Let's say you want to edit a single .java file on existing running machine. Let's say the file you want to edit is KickstartCommand.java (picked up by random):

• Create your temporary working dir

  cd /tmp
mkdir -p com/redhat/rhn/domain/kickstart/

• Transfer desired java file to your server

  scp yourmachine:/yourdir/java/src/com/redhat/rhn/domain/kickstart/KickstartCommand.java com/redhat/rhn/domain/kickstart/

• Compile using required libs

  javac -extdirs /var/lib/tomcat5/webapps/rhn/WEB-INF/lib/ com/redhat/rhn/domain/kickstart/KickstartCommand.java

• Add .class to rhn.jar

  jar uf /usr/share/rhn/lib/rhn.jar com/redhat/rhn/domain/kickstart/KickstartCommand.class /etc/init.d/tomcat5 restart