Question

I almost always have a Scala REPL session or two open, which makes it very easy to give Java or Scala classes a quick test. But if I change a class and recompile it, the REPL continues with the old one loaded. Is there a way to get it to reload the class, rather than having to restart the REPL?

Just to give a concrete example, suppose we have the file Test.scala:

object Test { def hello = "Hello World" }

We compile it and start the REPL:

~/pkg/scala-2.8.0.Beta1-prerelease$ bin/scala
Welcome to Scala version 2.8.0.Beta1-prerelease
(Java HotSpot(TM) Server VM, Java 1.6.0_16).
Type in expressions to have them evaluated.
Type :help for more information.

scala> Test.hello
res0: java.lang.String = Hello World

Then we change the source file to

object Test {
  def hello = "Hello World"
  def goodbye = "Goodbye, Cruel World"
}

but we can’t use it:

scala> Test.goodbye
<console>:5: error: value goodbye is not a member of object Test
       Test.goodbye
            ^

scala> import Test;
<console>:1: error: '.' expected but ';' found.
       import Test;

Answer 1

Class reloading is not an easy problem. In fact, it’s something that the JVM makes very difficult. You do have a couple options though:

• Start the Scala REPL in debug mode. The JVM debugger has some built-in reloading which works on the method level. It won’t help you with the case you gave, but it would handle something simple like changing a method implementation.
• Use JRebel (http://www.zeroturnaround.com/jrebel). JRebel is basically a super-charged class reloading solution for the
  JVM. It can handle
  member addition/removal, new/removed classes, definition changes, etc. Just about the only thing it can’t handle is changes in class hierarchy (adding a super-interface, for
  example). It’s not a free tool, but they do offer a complementary license which is limited to Scala compilation units.

Unfortunately, both of these are limited by the Scala REPL’s implementation details. I use JRebel, and it usually does the trick, but there are still cases where the REPL will not reflect the reloaded class(es). Still, it’s better than nothing.