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Example Web Application in Clojure

This is a simple web application using Component, Ring, Compojure, and Selmer connected to a local SQLite database.

Clojure beginners often ask for a "complete" example that they can look at to see how these common libraries fit together and for a long time I pointed them at the User Manager example in the Framework One for Clojure repo -- but since I EOL'd that framework and I'd already rewritten the example app to no longer use the framework, it's just confusing to point them there, so this is a self-contained repo containing just that web app example.

A variant using Integrant and Reitit (instead of Component and Compojure), inspired by this example repo, can be found in Michaël Salihi's repo.

A version of this application that uses the Polylith architecture is also available, on the polylith branch.

Requirements

This example assumes that you have a recent version of the Clojure CLI installed (at least 1.10.3.933), and provides a deps.edn file, and a build.clj file.

Clojure 1.10 (or later) is required. The "model" of this example app uses namespace-qualified keys in hash maps. It uses next.jdbc -- the "next generation" JDBC library for Clojure -- which produces namespace-qualified hash maps from result sets.

Usage

Clone the repo, cd into it, then follow below to Run the Application or Run the application in REPL or Run the tests or Build an Uberjar.

Run the Application

clojure -M -m usermanager.main

It should create a SQLite database (usermanager_db) and populate two tables (department and addressbook) and start a Jetty instance on port 8080.

If that port is in use, start it on a different port. For example, port 8100:

clojure -M -m usermanager.main 8100

Run the Application in REPL

Start REPL

$ clj

Once REPL starts, start the server as an example on port 8888:

user=> (require 'usermanager.main)                             ; load the code
user=> (in-ns 'usermanager.main)                               ; move to the namesapce
usermanager.main=> (def system (new-system 8888))              ; specify port
usermanager.main=> (alter-var-root #'system component/start)   ; start the server

Run the tests with:

clojure -T:build test

You should see something like this:

Running task for: test

Running tests in #{"test"}
2022-05-25 18:19:45.138:INFO::main: Logging initialized @6494ms to org.eclipse.jetty.util.log.StdErrLog

Testing usermanager.model.user-manager-test
Created database and addressbook table!
Populated database with initial data!

Ran 3 tests containing 9 assertions.
0 failures, 0 errors.

This uses the :build alias to load the build.clj file, based on tools.build, and run the test task. That in turn runs the run-tests task from my build-clj wrapper for tools.build, that provide "sane" defaults for the myriad options in tools.build so you can write simpler build.clj files.

Build an Uberjar

For production deployment, you typically want to build an "uberjar" -- a .jar file that contains Clojure itself and all of the code from your application and its dependencies, so that you can run it with the java -jar command.

The build.clj file -- mentioned above -- contains a ci task that:

  • runs all the tests
  • cleans up the target folder
  • compiles the application (sometimes called "AOT compilation")
  • produces a standalone .jar file
clojure -T:build ci

That should produce the same output as test above, followed by something like:

Cleaning target...

Skipping pom.xml because :lib and/or :version were omitted...
Copying src, resources...
Compiling usermanager.main...
2022-05-25 18:20:13.069:INFO::main: Logging initialized @3981ms to org.eclipse.jetty.util.log.StdErrLog
Building uberjar target/example-standalone.jar...

The target folder will be created if it doesn't exist and it will include a classes folder containing all of the compiled Clojure source code from the usermanager application and all of its dependencies including Clojure itself:

$ ls target/classes/
cheshire  clojure  clout  com  compojure  crypto  instaparse  json_html  layouts  medley  next  public  ring  selmer  usermanager  views

It will also include the standalone .jar file which you can run like this:

java -jar target/example-standalone.jar

This should behave the same as the Run the Application example above.

This JAR file can be deployed to any server that have Java installed and run with no other external dependencies or files.

Stuff I Need To Do

  • I might add a datafy/nav example.

License & Copyright

Copyright (c) 2015-2022 Sean Corfield.

Distributed under the Apache Source License 2.0.

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A little demo web app in Clojure, using Component, Ring, Compojure, Selmer (and a database)

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