Altering output file of FileAppender programatically

Question

so i have a thread that had this in the ctor:
DOMConfigurator.configure(".conf/log4j.properties.xml");
Appender a = Logger.getRootLogger().getAppender("FILE");
if(a != null){ //make a custom log file for the crawl
     RollingFileAppender r = (RollingFileAppender)a;
     File f = new File("./crawler/log/"+config.name+"_crawl.log");
     log.info("set log file for this crawl to:"+f.getAbsolutePath());
     r.setFile(f.getAbsolutePath());
}          i know that a is not null. ive stepped through this code and it runs. yet, the output of the logger is still going into the log file it was configured to log to in the log4j properties xml.
what gives? how do people change the log file programmatically?
also, note that i execute several of this thread with different params, and i want each thread to log to its own log file.

Answer 1

prob sol
add
r.activateOptions(); after changing stuff