URLClassLoader ClassNotFoundException

Question

I'm trying to write a code that loads classes dynamically at run time

public class URLDynClassLoader {

    public URLDynClassLoader(){
        try{
            loadclasses( new File("C:/Users/Miller/Desktop/test/") , "Shapes." );
        }
        catch(ClassNotFoundException e){
            e.printStackTrace();
            System.out.println(e.getMessage());
        }
        catch( Exception f ){
            System.out.println(f.getMessage());
            System.out.println("error");
        }
    }

    private ArrayList<String> getClassNames( File folder ){
        File[] listOfFiles = folder.listFiles();
        ArrayList<String> fileNames = new ArrayList<String>() ;
        for (File file : listOfFiles) {
            if (file.isFile()) {
                if( accept(file.getName()) ){
                    fileNames.add(file.getName());
                }
            }
        }
        return fileNames ;
    }

    ArrayList<Class> loadclasses( File folder , String packageName ) throws MalformedURLException, ClassNotFoundException{

        URLClassLoader load = URLClassLoader.newInstance( new URL[] { folder.toURL() })  ;
        ArrayList<Class> data = new ArrayList<Class>();
        ArrayList<String> names = getClassNames(folder);

        for(int i=0 ; i<names.size() ; ++i){;
            data.add(load.loadClass( fixName( packageName , names.get(i) ) ));
            System.out.println("i"+i);
        }

        return data ;
    }

    private String fixName(String packageName, String className ) {
        className = className.replaceAll(".class", "");
        return packageName+className;
    }

    public boolean accept(String arg) { 
        return arg.endsWith(".class"); 
    } 

    public static void main(String[] args) {
        new URLDynClassLoader();
    }

}

the problem is that this keeps giving me classNotFound exception , tho i'm pretty sure files .class exist in that directory , another thing I've tried loading already loaded classes and it worked , so it doesn't work only if my code doesn't have the class file !!

Answer 1

Problem seems to be with method getClassNames(). Can you please check fully qualified class name returned from fixName() before loading classes? It will give you better idea what's wrong with the code.

I ran your sample code with hard coded values passed to fixName() (without using getClassNames()) and it was able to load the class file.

Answer 2

Stupid answer perhaps, but for me the issue was that my project was in a folder which was somehow admin protected.

I tried so many "solutions", but I remembered when I tried to unzip a file to my project folder (some .jar lib I thought could fix it), it kept saying it couldn't extract because of admin rights or something.

Whenever I extracted it somewhere else and manually copy-pasted it, it asked for "admin consent" something, I clicked yes and I could copy the files.

My guess is, it's trying to find classes in a folder to which is does not have access to or something, and returns null. Which down the line, returns this error!

So if anyone ever stumbles upon issues like this with "URLClassLoader" and "PolicyException"(s), please also make sure it's in a correct folder!!!