i want to provide my one API to other company's server which will return a unique token number generated at my end and in response to that call, i will return the token number.
Should i use REST Web Service or Servlet for direct URL hit? or any other mode of communication?
I would recommend a REST web service, mostly because they are a lightweight industry standard. You're not giving us very many details though, and the data you are returning is so minimal that there is not likely a significant difference. Look into web server optimization practices, like enabling GZIP, which will make more of a difference.
