public class CustomClassLoader extends ClassLoader {

    @Override
    public Class findClass(String name) throws ClassNotFoundException {
        byte[] b = loadClassFromFile(name);
        return defineClass(name, b, 0, b.length);
    }

    private byte[] loadClassFromFile(String fileName)  {
        InputStream inputStream = getClass().getClassLoader().getResourceAsStream(
                fileName.replace(‘.‘, File.separatorChar) + ".class");
        byte[] buffer;
        ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
        int nextValue = 0;
        try {
            while ( (nextValue = inputStream.read()) != -1 ) {
                byteStream.write(nextValue);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        buffer = byteStream.toByteArray();
        return buffer;
    }

    public static void main(String[] args) {

        CustomClassLoader customClassLoader = new CustomClassLoader();
        try {
            Class clazz = Class.forName("com.xxx.Test1", true, customClassLoader);
            Object object = clazz.newInstance();
            Test1 test1 = (Test1) clazz.newInstance();
            test1.testMethod();

            Method helloMethod = clazz.getDeclaredMethod("testMethod", null);
            helloMethod.invoke(object, null);

        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }
}

public class Test1 {


    public void testMethod() {
        System.out.println("hello world");
    }

    public static void main(String[] args) {

        System.out.println("hello world");
    }
}

hello world
hello world