Debug information "Unknown Source" on Tomcat 6 server log

I install a Tomcat 6. When I run my application, the runtime exceptions debug on Tomcat server log did show any line number. It make me difficult to find the bug.

Can anyone tell me why the exception debug info don't show line number instead od "Unknown Source" when I use Tomcat 6? The debug information like:

java.lang.NullPointerException

at com.xxx.xxx.MailServlet.process(Unknown Source)

at com.xxx.xxx.MailServlet..doPost(Unknown Source)

...

at org.apache.catalina.xxxxxx

....

Actully it is common runtime error. Do I need confiugrate Tomcat 6 for that and I can see the excat line number?

[651 byte] By [rcd27a] at [2007-11-27 11:22:00]