configure log4j to log to custom file at runtime

You can also do this from the log4j.properties file. Using the sample file below I have added the system property ${logfile.name}:

# logfile is set to be a RollingFileAppender
log4j.appender.logfile=org.apache.log4j.RollingFileAppender
log4j.appender.logfile.File=${logfile.name}
log4j.appender.logfile.MaxFileSize=10MB
log4j.appender.logfile.layout=org.apache.log4j.PatternLayout
log4j.appender.logfile.layout.ConversionPattern=[%-5p]%d{yyyyMMdd@HH\:mm\:ss,SSS}\:%c - %m%n

The log file name can then be set two different ways:

1. As a command line, system property passed to java "-Dlogfile.name={logfile}"

2. In the java program directly by setting a system property (BEFORE you make any calls to log4j).

   System.setProperty("logfile.name","some path/logfile name string");

Adapted from the log4j documentation:

import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.SimpleLayout;
import org.apache.log4j.FileAppender;

public class SimpandFile {
   static Logger logger = Logger.getLogger(SimpandFile.class);
   public static void main(String args[]) {

      // setting up a FileAppender dynamically...
      SimpleLayout layout = new SimpleLayout();    
      FileAppender appender = new FileAppender(layout,"your filename",false);    
      logger.addAppender(appender);

      logger.setLevel((Level) Level.DEBUG);

      logger.debug("Here is some DEBUG");
      logger.info("Here is some INFO");
      logger.warn("Here is some WARN");
      logger.error("Here is some ERROR");
      logger.fatal("Here is some FATAL");
   }
}