///////    NetClassLoader.java    ///////
import java.net.*;
import java.io.*;
import java.util.Hashtable;
import java.lang.reflect.Method;

/** A class loader to Load name.class from a given url.
 * @author Paul S. Wang
 */
public class NetClassLoader extends ClassLoader
{/** Reads name.class at URL int byte array.
  * @param name    name.class is file to load
  * @return        array contain bytecodes of class
  */
   private byte[] loadFromUrl(String name)
          throws ClassNotFoundException
   {   try
       {  URL urlobj = new URL(urlstr+name+".class");
          BufferedInputStream in =
              new BufferedInputStream(urlobj.openStream());
          int len = in.available();
          if ( len <= 0 )
              throw new ClassNotFoundException("Empty Class");
          byte[] buf = new byte[len];
          in.read(buf);
          in.close();
          return buf;
       }  catch (IOException e)
          {   throw new ClassNotFoundException(e.toString());
          }
   }

 /** Coonstructor.
  * @param cb     the URL string where loader will find classes.
  */
   public NetClassLoader(String cb) // constructor
   {    urlstr = cb;   }

   protected NetClassLoader() {} 

 /** Overrides loadClass from ClassLoader.
  * @param name        name.class is file to load
  * @param resolve     whether to resolve loaded class
  * @return            Class object created
  */
   public synchronized Class
      loadClass(String name, boolean resolve)
          throws ClassNotFoundException
   {   Class cbj;
       try
       {   cbj = (Class) checkList.get(name);
           if (cbj == null)
           {   cbj = findSystemClass(name);
               if ( cbj != null ) return cbj;
           }
       } catch (ClassNotFoundException e)  { } 
       byte[] data = loadFromUrl(name);
       cbj = defineClass(null, data, 0, data.length);
       checkList.put(name, cbj);
       if (resolve) resolveClass(cbj);
       return cbj;
   }

/** main runs any remote application.
 * <tt> java NetClassLoader "http://SymbolicNet.mcs.kent.edu/" Factorial 6 </tt>
 *  gets the display:
 *  <tt> class loaded: Factorial </tt>
 *  <tt> 720 </tt>
 */
   public static void main(String[] args)
   {   if ( args.length < 2 )
       {   System.err.println(
             "Usage: Java NetClassLoader url ClassName arg1 ...");
           System.exit(1);
       }
       try
       {  NetClassLoader loader = new NetClassLoader(args[0]);
          Class cbj = loader.loadClass(args[1], true);
          System.out.println("class loaded: " + cbj.getName());
          Class[] param = new Class[1];
          param[0] = args.getClass();
          Method m = cbj.getMethod("main", param);
          int n = args.length-2;
          String[] arg = new String[n];
          if ( n > 0 ) System.arraycopy(args, 2, arg, 0, n);
          Object[] argArray = new Object[1];
          argArray[0] = arg;
          m.invoke(null, argArray);
       }  catch (Exception e)
          {  System.err.print(e.getMessage()); System.exit(1); }
   }
   private String urlstr;
   private Hashtable checkList = new Hashtable();
}