public class MyPidLayout implements CustomConversionLayout{

     char getConversionCharacter(){
            return 'y';
      }

     String getConversionValue(){
           return properties.getProperty("Pid"));
      }

}
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André Bergues