nrepl.el

(defun nrepl-port-from-file (file)
  "Attempts to read port from a file named by FILE."
  (let* ((dir (nrepl-project-directory-for (nrepl-current-dir)))
         (f (expand-file-name file dir)))
    (when (file-exists-p f)
      (with-temp-buffer
        (insert-file-contents f)
        (buffer-string)))))

(defun nrepl-default-port ()
  "Attempt to read port from .nrepl-port or target/repl-port.
Falls back to `nrepl-port' if not found."
  (or (nrepl-port-from-file ".nrepl-port")
      (nrepl-port-from-file "target/repl-port")
      nrepl-port))