It's jail for Marvin Barnes, but the Detroit Pistons needn't weep too uncontrollably: he gets to finish out the NBA season.

The not-so-Friarly Providence athlete, just getting into the NBA swing (21 points for the Pistons against Buffalo Saturday night), yesterday drew a one-year stretch in Cranston Correctional Institution for breaking Rhode Island probation. But it won't start, Judge Anthony A. Giannini decreed, until May 16 - later if the Pistons make the playoffs. And it could be over by the time next basketball season starts: Barnes will be eligible for parole in four months.

Last month, the judge gave Barnes a temporary reprieve after a handgun was found in his baggage at the Detroit airport and he was convicted of violating the three-year probation he was serving for assaulting Providence College teammate Larry Ketvirtis with a tire iron.

Barnes' lawyers wanted the 6-foot-9 forward - a regular "Pied Piper with children," said Piston coach Herb Brown - to make it up to society by working with troubled youngsters in high-crime areas. Judge Giannini decided Barnes wouldn't set that good an example.

But he may yet get to do his sociological thing. He still faces Michigan court action on the handgun arrest - and his attorney has asked that he be allowed to work off the time helping youths in Detroit's rugged 15th police precinct . . .

Now, if Marvin Barnes will be good henceforth, Jim Barnes of Washington - the original "Bad News" Barnes of the NBA - can rest easy. The rise of a rival claimant to the title, Jim Barnes protests to FanFare, is a "wrong that has been perpetrated on my reputation" and he asks us to help "clear the air of all doubts people might have about who the real "Bad News" Barnes is - the one whose Bad News all-purpose sauce you can find on the shelf of your supermarket.

Consider it done. We always knew who the real Bad News was, and it surprised us to learn during the current flap that young Marvin actually had adoapted the nickname while in college - went so far as to put "NEWS-24" on his jacket and his car license plate . . .