In my app, I am designing to allow user share messages to twitter. But those messages could already exceeded the 140 character constraint put on tweets, what will be the best way to work around that?
You can't work around the limit. If the 140 characters are exceeded twitter would not accept the message. I suggest you use a tweet-shortener like I do for my app.
What it basically does is if the number of tweets => 140, strip it in half, create a html page containing the full tweet, send back the shortened tweet and the link to the app, then paste it.
But there are already third party tweet-shorteners available so you can try out any one.
http://www.google.com/search?gcx=w&sourceid=chrome&ie=UTF-8&q=tweet+shortener
http://www.google.com/search?gcx=w&sourceid=chrome&ie=UTF-8&q=tweet+longer