Lesson 5 - A Web Server in Clojure

In our previous lessons we generated some web pages in clojure. However we just viewed those pages in our browser without a web server. Now let us roll out our own web server in clojure to view our own shiny little web site.


(use 'clojure.contrib.server-socket)


(create-server
  8080
  (fn [in out]
    (binding
      [*out* (java.io.PrintWriter. out)]
      (println "HTTP/1.0 200 OK")
      (println "Content-Type: text/html")
      (println "")
      (println "<h1>Wooo hooo hooo, my first web server!</h1>")
      (flush))))

Type the code above into a file called server.clj and run it. The program will not exit the terminal. Never mind. Now open a browser window and browse to "http://localhost:8080". You just rolled your first web server!

Now for some explanation. We use the server-socket library from clojure.contrib. The create-server function takes two parameters a port number and a callback function to call when a connection is made. The server listens to post 8080 and when a connection is made, the callback function is called with two arguments, a socket input stream and a socket output stream.
In the following line
[*out* (java.io.PrintWriter. out)]
we bind the stdout stream "*out*" to an instance of a java.io.PrintWriter class created with the socket output stream we received. We then print to the stdout and flush it. Thats it!

I had mentioned earlier that clojure can also call java functions and here we see
(java.io.PrintWriter. out)
whose equivalent in clojure is
(new java.io.PrintWriter out)
and in java
new java.io.PrintWriter(out)

Our shiny new web server is pretty rudimentary (to say the least).  We dump the output to anything that connects, without checking the request. So lets fix that first.


(use 'clojure.contrib.server-socket)
(import  '(java.io BufferedReader InputStreamReader PrintWriter))

(create-server
  8080
  (fn [in out]
    (binding
      [ *in* (BufferedReader. (InputStreamReader. in))
        *out* (PrintWriter. out)]
      (println "HTTP/1.0 200 OK")
      (println "Content-Type: text/html")
      (println "")
      (loop [line (read-line)]
        (println (str line "<br/>"))
        (if-not (empty? line)
          (recur (read-line))))
      (flush))))


Run the above program and browse to localhost:8080. The program simply returns all the lines of text in the input stream back to the browser with a little html formatting.

First, we use the import statement to import a few java classes into our namespace so that we can just use the class names without the namespace. We also bind the stdin "*in*" to our socket input via BufferedReader which takes an argument InputStreamReader which in turn takes our socket input stream "in" as argument.

Now let us look at the following function.
(loop [line (read-line)]
  (println (str line "<br/>"))
  (if-not (empty? line)
    (recur (read-line))))

The loop function takes a set of bindings as its arguments, and hitting a recur statement within its scope will return control to the top of the loop. Here we bind the value of (read-line) which will be the first line of *in* to a lexically scoped variable named "line". Next we print line to *out*.  Next we call the if-not function which in turn calls the empty? function to check if not "line" is empty. If it is not empty then we call recur function which will (read-line) and rebind "line" to the new line we read and pass control back to the top of the loop.

In the next lesson we look at parsing the HTTP Request.