ClassCastException Error while dynamically loading classes from a JAR file outside WAR at run time in JAVA?

I wanted to dynamically load jar files (and its classes) in my war code.

To do this I have written my object factory class as such:

import java.io.File;
import java.net.*;

public class ObjectFactory {

    private ClassLoader cl;

    public ObjectFactory(String jarFilePath) {

        try {
            File file= new File(jarFilePath);
            URL url = file.toURL();
            URL[] urls = new URL[]{url};
            cl = new URLClassLoader(urls);
        } catch (Exception e) {
            e.printStackTrace();
        }  


    }


    public <T> T getObject(String id){
        try {
            Class cls = cl.loadClass(id);
            T object =(T)cls.newInstance();

            return object;
        } catch (Exception e) {
            e.printStackTrace();
        } 
        return null;
    }

}

I have a Duck interface:

public interface Duck {
    public String quack(String arg);
}

And I have created service class to get Ducks I want at runtime:

public class DuckServiceClass {

    public static Duck getDuck(){
        try {
        String jarFilePath="\path\to\my\external_jar.jar"
        ObjectFactory of = new ObjectFactory(jarFilePath);
        Duck d1=of.getObject("implementationOfDuck.RobotDuck");     
        return d1;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

}

My jar has a copy of Duck interface and has various implementation of Duck : for example RobotDuck class:

package implementationOfDuck;
import Duck;

public class RobotDuck implements Duck {

    @Override
    public String quack(String arg) {
        return "Q U A N C K!!!" +arg;
    }

}

This service class works all fine in a main method:

public class WebAppTest {

    public static void main(String[] args) {
        Duck d1 =DuckServiceClass.getDuck();
        System.out.println(d1.getName()+">"+d1.quack("Hello"));
    }

}

But If I refer this ServiceClass method in my jsp page it gives me :

java.lang.ClassCastException: implementationOfDuck.RobotDuck cannot be cast to Duck

My JSP page is :

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<%@ page import="Duck" %>
<%@ page import="DuckServiceClass" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Duck-O-Gram</title>
</head>
<body>
<%
Duck d1=DuckServiceClass.getDuck();
%>
<h1><%=d1.quack("Hello")%></h1>
</body>
</html>

can any one guide me how to resolve this issue in a webapp ?

Solution

As @JimGarrison said, Duck is loaded by webapp class loader and URLClassLoader twice.

Specified the parent class loader in constructor of URLClassLoader could fix the problem. cl will search class/resource using parent class loader first, then load class/resource from given URLs. Since Duck is already loaded by webapp class loader, Duck.class inside external_jar.jar will not be loaded again.

public ObjectFactory(String jarFilePath) {
    try {
        File file= new File(jarFilePath);
        URL url = file.toURL();
        URL[] urls = new URL[]{url};
        cl = new URLClassLoader(urls, getClass().getClassLoader());
    } catch (Exception e) {
        e.printStackTrace();
    }  
}