Lenny Kravitz -- Exposes Junk ... After Leather Pants Rip Open!!! (PHOTO)

Lenny Kravitz had the worst wardrobe malfunction of all-time -- or maybe the best -- when his leather pants ripped completely open onstage ... and his junk fell out.

Luckily, Lenny had a back-up pair of pants during his Monday show in Stockholm -- so he was able to do a quick wardrobe change and finish the concert. Even luckier ... he's got no reason to be shy.

Encore!