Checking URL

I want to check if the user enters a valid url.

I have a form the user enters the URL, The URL has to be checked before stroing into the datbase.

eg:
The user enters http://www.yahoo.com , this is URL exits so i can store in the database.

The user enters http://www.testingurlsforbulletbaba.com this url does not exist so this is invalid and cannot be stored in the databse.

Please give ideas how to achieve this.

I am using struts framework

Thank you.