InputStream to Base64 stream

demo.clj

;; An example InputStream made of 128 bytes:
(defn bytes-is []
  (java.io.ByteArrayInputStream. (into-array Byte/TYPE (range 128))))

(defn b64encode [s]
  (String. (.encode (java.util.Base64/getEncoder) (.getBytes s))))

;; Now we read the input stream *in memory* (with slurp) and encode as base64:
(-> (bytes-is) slurp b64encode)
;; => "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJC...n8="