Java/Development Class/Applet Loader
Applet Loader Demo
<source lang="java">
import java.applet.Applet; import java.io.IOException; import java.io.InputStream; import java.net.MalformedURLException; import java.net.URL; import java.net.URLConnection; import java.util.Hashtable; import javax.swing.JFrame; public class MainClass {
public static void main(String args[]) {
String name = "http://urlWithClassName";
try {
if (!name.endsWith(".class")) {
System.err.println("That doesn"t look like a byte code file!");
return;
}
URL u = new URL(name);
URLClassLoader ucl = new URLClassLoader(u);
// parse out the name of the class from the URL
String s = u.getFile();
String classname = s.substring(s.lastIndexOf("/"), s.lastIndexOf(".class"));
Class AppletClass = ucl.loadClass(classname, true);
Applet apl = (Applet) AppletClass.newInstance();
JFrame f = new JFrame();
f.setSize(200, 200);
f.add("Center", apl);
apl.init();
apl.start();
f.setVisible(true);
} catch (Exception e) {
System.err.println(e);
}
}
} class URLClassLoader extends ClassLoader {
Hashtable cache = new Hashtable();
URL url;
public URLClassLoader(URL u) {
this.url = u;
}
public synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
Class cls = (Class) cache.get(name);
if (cls == null) {
try {
cls = findSystemClass(name);
} catch (ClassNotFoundException e) {
}
}
if (cls == null) {
byte classData[] = loadClassData(name);
cls = defineClass(classData, 0, classData.length);
cache.put(name, cls);
}
if (resolve) {
resolveClass(cls);
}
return cls;
}
private byte[] loadClassData(String name) throws ClassNotFoundException {
byte[] buffer;
InputStream theClassInputStream = null;
int bufferLength = 128;
try {
URL classURL = new URL(url, name + ".class");
URLConnection uc = classURL.openConnection();
uc.setAllowUserInteraction(false);
try {
theClassInputStream = uc.getInputStream();
} catch (NullPointerException e) {
System.err.println(e);
throw new ClassNotFoundException(name + " input stream problem");
}
int contentLength = uc.getContentLength();
// A lot of web servers don"t send content-lengths
// for .class files
if (contentLength == -1) {
buffer = new byte[bufferLength * 16];
} else {
buffer = new byte[contentLength];
}
int bytesRead = 0;
int offset = 0;
while (bytesRead >= 0) {
bytesRead = theClassInputStream.read(buffer, offset, bufferLength);
if (bytesRead == -1)
break;
offset += bytesRead;
if (contentLength == -1 && offset == buffer.length) { // grow the array
byte temp[] = new byte[offset * 2];
System.arraycopy(buffer, 0, temp, 0, offset);
buffer = temp;
} else if (offset > buffer.length) {
throw new ClassNotFoundException(name + " error reading data into the array");
}
}
if (offset < buffer.length) { // shrink the array
byte temp[] = new byte[offset];
System.arraycopy(buffer, 0, temp, 0, offset);
buffer = temp;
}
// Make sure all the bytes were received
if (contentLength != -1 && offset != contentLength) {
throw new ClassNotFoundException("Only " + offset + " bytes received for " + name
+ "\n Expected " + contentLength + " bytes");
}
} catch (Exception e) {
throw new ClassNotFoundException(name + " " + e);
} finally {
try {
if (theClassInputStream != null)
theClassInputStream.close();
} catch (IOException e) {
}
}
return buffer;
}
}
</source>
Dummy Applet Context
<source lang="java">
/**
* Copyright(c) 1996 DTAI, Incorporated (http://www.dtai.ru) * * All rights reserved * * Permission to use, copy, modify and distribute this material for * any purpose and without fee is hereby granted, provided that the * above copyright notice and this permission notice appear in all * copies, and that the name of DTAI, Incorporated not be used in * advertising or publicity pertaining to this material without the * specific, prior written permission of an authorized representative of * DTAI, Incorporated. * * DTAI, INCORPORATED MAKES NO REPRESENTATIONS AND EXTENDS NO WARRANTIES, * EXPRESS OR IMPLIED, WITH RESPECT TO THE SOFTWARE, INCLUDING, BUT * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND * FITNESS FOR ANY PARTICULAR PURPOSE, AND THE WARRANTY AGAINST * INFRINGEMENT OF PATENTS OR OTHER INTELLECTUAL PROPERTY RIGHTS. THE * SOFTWARE IS PROVIDED "AS IS", AND IN NO EVENT SHALL DTAI, INCORPORATED OR * ANY OF ITS AFFILIATES BE LIABLE FOR ANY DAMAGES, INCLUDING ANY * LOST PROFITS OR OTHER INCIDENTAL OR CONSEQUENTIAL DAMAGES RELATING * TO THE SOFTWARE. */
import java.awt.*; import java.applet.*; import java.net.*; import java.util.*; import java.io.*; /**
* DummyAppletContext - implements AppletContext and AppletStub to allow any * applet to easily run as an application. The only thing it can"t do is * access URL"s. Applet parameters are entered on the command line with * name as one word and value as the next. * * @version 1.1 * @author DTAI, Incorporated * * $Id: DummyAppletContext.java,v 1.5 1996/10/08 21:15:09 kadel Exp $ * * $Source: /cvs/java/classes/COM/dtai/gui/DummyAppletContext.java,v $ */
public class DummyAppletContext
extends Frame
implements AppletStub, AppletContext, URLStreamHandlerFactory {
private TextField status;
private Hashtable params = new Hashtable();
private Vector applets = new Vector();
private int initial_width;
private int initial_height;
/**
* Entry point into the standalone program.
