doFilter
public void doFilter(HttpExchange exchange)
              throws IOException
calls the next filter in the chain, or else
 the users exchange handler, if this is the
 final filter in the chain. The Filter may decide
 to terminate the chain, by not calling this method.
 In this case, the filter must send the
 response to the request, because the application's
 exchange handler will not be invoked.
- Parameters:
- exchange- the HttpExchange
- Throws:
- IOException- let exceptions pass up the stack
- NullPointerException- if exchange is- null