Re: How to set classpath in a program?

Dale King <"DaleWKing [at]gmail [dot] com">
Tue, 30 May 2006 19:11:22 -0400
Lion-O wrote:

I'm trying to grok reflection (java.lang.reflection) and so far I've come a
long way. After reading up on Class.forName() I wrote a test program which went
over the class files in the current directory to check and print their
properties. My first version works like a charm, obviously the next step is to
allow specifying the directory to look in.

And this is where I can't get my fingers behind whats happening... Here's the
snipplet of my code:

---[ snipplet ]---

public class GetClass {

  private static String nametoClass(String file) {
    return new File(file).getName().replace(".class", "");

  public static void main(String[] args) {

  Class c;
  File directory;
  String[] classes; // Array filled with names of classfiles + path
  Constructor[] cnsts;
  if (args.length > 0) {
    directory = new File(args[0]);
    String cp = System.getProperty("java.class.path");
    if (cp.equals(null)) {
      System.setProperty("java.class.path", ".:" + directory);
    } else System.setProperty("java.class.path", cp + ":" + directory);
  try {
    for (int i = 0; i < classes.length; i++) {
      c = Class.forName(nametoClass(classes[i]));
      System.out.print("Class " + c.getName() + " is part of package ");
      if (c.getPackage() == null) System.out.print("default");
      else System.out.print(c.getPackage().toString());
      System.out.println(" and contains:");
      cnsts = c.getConstructors();
      System.out.print("Constructor(s): ");
 } catch catch(ClassNotFoundException cnfe) {
   System.err.println("Error: Expected class was not found.\n");

---[ EOT ]---

Now... When I run this program to look into a javabot subdirectory (containing
a file 'javabot.class' which is part of a bot written using the PircBot API) I
get the following error:

Exception in thread "main" java.lang.NoClassDefFoundError: \

    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(

This is quite logical since 'Class.forName()' is basicly equal to using
"Class.forName(className, true, currentLoader)" where 'true' means that the
class should be initialized, which basicly means that it will also try to
determine the classes used (imported / extended) in the classes which I'm

So I tried turning this behaviour off by changing the above forName() line
into "c = Class.forName(nametoClass(classes[i]),false,null);".

This results in a ClassNotFoundException as can be expected from the method
(thats why it should be caught ;-)). But why can't it find this class even
though I have changed the classpath? The reason this confuses me is because it
can obviously find the class in my first version.

Any hints would be appreciated.

It isn't the classpath you would be dealing with after the program runs.
You need to create a classloader instance to read from where you want to
look for classes. The classpath is just used to set up the initial class
loaders and after that it isn't used anymore.

See URLClassLoader and give it some URL's pointing to the directories
you want to look at (note from the documentation you need to include a
trailing slash).

You can also use addURL to add URLs, but note there is no removeURL.

  Dale King

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