*
* @param args the command line arguments
*/
public static void main ( String args[] ) {
new DummyAppletContext( args );
}
/**
* Constructor for the main class, given an existing applet object.
*
* @param applet the applet embedded in this AppletContext
* @param args the command line arguments. Contains possibly
* height and width, and any applet parameters
*/
public DummyAppletContext( Applet applet, String args[] ) {
this( applet, 640, 480, args );
}
/**
* Constructor for the main class, given an existing applet object and a default
* frame (window) width and height.
*
* @param applet the applet embedded in this AppletContext
* @param default_width the default width of the window
* @param default_height the default width of the window
* @param args the command line arguments. Contains possibly
* height and width, and any applet parameters
*/
public DummyAppletContext( Applet applet, int default_width, int default_height,
String args[] ) {
super ( applet.getClass().getName() );
init( applet, default_width, default_height, args, 0 );
}
/**
* Constructor for the main class, from the command line arguments.
*
* @param args the command line arguments. Contains the name of the applet
* class, possibly height and width, and any applet parameters.
*/
public DummyAppletContext( String args[] ) {
super ( args[0] );
try {
Applet applet = (Applet)Class.forName( args[0] ).newInstance();
init( applet, 640, 480, args, 1 );
}
catch ( Exception e ) {
e.printStackTrace();
System.exit( 1 );
}
}
/*
* PRIVATE initialization function.
*
* @param applet the applet embedded in this AppletContext
* @param default_width the default width of the window
* @param default_height the default width of the window
* @param args the command line arguments. Contains possibly
* height and width, and any applet parameters
* @param startidx index in the args array at which to start parsing
*/
private void init( Applet applet, int default_width, int default_height,
String args[], int startidx ) {
URL.setURLStreamHandlerFactory( this );
applets.addElement( applet );
applet.setStub(this);
initial_width = default_width;
initial_height = default_height;
parseArgs( args, startidx );
status = new TextField();
status.setEditable( false );
add( "Center", applet );
add( "South", status );
appletResize( initial_width, initial_height );
show();
applet.init();
applet.start();
}
/**
* Parse the command line arguments. Get the initial width and height of
* the window if specified (-width [value] -height [value]), and the
* applet parameters (name value pairs).
*
* @param args the command line arguments. Contains possibly
* height and width, and any applet parameters
* @param startidx index in the args array at which to start parsing
*/
public void parseArgs( String args[], int startidx ) {
for ( int idx = startidx; idx < ( args.length - startidx ); idx+=2 ) {
try {
if ( args[idx].equals( "-width" ) ) {
initial_width = Integer.parseInt( args[idx+1] );
}
else if ( args[idx].equals( "-height" ) ) {
initial_height = Integer.parseInt( args[idx+1] );
}
else {
params.put( args[idx], args[idx+1] );
}
}
catch ( NumberFormatException nfe ) {
System.err.println("Warning: command line argument "+args[idx]+
" is not a valid number." );
}
}
}
/**
* Event handler to catch the Frame (window) close action,
* and exit the program.
*
* @param evt The event that occurred
* @return false if the event was not handled by this object.
*/
public boolean handleEvent( Event evt ) {
if ( evt.id == Event.WINDOW_DESTROY ) {
System.exit(0);
}
return super.handleEvent(evt);
}
/************ AppletStub methods *************/
/**
* Returns true if the applet is active.
*
* @return always true
*/
public boolean isActive() { return true; }
/**
* Gets the document URL.
*
* @return a "file:" URL for the current directory
*/
public URL getDocumentBase() {
URL url = null;
try {
File dummy = new File( "dummy.html" );
String path = dummy.getAbsolutePath();
if ( ! File.separator.equals( "/" ) ) {
StringBuffer buffer = new StringBuffer();
if ( path.charAt(0) != File.separator.charAt(0) ) {
buffer.append( "/" );
}
StringTokenizer st = new StringTokenizer( path, File.separator );
while ( st.hasMoreTokens() ) {
buffer.append( st.nextToken() + "/" );
}
if ( File.separator.equals( "\\" ) &&
( buffer.charAt(2) == ":" ) ) {
buffer.setCharAt( 2, "|" );
}
else {
}
path = buffer.toString();
path = path.substring( 0, path.length()-1 );
}
url = new URL( "file", "", -1, path );
}
catch ( MalformedURLException mue ) {
mue.printStackTrace();
}
return url;
}
/**
* Gets the codebase URL.
*
* @return in this case, the same value as getDocumentBase()
*/
public final URL getCodeBase() { return getDocumentBase(); }
/**
* Gets a parameter of the applet.
