redirect in a filter class

I have j2ee web app that uses filter to determine the timeout period. If the current time is longer than the last access time for a certain length. it will redirects to a logout page. But I got error: java.lang.IllegalStateException: Cannot forward after response has been committed. I know this error would occur if there is already some output of the page. But this is from a filter class. It's mapped to /* and is called before accessing any page. Why it's still complain this error?
Thanks.

When done correctly, it should not throw any exceptions.

Instead of calling doFilter() you send a redirect to break the chain.

Could you send some code?

I found that the error only occurs if I redirect it to an external website page. If I just redirect to another page within my web app, it's fine. But I need redirect it to an external webpage....

I found a solution. I use that redirected internal page to redirect again to the external webpage. Then it's working.