boot.user=> (binding [*out* (java.io.StringWriter.)] (println "hello") (str *out*))
"hello\r\n"
boot.user=> (source with-out-str)
(defmacro with-out-str
  "Evaluates exprs in a context in which *out* is bound to a fresh
  StringWriter.  Returns the string created by any nested printing
  calls."
  {:added "1.0"}
  [& body]
  `(let [s# (new java.io.StringWriter)]
     (binding [*out* s#]
       ~@body
       (str s#))))
nil
boot.user=> (with-out-str (println "hello"))
"hello\r\n"