Would go with the previous posts as to why your lens is not focusing right. There could be other reasons though.
It may have had a bad repair and the shims (spacers which seat the lens block properly on the barrel) either were lost, or improperly replaced. Latter means that there might be too less or too much. If you're getting front focus, it means the shims are too thick than the lens would have it. If you're getting back focus, then it's a case of having too less.
Repair might be easy. You need an extra screw mount RF camera with opening back- it could be a Canon or even a Leica M3 or even a FED-2/3/4/5. Any would do as long as they have proven 28.8mm lens flange to focal registers/distances.
Stick a focus screen on the film plane. Make one from an old discarded focusing screen or stretch 'magic tape'. MAKE SURE that 'screen' is exactly on film position- should be flat and should ride on the film rails.
MOunt lens, set scale to infinity and point to a very distant object (100 metres or more), and observe the focus on the 'screen'. Use a 5x loupe if necessary. If infinity seems to lie before the infinity mark on the scale, you'd need to increase shimming. If the lens doesn't seem to reach infinity, decrease shimming.
To work on the shims, you'd have to unscrew the optical block. Grip the front of the lens (preferably with a rubber mat) and unscrew. The optical block will screw off and you'd see some shimming in the form of metal rings. If you don't see any, you have likely found the reason why your lens isn't focusing right. Shim can be cut out of paper (various thickness) or even bits of wire.
What to use and how much would depend on what you see happening with your lens. Its really a TRIAL AND ERROR thing. You'd likely be taking the optic block on and off, each time with a variable amount of shimming, until you see the focus right.
Once you've found the correct value, reset your lens and reassembled it, you might see that the lens diaphragm scale no longer lines up properly with the index mark on the barrel. This is normal since the optic block has been slilghtly displaced. Adjust the diaphragm ring by loosening 2 or 3 TINY screws (-do not unscrew them all the way!) to loosen the tang. Set aperture to "f/2" and move the tang until the f/2 mark lines up with the index line.
The disassembly described here applies to the 'white' J-8 versions from the late 1950s to 1960s. The 70s, or black versions may be slightly different - the focusing mount had been significantly changed. The Earliest J-8 had focusing tabs- these too disassemble slightly differently. The optical block is locked in place by a tiny setscrew, and is usually difficult to work on.
Hope these help
Jay
