The illegalstate exception ( http://java.sun.com/j2se/1.3/docs/api/java/lang/IllegalStateException.html )could occur on different occasions in a servlet's execution. It is an runtime exception and used in other APIs also. The thumb rule is that if you try to change some basic properties of the response object after some content has been written, it would raise this exception.



If you try to change the content type, buffer size,etc during runtime this exception is raised.