Justin Bieber Defense -- I DIDN'T DO IT, IT WASN'T ME!

Justin Bieber has a pat defense these days ... as in, "I'm being framed," and we've learned that's exactly how he's attacking 2 criminal investigations targeting him.

CRIMINAL CASE #1:

The L.A. County District Attorney is currently deciding whether to prosecute Bieber for allegedly spitting in a neighbor's face and threatening to kill him after the neighbor confronted the singer for driving his Ferrari at breakneck speeds in the neighborhood.

Law enforcement sources tell us ... Bieber's people have told the D.A. they have a witness who will swear under oath ... Justin didn't spit. Our sources say the D.A. will not make a decision until they're able to interview the witness. FYI ... law enforcement sources tell us they have at least one witness -- aside from the alleged victim -- who will testify he saw the Biebs spit in the guy's face.

CRIMINAL CASE #2:

As we first reported ... Justin's being investigated for reckless driving after allegedly racing his white Ferrari Monday night on the streets of his gated community. Sources connected with the singer are quietly floating the defense that Justin was not driving.

Here's the problem. We broke the story ... Keyshawn Johnson chased Bieber down in his Prius and then blocked his driveway. We're told Keyshawn actually ran up the driveway into the garage and eyeballed Bieber jumping out of the car and running into the house. Law enforcement also tells TMZ ... multiple witnesses "clearly saw" Bieber driving the car.

We're told the L.A. County Sheriff's Dept. will refer the latest Ferrari case to the D.A. shortly. If things keep up, the D.A. is gonna need a special Bieber prosecutor.