multi-outstream.clj

(defmacro multi-outstream [var]
  (letfn [(outs [val] (if (instance? clojure.lang.Atom val) (first @val) val))]
    `(java.io.PrintStream.
      (proxy [java.io.BufferedOutputStream] [nil]
        (write
          ([b#]           (.write (~outs ~var) b#))
          ([b# off# len#] (.write (~outs ~var) b# off# len#)))
        (flush [] (.flush (~outs ~var)))))))

(defmacro with-outstream [bindings & forms]
  `(do (doseq [[var# outs#] (partition 2 ~bindings)]
         (swap! var# conj outs#))
       (binding ~bindings ~@forms)
       (doseq [[var# outs#] (partition 2 ~bindings)]
         (doall (swap! var# (partial remove #(= outs# %)))))))

(def *outs* (atom ()))
(def *errs* (atom ()))

(let [outs (multi-outstream *outs*)
      errs (multi-outstream *errs*)]
  (System/setOut outs)
  (System/setErr errs))

(with-outstream [*outs* outs, *errs* outs]
  (future (.println System/out "foo")))