How to reload a class or package in Scala REPL?
read-eval-print-loop, scala
Solution
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.
Problem
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; ```