Dynamic Code Reloading in the Scala Interpreter

A week ago I went to the Scala Liftoff Unconference, which was a pretty fun time. At some point I'll write about a conversation I had with Greg Meredith about Bayes and category theory, but only when I understand it all better myself.

The Platinum Sponsor, ZeroTurnaround, gave everyone there a free copy of JavaRebel, which automatically reloads classes on the JVM whenever you change them. No need to restart your java instance. Pretty sweet.

This got me to thinking: one of the things I love about Scala is the interpreter. In particular, one of the greatest advantages of the interpreter is ":load", which just compiles and executes your scala file and let's you play with the results. Unfortunately, the interpreter doesn't work so well when your code base gets over a certain size. But, JavaRebel fixes all that: just recompile your class files in your editor, and JavaRebel will autoreload them! Magic!

Example:

foo.scala:

class Foo {
def bar() = 3;
}

Then, compile it, and open up a modified jrscala interpreter (more on this below):

##########################################################

ZeroTurnaround JavaRebel 1.1
(c) Copyright Webmedia, Ltd, 2007. All rights reserved.

This product is licensed to David Hall

##########################################################

Welcome to Scala version 2.7.1.final (Java HotSpot(TM) Client VM, Java 1.5.0_13).
Type in expressions to have them evaluated.
Type :help for more information.

scala> val f = new Foo
f: Foo = Foo@950feb

scala> f.bar
res0: Int = 3

Now, open back up foo.scala, and change it:

class Foo {
def bar() = 10;
}

Recompile, and go back to your interpreter:

scala> f.bar
f.bar
JavaRebel: Reloading class 'Foo'.
res1: Int = 10

Magic! Ok, so how do I do this? Well, I just made a copy of the scala script (called jrscala) and added two options to the very last line: namely

"-noverify -javaagent:/path/to/your/javarebel.jar":

So it looks like:

${JAVACMD:=java} $JAVA_OPTS -cp "$TOOL_CLASSPATH" -noverify \
-javaagent:/path/to/your/javarebel.jar -Dscala.home="$SCALA_HOME" \
-Denv.classpath="$CLASSPATH" -Denv.emacs="$EMACS"  scala.tools.nsc.MainGenericRunner  "$@"

And you're done. Obviously I should be more principled/less hacky about it, but it's pretty slick.