Portal exception error recieved when using StringBuffer

Hello

I have created a portal service. I need to use StringBuffer or StringBuilder class as i do some text concatenation. Since the String is very resource intensive, this is the only best solution. But when i try to append a text to stringbuffer variable, iget a portal exception.

What could be the issue? Has anyone run into same issue? If some one could suggest an alternative, that would be fine too.

Regards

Kalpesh