"First up, it's a strong squad designed to win Test matches against quality opposition," said Johnson.
"We are taking the opportunity to have a look at some players and want to assess what they're like at this level.
"We need to be able to find out whether players who've done the business for their clubs can replicate that form in international footy.
"We also want to enable certain players who are known to us - for example Nick De Luca and Max Evans - to have the sort of off-season physical preparation that they've not had for a few years because of the demands of touring schedules."
Scotland leave for South Africa on 31 May and play their first match of a quadrangular tournament, against Samoa, on 8 June.
The Scots face the host nation a week later, with the tournament final and 3rd/4th place match on 22 June.
Backs: Alex Dunbar (Glasgow Warriors), Tom Heathcote (Bath Rugby), Peter Horne, Ruaridh Jackson (both Glasgow Warriors), Greig Laidlaw (Edinburgh Rugby), Sean Lamont, Peter Murchie, Henry Pyrgos (all Glasgow Warriors), Matt Scott (Edinburgh Rugby), Tommy Seymour (Glasgow Warriors), Duncan Taylor (Saracens), Greig Tonks and Tim Visser (both Edinburgh Rugby)
Forwards: John Barclay (Glasgow Warriors), Johnnie Beattie (Montpellier), Kelly Brown (Saracens) (captain), Geoff Cross, David Denton (both Edinburgh Rugby), Alasdair Dickinson (Sale Sharks), Ross Ford, Grant Gilchrist (both Edinburgh Rugby), Ryan Grant (Glasgow Warriors), Jim Hamilton (Gloucester Rugby), Alastair Kellock (Glasgow Warriors), Steven Lawrie (Edinburgh Rugby), Moray Low, Pat MacArthur (both Glasgow Warriors), Euan Murray (Worcester Warriors), Alasdair Strokosch (Perpignan), Tim Swinson and Ryan Wilson (both Glasgow Warriors).