public abstract class AbstractAction {  
    public abstract String action();  
}  
public class TestAction extends AbstractAction{  
    public String action() {  
        System.out.println("I am working ! ");  
        return "this ActionTest class";  
    }  
}  

 String s = "file://D:\Style.jar";***//jar所在的文件的URL*
 URL url = new URL(s);  
                  
URLClassLoader myClassLoader = new URLClassLoader(new URL[] { url }, Thread.currentThread()  .getContextClassLoader());  
Class<? extends AbstractAction> myClass = (Class<? extends AbstractAction>) myClassLoader.loadClass("com.java.jarloader.TestAction");  
AbstractAction action = (AbstractAction) myClass.newInstance();  
String str = action.action();  
System.out.println(str);