# What is Clojure?

Clojure is a dialect of the Lisp programming language. It is a general-purpose programming language with an emphasis on functional programming. It runs on the Java Virtual Machine, Common Language Runtime, and JavaScript engines. Like other Lisps, Clojure treats code as data and has a macro system.

> [wikipedia.org/wiki/Clojure](http://en.wikipedia.org/wiki/Clojure)

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# How to use this image

## Start a Lein/Clojure instance in your app

Since the most common way to use Clojure is in conjunction with [Leiningen (`lein`)](http://leiningen.org/), this image assumes that's how you'll be working. The most straightforward way to use this image is to add a `Dockerfile` to an existing Leiningen/Clojure project:

```dockerfile
FROM clojure
COPY . /usr/src/app
WORKDIR /usr/src/app
CMD ["lein", "run"]
```

Then, run these commands to build and run the image:

```console
$ docker build -t my-clojure-app .
$ docker run -it --rm --name my-running-app my-clojure-app
```

While the above is the most straightforward example of a `Dockerfile`, it does have some drawbacks. The `lein run` command will download your dependencies, compile the project, and then run it. That's a lot of work, all of which you may not want done every time you run the image. To get around this, you can download the dependencies and compile the project ahead of time. This will significantly reduce startup time when you run your image.

```dockerfile
FROM clojure
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app
COPY project.clj /usr/src/app/
RUN lein deps
COPY . /usr/src/app
RUN mv "$(lein uberjar | sed -n 's/^Created \(.*standalone\.jar\)/\1/p')" app-standalone.jar
CMD ["java", "-jar", "app-standalone.jar"]
```

Writing the `Dockerfile` this way will download the dependencies (and cache them, so they are only re-downloaded when the dependencies change) and then compile them into a standalone jar ahead of time rather than each time the image is run.

You can then build and run the image as above.

## Compile your Lein/Clojure project into a jar from within the container

If you have an existing Lein/Clojure project, it's fairly straightforward to compile your project into a jar from a container:

```console
$ docker run -it --rm -v "$PWD":/usr/src/app -w /usr/src/app clojure lein uberjar
```

This will build your project into a jar file located in your project's `target/uberjar` directory.
