Have some web app using java, jsp, servlet. too many times frequent queries to oracle 10g under load. Thinking to introduce memcached, Is it simple ? Can it be done without code change? are there any other solutions avaliable?
No, memcached is a new layer so it will require medium to large code changes.
While there are some fairly simple wrappers that allow minimal code changes they won't make much difference in performance as only a human can tell which queries should actually be cached.
Oracle really requires a good DBA to manage it, if you haven't got one that should be your first step.
Another option, if it's a site for anonymous users would be to use a reverse proxy like Squid or Varnish to cache the full page output instead.