Make an HTTP Server

• Add a dependency to the ring library to your deps.edn

  {:paths ["src"]
   :deps {org.clojure/clojure {:mvn/version "1.12.0"}
          ring/ring {:mvn/version "1.13.0"}}}
             

  Ring is the library most everyone in the Clojure world uses for HTTP servers.

• Edit your src/example/main.clj file so it has the following contents.

  (ns example.main
    (:require [ring.adapter.jetty :as jetty]))
  
  (defn handler
    [request]
    {:status 200
     :headers {"Content-Type" "text/html"}
     :body "Hello, world"})
  
  (defn -main []
    (jetty/run-jetty #'handler {:port 9999}))

  The handler function takes in an HTTP Request as outlined by The Ring Specification and returns a map representing an HTTP Response.

  When we call jetty/start-jetty we give it that handler function. A small wrinkle is that little #' in front. That makes it so that if you reload that function in a REPL the server will use the new definition right away.

  For more details on how that works you can dig into Clojure Vars. All you need to know is that when you want to pass a function as an argument and have it work with the REPL, you need to put #' in front.

• Run the server with just run

  You should see a warning like the following. Ignore it for now.

  SLF4J: No SLF4J providers were found.
  SLF4J: Defaulting to no-operation (NOP) logger implementation
  SLF4J: See https://www.slf4j.org/codes.html#noProviders for further details.

• Open your web browser and go to http://localhost:9999

  You should see Hello, world.