NSURLSession dataTaskForRequest:completion: unrecognized selector sent to instance

When trying to create my own session object NSURLSession() and request an url I get an unrecognized selector exception but when I use the shared session NSURLSession.sharedSession() everything works fine. How come?

var url = NSURL(string: "http:/www.google.com")
if url != nil {
    //throws unrecognized selector when dataTaskWithURL is called
    let session=NSURLSession()
    session.dataTaskWithURL(url!)

   //works
    let sharedSession=NSURLSession.sharedSession()
    sharedSession.dataTaskWithURL(url!)
}

You have to init URLSession with a configuration:

URLSession(configuration: .default)

or use shared session

URLSession.shared

In SWIFT 3.0 and up:

        URLSession.shared.dataTask(with: url, completionHandler:
        {
            (data, response, error) in

            //Your code
        }).resume()

Aside from the shared session NSURLSession must be initialized with one of these two methods

init(configuration configuration: NSURLSessionConfiguration)


init(configuration configuration: NSURLSessionConfiguration,
               delegate delegate: NSURLSessionDelegate?,
             delegateQueue queue: NSOperationQueue?)