Using Reflection on classes outside the classpath

Question

Hi all,
Im writing a tool that takes a directory containing class files (from other internal projects) and returns information such as the Class name, package, parent, constructors and public methods. These projects will not be on the classpath and will also use jars that are not on the classpath.
I decided that ClassLoaders were the way to go. So I created a SimpleClassLoader class that takes a file, reads it in as an array of bytes and calls defineClass:
public class SimpleClassLoader extends ClassLoader
     public SimpleClassLoader()
     public synchronized Class loadClass(String name, boolean resolveIt) throws ClassNotFoundException
          System.out.println("1) loading class:   " + name);
          return super.loadClass(name, resolveIt);
     public synchronized Class loadClass(File classFile) throws ClassNotFoundException
          System.out.println("2) Loading class: " + classFile.getName());
          try
               byte[] data = getClassData(classFile);
               return defineClass(name, data, 0, data.length);
          catch (IOException e)
               throw new ClassNotFoundException();
     byte[] getClassData(File classFile) throws IOException
          byte result[];
          try
               FileInputStream in = new FileInputStream(classFile);
               result = new byte[in.available()];
               in.read(result);
               in.close();
               return result;
          catch (IOException e)
               throw new IOException();
public class ClassParser
     public Class parse (File classFile)
          SimpleClassLoader loader = new SimpleClassLoader();
          Class reflectClass = null;
          try {
               return loader.loadClass(classFile);
          } catch (ClassNotFoundException cnfe) {
               System.out.println("Error loading class: " + classFile.getName() + "\n" + cnfe);
               return null;
}This worked for simple classes but when the superclass was in a jar or another file in the directory that hadnt already been loaded a ClassNotFoundException was thrown. So I then decided to try using URLClassLoader and loading in the jars and the entire source directory before calling findClass on it.
class MyURLClassLoader extends URLClassLoader
     public MyURLClassLoader(URL[] urls)
          super(urls);
     public Class findClass(String classname) throws ClassNotFoundException
          return super.findClass(classname);
public class ClassParser {
     private URL[] _classpath;
     private String _root;
     public ClassParser(File[] files, String root) {
          _root = root;
          _classpath = new URL[files.length];
          for (int i = 0; i < files.length; i++) {
               try {
                    URL url = new URL("jar", "", files.toURL() + "!/");
                    this._classpath[i] = url;
               } catch (MalformedURLException e) {
                    e.printStackTrace();
     public Class parse (File classFile)      {
          MyURLClassLoader urlLoader = new MyURLClassLoader(_classpath);
          Class reflectClass = null;
          String classname = "";
          // Try to load a Class instance from the file
          try {
               classname = getClassName(classFile);
               return urlLoader.findClass(classname);
          } catch (ClassNotFoundException cnfe) {
               System.out.println("Error loading class: " + classname + "\n" + cnfe);
               return null;
     String getClassName (File file) throws ClassNotFoundException {
          if (file.getAbsolutePath().startsWith(_root)) {
               String packagePath = file.getAbsolutePath().substring(_root.length(), file.getAbsolutePath().lastIndexOf("."));
               packagePath = packagePath.replace(File.separatorChar, '.');
               return packagePath;
          } else {
               throw new ClassNotFoundException();
     public static void main(String args[]) {
File [] files = new File [] {
new File("C:\\Projects\\app\lib\\jacob-1.6.jar"),
new File("C:\\Projects\\app\\lib\\xalan.jar"),
new File("C:\\Projects\\app\\lib\\xmlunit1.0.jar"),
new File("C:\\Projects\\app\\classes\\com\\foo\\bar\\swing\\")
parser = new ClassParser(files, "C:\\Projects\\app\\classes\\");
This seems to work when I call findClass on a class thats in one of the jars but not on a class in the classes directory.
Does anyone have any ideas what im doing wrong or whether im barking up the wrong tree completely??
Thanks,
Conor

Answer 1

The URLs that you supply to an URLClassLoader each define the location of the default package for that portion of the classpath for the ClassLoader.
This is the problem line
new File("C:\\Projects\\app\\classes\\com\\foo\\bar\\swing\\")I suspect that it should be..
new File( "C:\\Projects\\app\classes" )as this is the location of that default package.