The Scala supported IDE is one of the few pain points of developers who want to start using Scala in their Java project. On existing long term project developed by a team its hard to step in and introduce a new language that is not supported by the existing IDE. On way to go about it is to hid the fact that you use Scala from the Java world by using one way dependency injection. Still, if you wish to truly absorb Scala into your existing java environment then you’ll soon introduced cross language dependencies.

Most of our team is using Eclipse as the main IDE, its incrimental compilation in Java with its tight JUnit integration are great for fast TDD programming. Unfortunately the Eclipse Scala plugin is not there yet, it may hangs the IDE and messes up Java compilation – especially in large (more then 1000 source files) Java/Scala projects. Though the plugin is getting better over time some developers would find the plugin as a majore drag on their productivity.
For developers who do not write Scala at all or rather edit Scala with other editors, you can use this alternate path which lets them work on their Java or Scala code without messing with the plugin.

1. Add a compilation script to your project.
The Following script is using the Fast Scala Compiler (fsc). The fsc is a compilation server which always run in the background, as in a warm scalac always ready to receive new work. Is will reduce compilation time dramatically.
The classpath for compilation is taken from the Eclipse project .classpath file. You may take the source directory from there as well if you wish (exercise to the reader).
The params are not passed to the fsc in the command line since in my project’s case the line is too long for the OS to handle. The alternative is to put it into a file and let fsc handle it for you.

#!/bin/sh
# Create a classpath from the eclipse .classpath file
lib=`grep classpathentry .classpath | grep "kind="lib"" | tr -d 't' | sed 's/.*path="(.*[^src].jar)".*/1/' | grep -v classpathentry`
CLASSPATH=`echo ${lib} | sed 's/ /:/g' `  

# point SCALA_HOME to Scala home (might want to add it to your project as well)  
export SCALA_HOME=lib-tools/scala-2.8.0

# java opts for your compilation server
export JAVA_OPTS="-client -Xmx1024M -Xms256M -XX:PermSize=128m -Xss2M -XX:MaxPermSize=256m -Xverify:none -XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled"

DEST=target/bin-scala
mkdir -p $DEST

SCALAC_PROPERTIES=scalac.properties
echo "-classpath $CLASSPATH:target/bin" > $SCALAC_PROPERTIES
echo "-d $DEST -deprecation" >> $SCALAC_PROPERTIES
find src srctest -name *.java -o -name *.scala >> scalac.properties

time $SCALA_HOME/bin/fsc @$SCALAC_PROPERTIES &

2. Eclipse classpath
Add the scalac destination directory ($DEST) to the .eclipse classpath

3. Add the fsc builder to your project
Open the project properties and add the script above to the list of builders.
Set the working directory to be your project root directory.

5. Set the builder properties
Run it in the background when needed: