What is the etymology of Johnny on the spot?

The Dictionary of American Slang says it's been around since about 1895 but gives no further explanation of its origin. The name John and its relatives (Jacques, Jack, Johnny . . . ) turn up in many phrases to mean an unspecified male, as in John Doe. JOHNNIE -- "Johnnie meant fellow, chap in English by the 17th century and a man-about-town in the 1880s. 'Johnny-Come-Lately' was in use in America by the 1830s. 'Johnny-on-the-spot' by the 1890s and 'stage-door Johnny' by 1912.'" From "John and Mary:Common First Names," a chapter in "Listening to America" by Stuart Berg Flexner (Simon and Schuster, New York, 1982). Johnny-on-the-spot is a noun that means "an unusually alert fellow who is capable of decisive action, seizing an opportunity, etc. Also (obs.) 'Johnny-on-the-job.' 1896 Ade 'Artie' 19: She was settin' over in the corner, and a Johnny-on-the-spot, with a big badge, marked 'Committee,' was tryin' to keep cases on her. Ibid. 63. I'll be Johnny-on-the-spot to see that everything's on the level.'" From the "Random House Historical Dictionary of American Slang, Volume 1, H-O" by J.E. Lighter, Random House, New York, 1994.