There are many mysteries in life, but few are as perplexing as the eternal question “What exactly happens during cider-jack-in?”. Today it’s time to unravel this mystery!

The Basics

At its heart cider-jack-in does something super simple - it just executes a shell command (e.g. lein repl) that starts an nREPL server, waits for this server to come to life, and then connects CIDER to it. It’s really as simple as this. If I had to explain it in more details I’d probably do it like:

  • CIDER shells out and runs a command like lein repl :headless.
  • CIDER waits for the nREPL server to start. CIDER figures out this by parsing the output from the command and waiting for a line like nREPL server started on port 53005 on host localhost - nrepl://localhost:53005 to appear there.
  • CIDER extracts the port of the nREPL from the preceding message.
  • It connects to the running nREPL server.

Basically it’s a combination of auto-starting an nREPL server and doing cider-connect to it afterwards. 1 Or is it? If it were so simple why does the shell command executed by cider-jack-in for Leiningen project look like:

[nREPL] Starting server via /usr/local/bin/lein update-in :dependencies conj \[nrepl\ \"0.6.0\"\] -- update-in :plugins conj \[cider/cider-nrepl\ \"0.23.0-SNAPSHOT\"\] -- repl :headless :host localhost

It’s time to explain another confusing concept - namely auto-injection of Clojure dependencies.

Auto-Injecting Dependencies

While CIDER’s core functionality requires nothing more than an nREPL server, there are many advanced features2 that depend on the presence of additional nREPL middleware (e.g. cider-nrepl). In the early versions of CIDER (up to CIDER 0.11) users had to add those dependencies themselves, which was a painful and error-prone process. Fortunately today that’s handled auto-magically when you’re using cider-jack-in.

If your project uses lein, boot or tools.deps (deps.edn), CIDER will automatically inject all the necessary nREPL dependencies (e.g. cider-nrepl or piggieback) when it starts the server. The injection process is extremely simple - CIDER simply passes the extra dependencies and nREPL configuration to your build tool in the command in runs to start the nREPL server. Here’s how this looks for tools.deps:

$ clojure -Sdeps '{:deps {nrepl {:mvn/version "0.6.0"} cider/cider-nrepl {:mvn/version "0.22.4"}}}' -m nrepl.cmdline --middleware '["cider.nrepl/cider-middleware"]'

Here’s a tip for you - if you don’t want cider-jack-in to inject dependencies automatically, set cider-inject-dependencies-at-jack-in to nil. Note that you’ll have to setup the dependencies yourself (e.g. by adding them to your project) just as in CIDER 0.10 and older. There are few reason why you’d want to do something like this, but the option exists.

Normally cider-jack-in would inject only cider-nrepl and cider-jack-in-cljs would add piggieback as well. The injection, however, mechanism is configurable and you can easily add more libraries there. Some CIDER extensions (e.g. sayid and clj-refactor) would use this mechanism to auto-inject their own dependencies.

CIDER would also inject the most recent version of nREPL that it supports. This is a simple trick to override the version of nREPL bundled with your build tool, so you can gain access to the newest nREPL features. Generally that’s one aspect of CIDER’s inner workings that end-users will rarely have to think about.

CIDER can also inject a Clojure dependency into your project, which is useful, for example, if your project defaults to an older version of Clojure than that supported by the CIDER middleware. Set cider-jack-in-auto-inject-clojure appropriately to enable this.

There are other nuances of the auto-injection - e.g. for Leiningen you can inject both dependencies and plugins, but that’s outside the scope of this article.

Now it’s time for a little quiz! Can you guess why CIDER does not currently support dependency auto-injection for Gradle projects? Yeah, you’re totally right! Unfortunately there’s no way to pass extra dependencies to Gradle via its command-line interface.

Jacking-in without a Project

If you try to run cider-jack-in outside a project directory, CIDER will warn you and ask you to confirm whether you really want to do this; more often than not, this is an accident. If you decide to proceed, CIDER will invoke the command configured in cider-jack-in-default. Prior to CIDER 0.17, this defaulted to lein but was subsequently switched to clj, Clojure’s basic startup command.

Here’s another useless tip - you can set cider-allow-jack-in-without-project to t if you’d like to disable the warning displayed when jacking-in outside a project. I never run CIDER outside of projects, but you might have a different style and, as you probably noticed by now, CIDER is infinitely configurable.

Customizing the Jack-in Command Behaviour

We now made it to my favourite part of our exploration of CIDER’s jack-in - how can we tweak the shell command it runs? There are many reasons to want to do this - most often you’d like to include some extra profiles (e.g. the famous fig profile if you are using tools.deps), but I can imagine some people would have to do more creative tweaks as well.

Let’s start with the simplest possible option, that’s idea for one-off customizations. You can use C-u M-x cider-jack-in (C-u C-c C-x j j) to specify the exact command that cider-jack-in would run. This option is very useful is you want to specify a something like a lein or deps.edn profile.

Alternatively you can C-u C-u M-x cider-jack-in, which is a variation of the previous command. This command will first prompt you for the project you want to launch cider-jack-in in, which is pretty handy if you’re in some other directory currently. This option is also useful if your project contains some combination of project.clj, build.boot and deps.edn and you want to launch a REPL for one or the other.

Keep in mind that the examples here use only cider-jack-in, but this behaviour is consistent for all cider-jack-in-* commands.

You can further customize the command line CIDER uses for cider-jack-in by modifying the following string options:

  • cider-lein-global-options, cider-boot-global-options, cider-clojure-cli-global-options, cider-gradle-global-options: these are passed to the command directly, in first position (e.g., -o to lein enables offline mode).
  • cider-lein-parameters, cider-boot-parameters, cider-clojure-cli-parameters, cider-gradle-parameters: these are usually task names and their parameters (e.g., dev for launching boot’s dev task instead of the standard repl -s wait).

Don’t forget that those variables can be configured on a per-project basis.

A Note about tools.deps and Windows

You’ve probably figured out by now that using tools.deps with Windows can be a bit painful. Fortunately CIDER can help with that.

To use cider-jack-in with tools.deps on Windows set the cider-clojure-cli-command to "powershell". This happens by default if you are on Windows and no clojure executable is found. Using "powershell" will Base64 encode the clojure launch command before passing it to PowerShell and avoids shell-escaping issues.

You might be wondering why CIDER uses clojure instead of clj, right? It’s pretty simple actually - clj simply wraps clojure with some terminal-friendly functionality (rlwrap), that explodes if you’re not in an actual terminal. Another great mystery has been resolved!


Phew! That’s a lot of information! I hope now you’ve got a much better idea how the jack-in process is working and that this will help you use CIDER more effectively down the road.

There’s a lot more that can be said about jack-in, but I do feel we’ve covered everything essential. I’m also a bit lazy and I don’t feel like writing anymore. :-)

That’s all I have for you today. This episode was brought to you by Clojurists Together. They are awesome and so are all of you! Keep hacking!

  1. With a pretty confusing Neuromancer/Matrix-inspired name. 

  2. “Advanced” is a bit of stretch here - there’s no ClojureScript support without Piggieback.