Creating a jar file from a Scala file

Question

I'm new to Scala and don't know Java. I want to create a jar file out of a simple Scala file. So I have my HelloWorld.scala, generate a HelloWorld.jar.

Manifest.mf:

Main-Class: HelloWorld

In the console I run:

fsc HelloWorld.scala
jar -cvfm HelloWorld.jar Manifest.mf HelloWorld\$.class HelloWorld.class
java -jar HelloWorld.jar 
  => "Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld/jar"

java -cp HelloWorld.jar HelloWorld 
  => Exception in thread "main" java.lang.NoClassDefFoundError: scala/ScalaObject
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:675)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
    at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:316)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:280)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
    at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:374)
    at hoppity.main(HelloWorld.scala)

Answer 1

Because Scala scripts require the Scala libraries to be installed, you will have to include the Scala runtime along with your JAR.

There are many strategies for doing this, such as jar jar, but ultimately the issue you're seeing is that the Java process you've started can't find the Scala JARs.

For a simple stand-alone script, I'd recommend using jar jar, otherwise you should start looking at a dependency management tool, or require users to install Scala in the JDK.

Answer 2

Sample directory structure:

X:\scala\bin
X:\scala\build.bat
X:\scala\MANIFEST.MF
X:\scala\src
X:\scala\src\foo
X:\scala\src\foo\HelloWorld.scala

HelloWorld.scala:

//file: foo/HelloWorld.scala
package foo {
  object HelloWorld {
    def main(args: Array[String]) {
      println("Hello, world!")
    }
  }
}

MANIFEST.MF:

Main-Class: foo.HelloWorld
Class-Path: scala-library.jar

build.bat:

@ECHO OFF

IF EXIST hellow.jar DEL hellow.jar
IF NOT EXIST scala-library.jar COPY %SCALA_HOME%\lib\scala-library.jar .

CALL scalac -sourcepath src -d bin src\foo\HelloWorld.scala

CD bin
jar -cfm ..\hellow.jar ..\MANIFEST.MF *.*
CD ..

java -jar hellow.jar

---

In order to successfully use the -jar switch, you need two entries in the META-INF/MANIFEST.MF file: the main class; relative URLs to any dependencies. The documentation notes:

-jar

Execute a program encapsulated in a JAR file. The first argument is the name of a JAR file instead of a startup class name. In order for this option to work, the manifest of the JAR file must contain a line of the form Main-Class: classname. Here, classname identifies the class having the public static void main(String[] args) method that serves as your application's starting point. See the Jar tool reference page and the Jar trail of the Java Tutorial for information about working with Jar files and Jar-file manifests.

When you use this option, the JAR file is the source of all user classes, and other user class path settings are ignored.

• java command line usage
• manifest spec

(Notes: JAR files can be inspected with most ZIP applications; I probably neglect handling spaces in directory names in the batch script; Scala code runner version 2.7.4.final .)

---

For completeness, an equivalent bash script:

#!/bin/bash

if [ ! $SCALA_HOME ]
then
    echo ERROR: set a SCALA_HOME environment variable
    exit
fi

if [ ! -f scala-library.jar ]
then
    cp $SCALA_HOME/lib/scala-library.jar .
fi

scalac -sourcepath src -d bin src/foo/HelloWorld.scala

cd bin
jar -cfm ../hellow.jar ../MANIFEST.MF *
cd ..

java -jar hellow.jar

Answer 3

You can also use maven and the maven-scala-plugin. Once you set up maven, you can just do mvn package and it will create your jar for you.

Answer 4

One thing which may cause a similar problem (although it's not the problem in the initial question above) is that the Java vm seems to demand that the main method returns void. In Scala we can write something like (observe the =-sign in the definition of main):

object MainProgram {

  def main(args: Array[String]) = {
    new GUI(args)
  }
}

where main actually returns a GUI-object (i.e. it's not void), but the program will run nicely when we start it using the scala command.

If we package this code into a jar-file, with MainProgram as the Main-Class, the Java vm will complain that there's no main function, since the return type of our main is not void (I find this complaint somewhat strange, since the return type is not part of the signature).

We would have no problems if we left out the =-sign in the header of main, or if we explicitly declared it as Unit.