Johnson has signed a new contract tying him to the club until June 2016.
"I'm absolutely delighted. As soon as I heard that the club wanted to sit down and sort a new deal I just wanted to get it done and I'm delighted that it has," he told the club's official website.
"I think I'm playing the best football of my career and I'm playing in the best league in the world. I think this is a club that is moving forward and I want to be a part of that. We've proved that we're good enough to be here and we know how hard it is to stay in this league, but we work as hard as we can to do exactly that."