Editing programs “while they are running”? How?

Question

This question is a corollary to: Editing programs “while they are running”? Why?

I\'m only recently being exposed to the world of Clojure and am fascinated by a few exam

Answer 1

There are a lot of good answers here, and I'm not sure I can improve on any of them, but I wanted to add some comments around Clojure and Java.

First off, Clojure is written in Java, so you can definitely build a live-coding environment in Java. Just think of Clojure as a specific flavor of live-coding environment.

Basically, live coding in Clojure works via the read function in main.clj and the eval function in core.clj (src/clj/clojure/main.clj and src/clj/clojure/core.clj in the github repository). You read in the forms and pass them to eval, which calls the clojure.lang.Compiler (src/jvm/clojure/lang/Compiler.java in the repo).

Compiler.java converts Clojure forms into JVM bytecode using the ASM library (ASM website here, documentation here). I'm not sure what version of the ASM library is used by Clojure. This bytecode (an array of bytes => byte[] bytecode is the member of the Compiler class that will ultimately hold the bytes generated by the clojure.asm.ClassWriter class via ClassWriter#toByteArray) must then be converted to a class and linked into the running process.

Once you have a representation of a class as a byte array, it's a matter of getting ahold of a java.lang.ClassLoader, calling defineClass to turn those bytes into a Class, and then passing the resulting Class to the resolve method of the ClassLoader to link it to the Java runtime. This is basically what happens when you define a new function, and you can see the internals of the compiler in Compiler$FnExpr which is the inner class that generates the bytecode for function expressions.

There's more going on than that with respect to Clojure, such as the way in which it handles namespace and symbol interning. I'm not completely sure how it gets around the fact that the standard ClassLoader will not replace a linked Class with a new version of that Class, but I suspect it has to do with how classes are named and how symbols are interned. Clojure also defines its own ClassLoader, a certain clojure.lang.DynamicClassLoader, which inherits from java.net.URLClassLoader, so that might have something to do with it; I'm not sure.

In the end, all the pieces are there to do live-coding in Java between ClassLoaders and bytecode generators. You just have to provide a way to input forms into a running instance, eval the forms, and link them up.

Hope this sheds a little more light on the subject.