flymake mode for json using emacs' builtin json-parse-buffer
Throw json-simple-flymake.el into your ~/.emacs.d/ and add
(load "~/.emacs.d/json-simple-flymake.el")
at the end of your ~/.emacs.
M-x customize-group RET json-simple-flymake-group
only ever marks but one error, because we use json-parse-buffer, which doesn't do more. But hey, more than nothing for a devop who want's to edit some json configs and not install a zillion jiggabytes of npm madness for jsonlint, right?