Using the Clojure REPL with Java or Scala

Clojure is a tool that enables interactive development and runtime inspection. Even when we work in other programming languages, Clojure can still be useful. Especially when that other language lives on the JVM.

Let's take Scala for example. Scala has a REPL. The REPL can be used to test-drive software in development. But it doesn't really let you inspect a running program when you didn't start it with `sbt console`. So let's use Clojure for that. We will walk through a simple Scala program that allows runtime inspection of an otherwise unknown state.

We'll need an sbt project for this example. Make a directory and put a build.sbt in it. The only dependency in this example is Clojure.

scalaVersion := "2.11.8"

libraryDependencies ++= Seq(
  "org.clojure" % "clojure" % "1.8.0"

In src/main/scala/example.scala we add the following imports:

import{`var` => cvar}

We'll be using Clojure's Java API. In Scala var is a reserved keyword, so I'm renaming it to cvar, since I don't like the backticks in my code.

Next, let's create an object that will contain some random value:

object BusinessLogic {
  val x = Math.random() // I wonder what this value is at runtime... 

Also, let's create an App so we can run our program with sbt run:

object Main extends App {

If we would execute sbt run, we would never know the value of x in BusinessLogic. We could add a println, but what if x was a var and it's value would change over time? Clojure lets us inspect this value at any given point in time. We'll start a socket server that is available since Clojure 1.8.0.

object Main extends App {
  val require = cvar("clojure.core","require")
  val startServer = cvar("clojure.core.server","start-server")
  val options ="""{:port 4555 :accept clojure.core.server/repl 
    :name repl :server-daemon false}""")

This may seem a little intimidating, so I'll explain it line by line. On line 2 we get a reference to Clojure's require so we can... yes, require namespaces. On line 3 we read a string so we get the symbol that require needs to load the clojure.core.server namespace. On line 4 we get a reference to the start-server var. On line 5 we define a bunch of settings. Their meaning can be found here.

In Clojure this would read as:

(require 'clojure.core.server)
{:port 4555
:accept clojure.core.server/repl
:name 'repl
:server-daemon false}

but since we're coming from Scala and have to use Clojure's Java API, it looks a bit more involved.

On the last line we invoke start-server with said options. When we execute sbt run again, the process will block, because :server-daemon was set to false:

~/dev/scala/cljrepl $ sbt run
[info] Loading global plugins from /Users/Borkdude/.sbt/0.13/plugins
[info] Set current project to cljrepl (in build file:/Users/Borkdude/Dropbox/dev/scala/cljrepl/)
[info] Compiling 1 Scala source to /Users/Borkdude/Dropbox/dev/scala/cljrepl/target/scala-2.11/classes...
[info] Running Main

This gives us the chance to connect to the socket repl with good ol' telnet:

~ $ rlwrap telnet localhost 4555
Connected to localhost.
Escape character is '^]'.
user=> (import 'BusinessLogic)
user=> (BusinessLogic/x)
0.722431948099764 ;; <-- aah! 
user=> :repl/quit
Connection closed by foreign host.

See what we did there?

Clojure's socket REPL also supports initialization via a JVM property. To try this, add these lines to build.sbt:

fork := true
javaOptions := Seq("-Dclojure.server.repl={:port 4555 :accept clojure.core.server/repl :server-daemon false}")

The App can now be reduced to something like:

object Main extends App {
  val require = cvar("clojure.core","require")

This doesn't do much except taking care that Clojure is initialized (for more info, read the last paragraph in this Stackoverflow answer).

This Scala example translates fairly straightforward to Java. Now don't tell your boss you're using a different programming language. After all, Clojure is just a Java library that gives you superpowers :-).

PS: it may be wise to turn this off in production because of the security risk; on the other hand, a Clojure REPL has saved my day more than once in the past!

Discuss this post here.

Published: 2016-07-26