tomcat6: cannot create directory for logging in linux

Question

i use ubuntu 9.04 for testing platform. I have created a webapp that log a hardcoded string whenever someone visit index.jsp. I have enable SECURITY_MANAGER in /etc/init.d/tomcat6. And I have grant permission for the webapp named w1 with

grant codeBase "file:${catalina.base}/webapps/w1/-" {
    permission java.security.AllPermission;
};

however, when i access the index.jsp with a browser, a

java.io.FileNotFoundException: /Log/10127-2010-10-14.log (No such file or directory)

is presented in catalina.out. The situation is that the directory of Log is not exist nor the log file.

If I create the directory myself, the log file will be created but with no context inside.

And I found it runs successfully in tomcat5.5 in another 8.04 box and tomcat6 in windows xp box.

Any ideas?