A retired judge and former district attorney said he was relieved yesterday after a Northampton jury found him not guilty of striking a police officer. The jury deliberated 90 minutes before acquitting W. Michael Ryan in Northampton District Court, where he was once presiding justice. The former Northwestern district attorney was charged with disorderly conduct and assault and battery on an officer. Police said Ryan and another person were in a parking lot at night in October trying to change a tire when an officer approached and asked them questions. Police allege Ryan became “argumentative and belligerent’’ and struck the officer’s wrist. Ryan denied the charges.