Wishful Coding

Didn't you ever wish your computer understood you?

NIO in Clojure

So, I’m playing with the idea of writing a little webserver in Clojure, much like those asynchronous Python ones like Twisted and Tornado, but actually I’m just writing this to play with my new blog.

There you have the code, not as a gist this time. The code actually lives in a separate file on Github, and is included here by a few lines of Liquid markup.

This code was modeled after this Java code. If you have anything interesting to say about this code, NIO, webservers or Clojure, I’d love to hear from you.

(ns nio
  (:import
     [java.net
      InetSocketAddress]
     [java.nio
      ByteBuffer
      CharBuffer]
     [java.nio.channels
      ServerSocketChannel
      Selector
      SelectionKey]
     [java.nio.charset
      Charset]))

(def *buffer* (ByteBuffer/allocate 16384))

(defn selector [server-socket-channel]
  (let [selector (Selector/open)]
    (.register server-socket-channel selector SelectionKey/OP_ACCEPT)
    selector))

(defn setup [port]
  (let [server-socket-channel (ServerSocketChannel/open)
        _ (.configureBlocking server-socket-channel false)
        server-socket (.socket server-socket-channel)
        inet-socket-address (InetSocketAddress. port)]
    (.bind server-socket inet-socket-address)
    [(selector server-socket-channel)
     server-socket]))

(defn state= [state channel]
  (= (bit-and (.readyOps channel) state) state))

(defn buffer->string
  ([byte-buffer]
   (buffer->string byte-buffer (Charset/defaultCharset)))
  ([byte-buffer charset]
   (.toString (.decode charset byte-buffer))))

(defn string->buffer
  ([string]
   (string->buffer string (Charset/defaultCharset)))
  ([string charset]
   (.encode charset string)))

(defn accept-connection [server-socket selector]
  (let [channel (-> server-socket (.accept) (.getChannel))]
    (println "Connection from" channel)
    (doto channel
      (.configureBlocking false)
      (.register selector SelectionKey/OP_READ))))

(defn read-socket [selected-key]
  (let [socket-channel (.channel selected-key)]
    (.clear *buffer*)
    (.read socket-channel *buffer*)
    (.flip *buffer*)
    (if (= (.limit *buffer*) 0)
      (do
        (println "Lost connection from" socket-channel)
        (.cancel selected-key)
        (.close (.socket socket-channel)))
      (.write socket-channel *buffer*))))

(defn react [selector server-socket]
  (while true
    (when (> (.select selector) 0)
      (let [selected-keys (.selectedKeys selector)]
        (doseq [k selected-keys]
          (condp state= k
            SelectionKey/OP_ACCEPT
            (accept-connection server-socket selector)
            SelectionKey/OP_READ
            (read-socket k)))
        (.clear selected-keys)))))

(defn run []
  (apply react (setup 2323)))
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