NoClassDefFoundError using Scala class from Java

Question

I have no experience with Scala, so this question may be elementary. I am trying to use Scala class from within Java, based on the "Person" example in this tutorial: http://www.codecommit.com/blog/java/interop-between-java-and-scala

I create two source files, one Scala and one Java, as follows.

Person.scala:

class Person {
  def getName() = "Daniel Spiewak"
}

Test.java:

public class Test {
  public static void main(String[] args) {
    Person p = new Person();
    p.getName();
  }
}

I can compile (with a warning I don't understand), but I get a ClassNotFoundException when I try to run the program.

$ scalac Person.scala
$ javac Test.java
./Person.class: warning: Cannot find annotation method 'bytes()' in type 'ScalaSignature': class file for scala.reflect.ScalaSignature not found
1 warning
$ java Test
Exception in thread "main" java.lang.NoClassDefFoundError: scala/ScalaObject
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:787)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:447)
        at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
        at Test.main(Test.java:3)
Caused by: java.lang.ClassNotFoundException: scala.ScalaObject
        at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
        ... 13 more

Any idea what I'm doing wrong?

Answer 1

You need to include the Scala runtime library to be able to use Scala classes. This is the cause of both the compiler warning and the error at runtime.

Try this

 java -cp .:scala-library.jar Test

(the name of the jar file might be different, but the point is to add it to your classpath).

Answer 2

Did you put scala-library.jar in your classpath? Seems you forgot this, so JVM cannot find some internal class for Scala runtime.

In your link http://www.codecommit.com/blog/java/interop-between-java-and-scala:

Just stick scala-library.jar on your classpath and all of your Scala classes should be readily available within your Java application.