A -main function to run Jetty web server

The Ring Jetty adaptor is used to run an instance of Jetty. The -main function contains an anonymous function that takes any request and returns a response map.

The -main function takes a port number as an argument which we pass when running the application.

Ring - Adaptor and anonymous function

A response map contains the following key / value pairs

  • :status - the result of the request, eg. 200 OK, 401 Not Found, etc
  • :body - the content to be returned (web page, json, etc)
  • :headers - a map of standard headers included in any web browser response

Add a function called -main to the src/todo_list/core.clj file.

Edit the src/todo_list/core.clj file and add the following function, after the namespace definition.

(defn -main
  "A very simple web server using Ring & Jetty"
    (fn [request]
      {:status  200
       :headers {}
       :body    "<h1>Hello, Clojure World</h1>
                 <p>Welcome to your first Clojure app.
                 This message is returned regardless of the request, sorry</p>"})
    {:port  (Integer. port-number)
     :join? false}))

Explaining the new function

Using a - at the start of the -main function is a naming convention, helping you see which function is the entry point to your program. Leiningen also looks for this -main function by default when running your application.

The webserver/run-jetty function takes two arguments. In our example, the first argument is an anonymous function that returns a map (the response to the browser request); the second argument is a port number to run the jetty server on expressed as a Java Integer object.

The Integer. function is a call to java.lang.Integer. The . is a special form that tells Clojure to treat this name as a call to Java. See coersing types and java.lang

The :join? false setting enables the REPL prompt to run after the web server starts. By default the join setting is true and the running server would block access to the REPL prompt.

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