Add main function as per Heroku guide to running on it

Procfile

@@ -1 +1 @@
-web: java $JVM_OPTS -jar target/todo-backend-standalone.jar $PORT
+web: java $JVM_OPTS -jar target/todo-backend-standalone.jar

project.clj

@@ -4,13 +4,15 @@
   :min-lein-version "2.0.0"
   :aot :all
   :java-target "1.8"
+  :main ^:skip-aot todo-backend-clojure.handler
   :uberjar-name "todo-backend-standalone.jar"
   :resource-paths ["resources"]
   :dependencies [[org.clojure/clojure "1.8.0"]
                  [org.clojure/data.json "0.2.6"]
                  [org.clojure/java.jdbc "0.6.1"]
                  [compojure "1.5.1"]
                  [ring/ring-defaults "0.2.1"]
+                 [ring/ring-jetty-adapter "1.5.0"]
                  [ring/ring-json "0.4.0"]
                  [jumblerg/ring.middleware.cors "1.0.1"]
                  [com.h2database/h2 "1.3.170"]]

src/todo_backend_clojure/handler.clj

@@ -2,10 +2,12 @@
   (:require
     [compojure.core :refer :all]
     [compojure.route :as route]
+    [ring.adapter.jetty :as ring]
     [ring.middleware.json :as json-middleware]
     [ring.middleware.defaults :refer [wrap-defaults api-defaults]]
     [todo-backend-clojure.todo_routes :as routes]
-    [todo-backend-clojure.middleware :as todo-middleware]))
+    [todo-backend-clojure.middleware :as todo-middleware])
+  (:gen-class))
 
 (defroutes app-routes
   routes/todo-routes)
@@ -19,3 +21,11 @@
     (json-middleware/wrap-json-response)
     (json-middleware/wrap-json-body)
     (wrap-defaults api-defaults)))
+
+(defn start [port]
+  (ring/run-jetty app {:port  port
+                               :join? false}))
+
+(defn -main []
+  (let [port (Integer. (or (System/getenv "PORT") "8080"))]
+    (start port)))