My own ClassLoader didn't work.

[snooze-g]

Hi all.
I have a problem when I trying to get a instance of my TestClass object.
System get me a "cannot find symbol...".
I think that i have a problem with resolveClass method, as i know that method going to set Link to my class.
Please help.

Thanks Snooze-g.

Code:

import java.io.FileInputStream;

import java.util.Hashtable;


public class SimpleClassLoader extends ClassLoader {
    private Hashtable classes = new Hashtable();

    public SimpleClassLoader() {

    }

    private byte getClassImplFromDataBase(String className)[] {
        System.out.println("        >>>>>> Fetching the implementation of " +
                           className);
        byte result[];
        try {
            FileInputStream fi = new FileInputStream("c:\\bindata\\" +
                    className + ".class");
            result = new byte[fi.available()];
            fi.read(result);
            return result;
        } catch (Exception e) {

            return null;
        }
    }


    public Class loadClass(String className) throws ClassNotFoundException {
        return (loadClass(className, true));
    }


    public synchronized Class loadClass(String className, boolean resolveIt) throws
            ClassNotFoundException {
        Class result;
        byte classData[];

        System.out.println("        >>>>>> Load class : " + className);

        result = (Class) classes.get(className);
        if (result != null) {
            System.out.println("        >>>>>> returning cached result.");
            return result;
        }

        try {
            result = super.findSystemClass(className);
            System.out.println(
                    "        >>>>>> returning system class (in CLASSPATH).");
            return result;
        } catch (ClassNotFoundException e) {
            System.out.println("        >>>>>> Not a system class.");
        }

        classData = getClassImplFromDataBase(className);
        if (classData == null) {
            throw new ClassNotFoundException();
        }

        result = defineClass(className, classData, 0, classData.length);
        if (result == null) {
            throw new ClassFormatError();
        }

        if (resolveIt) {
            resolveClass(result);
        }

        classes.put(className, result);

        System.out.println("        >>>>>> Returning newly loaded class.");
        return result;

    }

    public static void main(String[] args) {

        SimpleClassLoader simp = new SimpleClassLoader();
        try {
            Class c = simp.loadClass("TestClass");

            TestClass t = null;

            try {
                t = (TestClass) c.newInstance();           
 } catch (IllegalAccessException ex) {
            } catch (InstantiationException ex) {
            }

            System.out.println(t.toString());
            System.out.println(c.getName());

        } catch (ClassNotFoundException e) {

            e.printStackTrace();
        }

    }
}

[JavaBean]

This is a compiler array. The compiler complains because it can not find the class you want to use but i understand that you want to load that class dynamically without knowing its implementation details!

As far as i see, there is only one solution:

1. You will create an interface and include that interface to this project. Lets call it TestInterface.

2. The interface will have all the methods you want to call from your TestClass.

3. Your TestClass will implement this interface. (e.g. TestClass implements TestInterface)

4. Modify your code and cast your dynamically created class into TestInterface object (not TestClass object). Like this:

Code:

SimpleClassLoader simp = new SimpleClassLoader();
        try {
            Class c = simp.loadClass("TestClass");

            TestInterface t = null;

            try {
                t = (TestInterface) c.newInstance();            } catch (IllegalAccessException ex) {
            } catch (InstantiationException ex) {
            }

            System.out.println(t.toString());
            System.out.println(c.getName());

        } catch (ClassNotFoundException e) {

            e.printStackTrace();
        }

5. You will successfully compile your code in this case and use the methods you want to call from your project without knowing implementation details of TestClass.