There is a Java library, called zxing, that was particularly designed for the mobile use-case. I worked with it to develop a web application that will both generate and scan in QR codes and found it easy to work with. There are also complete code samples on the internet for scanning QR codes with the phone’s camera and decoding them with zxing, as is discussed here:
http://stackoverflow.com/questions/23156399/how-to-decode-a-barcode-from-camera-preview-using-zxing-…
From there you could open a URL, passing the QR code’s value in as a query string parameter, for example.
