Dynamic file name in log4j’s file appender

In your log4j.xml, set the file name as a variable

 	<appender name="my-file-appender" class="org.apache.log4j.FileAppender">
		<param name="file" value="${myFilePath}" />
		<!-- ... --> 
	</appender> 

Then in your java code, make sure the following is called before the first getLogger() call

 	System.setProperty("myFilePath", someFilePath);