Question

Alright, so I created a JavaScript file named HelloWorld.js with the following contents:

java.lang.System.out.println("Hello World!");

Now, I compiled it using the Rhino JavaScript Compiler using the following command (the js.jar file is in my classpath):

java org.mozilla.javascript.tools.jsc.Main HelloWorld.js

It compiled the JavaScript file and created the Java class file as expected. Then I tried to execute the Java class file by calling java HelloWorld. However, it generated the following error message:

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld
Caused by: java.lang.ClassNotFoundException: HelloWorld
    at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: HelloWorld. Program will exit.

I tried to understand what caused the java.lang.NoClassDefFoundError to be thrown, and from what I read in this blog post I learned that the java.lang.NoClassDefFoundError is thrown if a class was present during compile time but not available in Java classpath during runtime.

So I ran the javap HelloWorld command to check what the problem is, and this is what I got:

public class HelloWorld extends org.mozilla.javascript.NativeFunction implements org.mozilla.javascript.Script {
    public HelloWorld();
    public static void main(java.lang.String[]);
    public final java.lang.Object exec(org.mozilla.javascript.Context, org.mozilla.javascript.Scriptable);
    public final java.lang.Object call(org.mozilla.javascript.Context, org.mozilla.javascript.Scriptable, org.mozilla.javascript.Scriptable, java.lang.Object[]);
    public int getLanguageVersion();
    public java.lang.String getFunctionName();
    public int getParamCount();
    public int getParamAndVarCount();
    public java.lang.String getParamOrVarName(int);
    public boolean getParamOrVarConst(int);
}

Now, what I understand from this is that the HelloWorld class is present and is declared as public. Hence there shouldn’t be any reason why the Java Virtual Machine can’t find it. This is where I’m confused. I don’t know where to go from here, nor what to do to resolve this problem.

I found out that I could execute the Java class file if I invoked Rhino to call the exec method on an instance of HelloWorld as follows:

java -jar /usr/share/rhino/js.jar HelloWorld.class

However, I would like to execute the Java class file using the java HelloWorld command directly since the js.jar file already in my classpath. I would like to understand what the problem is so that I know what’s really happening behind the scenes.

Answer 1

Are you sure the class file is on the classpath (as well as js.jar)?

Try

java -cp .;js.jar HelloWorld

(assuming HelloWorld.class in the current directory, otherwise something like -cp build;js.jar).