Clojure Docker Web Apps

If you’re interested in making a commercial grade Clojure web app that deploys as a Docker container, this is the tutorial for you.

Prerequisites: You will need Leiningen and Docker installed.

First step needed is to create a Clojure web app template project. I used the following command.

This creates a Clojure web app using the Luminus Framework, Reitit for mapping of incoming requests to the proper handlers, HTTP Kit for HTTP handling, MySQL for JDBC connections to our MySQL database, Swagger for API support, and Buddy for authentication middleware. Your new web app is in the directory, mywebapp .

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JAVA_CMD for lein

Sometimes you need to configure lein  to use a custom JDK. On Linux, set the environment variable JAVA_CMD  to point to the java  executable you want lein  to use.

For example, you might add the following lines to your .bash_profile

Once you update your bash profile, you can start using your configuration by using the following command.


Clojure Record Heirarchy

In Clojure, if you want a basic OOP behavior without killing yourself to fully implement Java classes and interfaces in Clojure, the preferred technique is to use records. Sometimes implementing protocols in records suits your needs, mostly due to speed of code execution. However, if you want polymorphism with your records, then multimethods are where it’s at.

Admittedly, multimethods are very flexible, so the technique I’ll show you is one of many techniques you could use. The basic pattern I use for inheritance with records, is to use the derive  command with keywords that are kept as a value in the record. For example,

type  is used to track isa  relationships of ::food  and ::fruit . derive  is used to set ::food  as the parent of ::fruit . Notice the use of custom constructors to set default values for type  in the records.

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Clojure Agents

Most of the time, Clojure does not need locks like Java and other languages. This is because of atom s, ref s, and agent s. I’ll focus on Clojure agents in this short post.

Agents are handled in  single-threaded queues. By lining up calls of a function in a queue, no locks are needed. When combined with send-off , deadlocks won’t happen.

Here’s a quick example.

A few things are going on here, so let’s look at them.

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Quick OOP Example

Want some Object Oriented Programming in Clojure? Here’s a quick example.

Let’s create a dog object.

Now let’s use the dog object.

See, OOP in Clojure is fairly simple. This sample uses protocols with records, but you can add polymorphism with just a little extra work. For polymorphism, use records and multimethods.

Razvan’s ‘defrecord’ Example

I absolutely love the user razvan’s Clojure defrecord example over at stackoverflow, so I copied it here. I know I’ll never find it again if I don’t. Look at the original over on stackoverflow at this link.

Here’s the example …

Clojure defrecord example:

If you’re looking for more information on combining protocols with records in Clojure, then Matthew Boston has the blog for you!

pr and prn

NOTE: All the code in this tutorial can be quickly tested at the REPL using the command lein repl  to start a REPL to test with.

One of the first things you learn when learning clojure is print  and println . For example,

That’s probably the first Clojure program you ever wrote. Maybe you wrote this variation.

Because println  or print are the first ways learned of displaying output in Clojure, most Clojure developers continue using them when debugging code. Nothing wrong with that. But sometimes it is better to use a more specialized tool.

In steps the prn  and pr  commands. These two commands are tailored for printing objects. At first glance, they appear to work the same as print  and println . However, they actually show more information in some instances.

If using print  or println , the \n  in the following code quietly gets replaced by the action of a new line being printed.

When pr  or prn  are used, the two characters \  and n  are displayed instead of an actual new line. This is very handy when hidden characters are fowling up your layout, or needed to be visible when debugging code.

Final note: The only difference between pr  and prn , is that prn  prints a new line after the objects it is printing out.