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);