Automatic reloading with wrap-reload middleware

wrap-reload is a ring middleware function that will push all our code changes to the application each time we save.

Ring - wrap-reload middleware

Include wrap-reload in the namespace of our project

Require the wrap-reload directly into the namespace

(ns todo-list.core
  (:require [ring.adapter.jetty :as jetty]
            [ring.middleware.reload :refer [wrap-reload]]))

Define a function to use wrap-reload

A function called -dev-main will run the reloading web server when we are developing, ensuring we only use the wrap-reload function during development.

Create a -dev-main function

The -dev-main funciton is the same as -main, except we use the wrap-reload middleware around the welcome function. Each time you change the welcome function definition it will be reloaded.

Using the quote reader macro, #' in front of the welcome function name tells Clojure to skip evaluation of the function and reference the name of the function instead. This allows the wrap-reload middleware to decide when to evaluate the welcome function.

(defn -dev-main
  "A very simple web server using Ring & Jetty,
  called via the development profile of Leiningen
  which reloads code changes using ring middleware wrap-reload"
  [port-number]
  (webserver/run-jetty
    (wrap-reload #'welcome)
    {:port  (Integer. port-number)
     :join? false}))

Note::Tweak the -main function for production

The -main function is typically called on the command line when run in production, so we want to be connected to the output of the webserver.

Remove the :join? false option for the embedded Jetty server, so the output of the server is displayed

(defn -main
  "A very simple web server using Ring & Jetty
  Production mode operation, no reloading."
  [port-number]
  (webserver/run-jetty
    welcome
    {:port (Integer. port-number)}))

Configure the dev profile in your project

When you start your Clojure webapp with lein run it looks for main class to run in the :dev profile first. So we need to create a :dev profile.

:dev profile that sets -dev-main to be the starting point of our application. This

Add profile to project configuration

Edit the project.clj and create a :dev profile to define the initial function to call when starting our webapp.

:profiles {:dev
            {:main todo-list.core/-dev-main}}

The project.clj file should look like the following:

(defproject todo-list "0.1.0-SNAPSHOT"

  :description "A Todo List server-side webapp using Ring & Compojure"
  :url "https://github.com/practicalli/clojure-todo-list-example"

  :license {:name "Creative Commons Attribution Share-Alike 4.0 International"
            :url  "https://creativecommons.org"}

  :dependencies [[org.clojure/clojure "1.10.1"]
                 [ring "1.8.0"]]

  :repl-options {:init-ns todo-list.core}

  :main todo-list.core

  :profiles {:dev
             {:main todo-list.core/-dev-main}})

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