The couple has been dating for over six months.
"He loves that she's so independent and doesn't cling to him like a lovesick barnacle. Katy, of course, loves a bad boy but she's not going to let a man stomp all over her," okmagazine.com quoted a source as saying.
"She's really quite sassy with John and he loves it. He needs someone to take him down a notch," the source added.