Test.java

import com.hs.acm.client.message.in.AcmClientReqMsg;
import com.hs.acm.client.message.in.JoinCallReqMsg;
import java.net.URL;
import java.net.URLClassLoader;

public class Test {

    public static void main(String[] args) throws Exception {
        URL[] urls = new URL[]{new URL("file:./lib/ACM_0.1.0.jar"),
                                       new URL("file:./lib/ActiveStudy.Utility.2.0.1.jar")};
        /*
        create an URLClassLoader, which only load class from /lib/ACM_0.1.0.jar and
        lib/ActiveStudy.Utility.2.0.1.jar
        I set its parent null because I just only want to load class in two jars provided
        */
        URLClassLoader clsLoader = URLClassLoader.newInstance(urls, null);
      
        /*set it as context class loader for current thread */
        Thread.currentThread().setContextClassLoader(clsLoader);
      
        /*load an test class */
        Class<?> clazz = clsLoader.loadClass("com.hs.acm.client.message.in.JoinCallReqMsg");
      
        /*create new instance*/
        Object msgObj = clazz.newInstance();
      
        /*
        Problem arise here. If I create an URLClassLoader with null parent as above,
        this line of code fire an exception (below). If I don't explicit set it, no problem at all
        */
        JoinCallReqMsg obj = (JoinCallReqMsg) msgObj;
        System.out.println(obj instanceof JoinCallReqMsg);
        System.out.println(obj instanceof AcmClientReqMsg);
        System.out.println(obj.getClass().getSimpleName());
    } 
}