How do I compile and run this Java Native Access (JNA) example?

Question

I can compile this JNA example code (from step 2 of https://jna.dev.java.net/#getting_started):

package com.sun.jna.examples;

import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Platform;

/** Simple example of JNA interface mapping and usage. */
public class HelloWorld {

    // This is the standard, stable way of mapping, which supports extensive
    // customization and mapping of Java to native types.
    public interface CLibrary extends Library {
        CLibrary INSTANCE = (CLibrary)
            Native.loadLibrary((Platform.isWindows() ? "msvcrt" : "c"),
                               CLibrary.class);

        void printf(String format, Object... args);
    }

    public static void main(String[] args) {
        CLibrary.INSTANCE.printf("Hello, World\n");
        for (int i=0;i < args.length;i++) {
            CLibrary.INSTANCE.printf("Argument %d: %s\n", i, args[i]);
        }
    }
}

...using javac -classpath .:jna.jar -g HelloWorld.java without error. (I downloaded jna.jar and put it in the same directory as HelloWorld.java for now.)

But when I run it using java -classpath .:jna.jar HelloWorld, I get:

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld (wrong name: com/sun/jna/examples/HelloWorld)
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
    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$1.run(URLClassLoader.java:195)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
    at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)

I get the exact same exception on Mac OS X and Linux.

How do I get this to run?

Answer 1

This example (as well as vast majority of java classes) uses packages:

package com.sun.jna.examples;

In order to compile / run it properly you need to run javac / java from the "root" folder (e.g. folder where "com" is located):

Let's say you have a folder called examples. You'd put both the jna.jar and the source code in it preserving folder structure:

/examples
 jna.jar
 /com
   /sun
      /jna
         /examples
           HelloWorld.java

You compile and run using:

javac -classpath .:jna.jar -g com/sun/jna/examples/HelloWorld.java

java -classpath .:jna.jar com.sun.jna.examples.HelloWorld

Note the path separators in the former case and dots in the latter.

Answer 2

Either just remove this line and recompile (which is fine in this case as you just try out some sample)

package com.sun.jna.examples;

or read up on what packages in java are and how they have to be handled (ChssPly76s Posts as a starter).

Better choose the second option as sooner or later (probably sooner) you will have to deal with packages anyway. So just take the time now to read up on it.