Having trouble with creating objects from instances created with ClassLoade

Question

Hi.
I'm having a bit of trouble with casting an instance of an object from a custom ClassLoader. Don't worry - the code isn't for anything sinister - it's for one of those life simulation thingies, but I want to make it modular so people can write their own 'viruses' which compete for survival. You know the score.
Anyway. I've got the beginnings of my main class, which seems to load the class data for all 'virus' classes in a folder called 'strains'. There is a abstract class called AbstractVirus which declares the method calls for how the viruses should behave and to get textual descriptions, etc. AbstractVirus is to be subclassed to create working virus classes and these are what my main class is trying to load instances of.
Unfortuantely, I can't cast the instances into AbstractVirus objects. The error I've been getting is 'ClassCastException' which I presume is something to do with the fact that my ClassLoader and the Bootstrap ClassLoader aren't seeing eye-to-eye with the class types. Can anyone help? This line of programming is really new to me.
My code for the main class is below:
/* LifeSim.java */
public class LifeSim {
  public LifeSim() {
    /* Get a list of all classes in the 'strains' directory and store non-
     * abstract classes in an array. */
    Class virusClasses[] = null;
    try {
      /* Get a reference to the file folder 'strains' and make sure I can read
       * from it. */
      java.io.File modulesFolder = new java.io.File("strains");
      if (!modulesFolder.isDirectory() || !modulesFolder.canRead()) {
     System.out.println("Failed to find accessible 'strains' folder");
     System.exit(-1);
      /* Get a list of all the class files in the folder. */
      String virusFiles[] = modulesFolder.list(new ClassFileFilter());
      if (virusFiles.length == 0) {
     System.out.println("No virus strains in 'strains' folder");
     System.exit(-1);
      /* Create an array of class objects to store my Virus classes. Ignore the
       * abstract class as I cannot instantiate objects directly from it.*/
      virusClasses = new Class[virusFiles.length];
      VirusClassLoader classLoader = new VirusClassLoader();
      int j = 0;
      for (int i = 0; i < virusFiles.length; i++) {
     String virusName = "strains/" + virusFiles;
     Class tempClass = classLoader.loadClass(virusName);
     if (tempClass.getName().compareToIgnoreCase("strains.AbstractVirus") != 0) {
     virusClasses[j++] = tempClass;
} catch (ClassNotFoundException ncfe) {
System.out.println("Failed to access virus class files.");
ncfe.printStackTrace();
System.exit(-1);
/* TEST CODE: Create an instance of the first virus and print its class
* name and print details taken from methods defined in the AbstractVirus
* class. */
if (virusClasses.length > 0) {
try {
     // Print the class name
     System.out.println(virusClasses[0].getName());
     Object o = virusClasses[0].newInstance();
     strains.AbstractVirus av = (strains.AbstractVirus) o;
     // Print the virus name and it's description
     System.out.println(av.getQualifiedName());
     System.out.println(av.getDescription());
} catch (InstantiationException ie) { ie.printStackTrace(); }
     catch (IllegalAccessException iae) { iae.printStackTrace(); }
public static void main(String args[]) {
new LifeSim();
class ClassFileFilter implements java.io.FilenameFilter {
public boolean accept(java.io.File fileFolder, String fileName) {
if (_fileName.indexOf(".class") > 0) return true;
return false;
class VirusClassLoader extends ClassLoader {
private String legalClassName = null;
public VirusClassLoader() {
super(VirusClassLoader.class.getClassLoader());
public byte[] findClassData(String filename) {
try {
java.io.File sourcefile = new java.io.File(filename);
legalClassName = "strains." + sourcefile.getName().substring(0,sourcefile.getName().indexOf("."));
java.io.FileInputStream fis = new java.io.FileInputStream(sourcefile);
byte classbytes[] = new byte[fis.available()];
fis.read(classbytes);
fis.close();
return classbytes;
} catch (java.io.IOException ioex) {
return null;
public Class findClass(String classname) throws ClassNotFoundException {
byte classbytes[] = findClassData(classname);
if (classbytes == null) throw new ClassNotFoundException();
else {
return defineClass(legalClassName, classbytes, 0, classbytes.length);
Thank you in advance
Morgan

Answer 1

Two things:
I think your custom ClassLoader isn't delegating. In general a ClassLoader should begin by asking it's parent ClassLoader to get a class, and only if the parent loader fails get it itself. AFAIKS you could do what you're trying to do more easilly with URLClassLoader.
Second, beware that a java source file can, and often does, generate more than one class file. Ignore any class files whose names contain a $ character. It's possible you are loading an internal class which doesn't extend your abstract class.