"There are a couple of Englishmen who have had great seasons at Portsmouth and Glen certainly fits into that category," Adams said.
Johnson was branded an expensive flop after a four-year spell at Chelsea, during which he made just 41 Premier League appearances. He has five caps but has not appeared in an England shirt since a calamitous 4-1 friendly defeat by Denmark three years ago.
Adams added: "He's playing regularly and we give him confidence. Glen knows he is one of the main men at the club, which I don't think he felt at Chelsea.
"We've created a happy environment for him to go and express himself. It's up to the player to show what he can do and he has certainly not let us down in that respect.
"He's always been an attacking threat, but he's improved defensively no doubt. You only have to look at the FA Cup win against Manchester United - I thought he was exceptional in that game."