A Simple Web Server in Clojure

Clojure has no built-in HTTP server, but the de facto standard for serving basic, synchronous HTTP requests is the Ring library.

Ring is the basis for most web applications in Clojure. It provides a low-level and straightforward request/response API, where requests and responses are plain old Clojure maps. Ring applications are architected around handlers: functions that accept requests and return responses.

For more details check the Ring API Documentation.

The following example was adapted from parts of chapter 7 of the “Clojure Cookbook” by Luke VanderHart and Ryan Neufeld.

Create a Leiningen application called webapp1. At the terminal type:
```
lein new app webapp1
```
Run your application for the first time:
```
lein run
```
The output should just be:
```
Hello, World!
```
Modifiy the webapp1/project.clj file. Update the Clojure version after :dependencies key and also add the ring and compojure libraries, like this:
File: webapp1/project.clj
```
:dependencies [[org.clojure/clojure "1.9.0"]
               [ring "1.6.3"]
               [compojure "1.6.1"]]
```

Replace the full content of the webapp1/src/webapp1/core.clj file with the following code:

File: webapp1/src/webapp1/core.clj

;;;; A Simple Web Server in Clojure.
;;;; Uses the Ring library.

(ns webapp1.core
  (:require [ring.adapter.jetty :refer [run-jetty]]
            [ring.middleware.file :refer [wrap-file]]))

(def port 8080)

(defn not-found [request] ; (1)
  {:status   404
   :headers  {"content-type" "text/plain; charset=utf-8"}
   :body     (str "Resource not found: " (:uri request))})

(def app (wrap-file not-found "public")) ; (2)

(defn -main [] ; (3)
  (run-jetty app {:port port}))

1	This is a handler that produces a response when a requested resource was not found.
2	The directory at the given root-path `"public"` is checked for a static file with which to respond to the request, proxying the request to the `not-found` handler if such a file does not exist.
3	This is the application’s (web server) entry point.

Create a public directory inside the the root webapp1 directory. In the terminal type:
```
cd webapp1
```
```
mkdir public
```
All public static content should be placed in this public directory.

Create a webapp1/public/index.html file with the following content:

File: webapp1/public/index.html

<!DOCTYPE html>
<!-- A simple default web page. -->
<html>
  <head lang="en">
    <meta charset="utf-8">
    <title>Test</title>
  </head>
  <body>
    <h1>Test</h1>
    <p>This is a web page.</p>
  </body>
</html>

Run the application (web server). From the root webapp1 directory, type at the terminal:
```
lein run
```
Test the web server. In your browser’s address bar, type the corresponding URL:
```
http://server-name:8080
```
where server-name is the name or IP address of the web server.
If you’re using a Cloud9 environment, you can obtain this name with the following terminal command:

echo $C9_HOSTNAME
You should see the index.html document correctly rendered in your web browser.