Wednesday, November 4, 2009

Embedded Scala interpreter

The Scala interpreter is proof that a statically typed language can have features most people only expect from a dynamically-typed language. One of the cool uses of an interpreter is embedding it within your application. This allows you to conveniently experiment with Scala and probably even interact with object instances in your running system. I won't explain how the interpreter works here, but I will try to show you a simple way of embedding the interpreter.

As it usually happens, someone beat me to it. Josh Suereth explains in great detail how to embed an interpreter, but he has done so many customizations that his solution would probably fit on several pages.

I wanted a simpler solution which one could understand at a glance. The code for Lord of the REPLs is much shorter although it doesn't customize much of what the standard interpreter offers.

I tried to come up with the shortest working version you could possibly get away with. Provided I create the settings properly, this is what I could muster:


val out = new java.io.StringWriter()
val interpreter = new scala.tools.nsc.Interpreter(settings, new PrintWriter(out))
interpreter.interpret(code)


Not too much code, is it? (Half of it is probably due to the full package names). Now you could collect your output from the "out" stream and probably convert to String if you need using "out.toString".

Not so fast, though. I said this works if I have the appropriate settings:


val settings = new scala.tools.nsc.Settings()


The problem here is that the interpreter doesn't find two of the crucial jars needed for its proper functioning: scala-compiler.jar and scala-library.jar. When it doesn't it spits out the following error:


scala.tools.nsc.FatalError: object scala not found.


Thanks to the following discussion by Eric Torreborre (author of Specs) I managed to find out that one needs to add to the bootclasspath of the settings object:


val origBootclasspath = settings.bootclasspath.value
val pathList = List(jarPathOfClass(compilerPath),
jarPathOfClass(libPath))
settings.bootclasspath.value = (origBootclasspath :: pathList).mkString(java.io.File.separator)


One could hardcode the path to these two jars, but that's not too flexible. If we want to do it right, we might create a function which discovers the path to the jar from the name of a class that's in it:


def jarPathOfClass(className: String) = {
Class.forName(className).getProtectionDomain.getCodeSource.getLocation
}


Now you could find the paths to these jars like this:


val compilerPath = jarPathOfClass("scala.tools.nsc.Interpreter")
val libPath = jarPathOfClass("scala.ScalaObject")


I've read that getProtectionDomain.getCodeSource returns null in some classloaders and might have problems specifically with OSGi. In that case, one might need to resort to the following hack:


def jarPathOfClass(className: String) = {
val resource = className.split('.').mkString("/", "/", ".class")
val path = getClass.getResource(resource).getPath
val indexOfFile = path.indexOf("file:")
val indexOfSeparator = path.lastIndexOf('!')
path.substring(indexOfFile, indexOfSeparator)
}


With the last ugly piece of code creating an interpreter is not so concise anymore, but sometimes you can't be both robust and concise.

If you want to see the above snippets assembled in one piece you can check out Apache ESME's source code for the ScalaInterpreter action.

Warning: interpreting code directly in your application is a huge security risk and might not always be a good idea.

4 comments:

phil said...

Hi, if I interpret some code in here, does my main program have access to the class interpreted here?
Also does the opposite hold true, if I load a class, does the interpreted class have access to my project classes?Thanks, Phil

Kells said...

Did you know that a Scala interpreter is an object-oriented programming language that blends with a imperative and functional programming styles.

Fredjack said...

Bravo! This is exactly the information I needed!

Unknown said...

After a lot of searching, I finally came across your post and was able to implement it. Thanks so much