Neil Lennon's side thought that they had the lead on half-hour mark, but Griffiths's effort was ruled out for offside.
The forward was unfortunate not to break the deadlock moments later when he rounded the goalkeeper only to see his shot blocked on the line.
With the first half petering out, Celtic struck through Johansen after a pinpoint delivery from Griffiths.
Griffiths's hard work was finally rewarded on the hour mark when Stokes avoided a couple of challenges before laying in his teammate, who made no mistake with the finish.
Kris Commons went close with a stinging effort minutes later before he struck the post with a similarly powerful strike on 64 minutes.
Stokes then put the gloss on the result in stoppage time to edge them closer to the title.