Load Jar and access a class in jar at run time

Question

I need help from you.
How to load a Jar and access a class in the jar at run time?
When i try the following code it works fine while running in Java (Jdk1.5).If iam running the same code in servlet,ClassCastException occurs.
Error Message : ClassCastExcption : jartest1 cannot be cast to Thing
test.jar contains jartest.class and Thing.class
jartest1.java
try{
File file =new File("test.jar");
String lcStr ="jartest";
URL jfile = new URL("jar", "", "file:" + file.getAbsolutePath() +"!/");
URLClassLoader cl = URLClassLoader.newInstance(new URL[] { jfile });
Class loadedClass = cl.loadClass(lcStr);
Thing t=(Thing)loadedClass.newInstance();
t.execute();
catch(Exception e)
System.err.println(e);
Thing.java
public interface Thing
void execute();
jartest.java
public class jartest implements Thing
public void exceute()
System.out.println("Welcome");
Thanks and Regards
V.Senthil Kumar
Edited by: senthilv_sun on Dec 16, 2008 8:30 PM

Answer 1

senthilv_sun wrote:
I need help from you.
How to load a Jar and access a class in the jar at run time?
When i try the following code it works fine while running in Java (Jdk1.5).If iam running the same code in servlet,ClassCastException occurs.
Error Message : ClassCastExcption : jartest1 cannot be cast to ThingPresumably we can only hope that that is a transciption error. It always helps to use copy and past actual errors and code rather than manually typing them.
test.jar contains jartest.class and Thing.classWrong.
The interface class and plugable class must not be in the same jar.
A plugable interface requires two components
- Interface (generic sense)
- Functional components.
The Interface must be independant (own jar) so that it is available to the framework (user of plugin) and to the functional components. And the plugable component must not be on the java class path.
This also means that we know for certain that the plugable component is also on the system class path. That is a bad idea as well.
Given that it is pretty pointless to even speculate as to why this error is showing up. Create the correct jar layout. Test using the command line. Then test using servlets. Insure that the plugable jar is NOT on the java classpath for both tests.