*
* @param name the name of the parameter
* @return the value, or null if not defined
*/
public final String getParameter( String name ) {
return (String)params.get( name );
}
/**
* Gets a handler to the applet"s context.
*
* @return this object
*/
public final AppletContext getAppletContext() { return this; }
/**
* Called when the applet wants to be resized. This causes the
* Frame (window) to be resized to accomodate the new Applet size.
*
* @param width the new width of the applet
* @param height the new height of the applet
*/
public void appletResize( int width, int height ) {
Insets insets = insets();
resize( ( width + insets.left + insets.right ),
( height + status.preferredSize().height +
insets.top + insets.bottom ) );
}
/************ AppletContext methods *************/
/**
* Gets an audio clip. (There doesn"t seem to be a "Toolkit" for
* audio clips in my JDK, so this always returns null. You could
* implement this differently, returning a dummy AudioClip object
* for which the class could be defined at the bottom of this file.)
*
* @param url URL of the AudioClip to load
* @return the AudioClip object if it exists (in our case,
* this is always null
*/
public final AudioClip getAudioClip( URL url ) { return null; }
/**
* Gets an image. This usually involves downloading it
* over the net. However, the environment may decide to
* cache images. This method takes an array of URLs,
* each of which will be tried until the image is found.
*
* @param url URL of the Image to load
* @return the Image object
*/
public final Image getImage( URL url ) {
return Toolkit.getDefaultToolkit().getImage( filenameFromURL( url ) );
}
/*
* PRIVATE utility function. Ignores the protocol, and returns a
* filename for a file on the local filesystem (which may or may
* not exist, of course).
*
* @param url URL to be converted to a filename on the local
* filesystem.
* @return the filename
*/
private String filenameFromURL( URL url ) {
String filename = url.getFile();
if ( filename.charAt(1) == "|" ) {
StringBuffer buf = new StringBuffer( filename );
buf.setCharAt( 1, ":" );
filename = buf.toString();
}
else if ( filename.charAt(2) == "|" ) {
StringBuffer buf = new StringBuffer( filename );
buf.setCharAt( 2, ":" );
filename = buf.toString();
}
return filename;
}
/**
* Gets an applet by name.
*
* @param name the name of the applet
* @return null if the applet does not exist, and it never
* does since we never name the applet.
*/
public final Applet getApplet( String name ) { return null; }
/**
* Enumerates the applets in this context. Only applets
* that are accessible will be returned. This list always
* includes the applet itself.
*
* @return the Enumeration -- contains ONLY the applet created with
* this DummyAppletContext
*/
public final Enumeration getApplets() { return applets.elements(); }
/**
* Shows a new document. This may be ignored by
* the applet context (and in our case, it is, but we"ll show the
* user, in the status area, that the document was requested and
* WOULD be loaded if in a browser).
*
* @param url URL to load
*/
public void showDocument( URL url ) {
status.setText( "AppletContext request to show URL " +
url.toString() );
}
/**
* Show a new document in a target window or frame. This may be ignored by
* the applet context. (Again, it is ignored, but we"ll show the
* request information to the user in the status area.)
*
* This method accepts the target strings:
* _self show in current frame
* _parent show in parent frame
* _top show in top-most frame
* _blank show in new unnamed top-level window
* <other> show in new top-level window named <other>
*
* @param url URL to load
* @param target the target string
*/
public void showDocument( URL url, String target ) {
status.setText( "AppletContext request to show URL " +
url.toString() +
" in target: " + target );
}
/**
* Show a status string in the status area (the Text object at the bottom
* of the window.
*
* @param text the text to display
*/
public void showStatus( String text ) { status.setText( text ); }
/************ URLStreamHandlerFactory methods *************/
/**
* Creates a new URLStreamHandler instance with the specified protocol.
*
* @param protocol the protocol to use (ftp, http, nntp, etc.).
* THIS PROTOCOL IS IGNORED BY THIS APPLET CONTEXT
*/
public URLStreamHandler createURLStreamHandler( String protocol ) {
return new DummyURLStreamHandler();
}
} /*
* A URL stream handler for all protocols, used to return our * dummy implementation of URLConnection to open up a local * file when called upon. */
class DummyURLStreamHandler extends URLStreamHandler {
protected final URLConnection openConnection( URL u ) throws IOException {
return new DummyURLConnection( u );
}
} /*
* Our dummy implementation of URLConnection used to open up a local * file when called upon with a given URL of ANY protocol type. This * allows the applet to easily use the "getInputStream()" function. */
class DummyURLConnection extends URLConnection {
boolean connected = false;
InputStream instream;
/*
* Constructor for the DummyURLConnection
*/
protected DummyURLConnection( URL url ) {
super( url );
}
/*
* open the local file
*/
public void connect() throws IOException {
if ( ! connected ) {
String filename = url.getFile();
if ( filename.charAt(1) == "|" ) {
StringBuffer buf = new StringBuffer( filename );
buf.setCharAt( 1, ":" );
filename = buf.toString();
}
else if ( filename.charAt(2) == "|" ) {
StringBuffer buf = new StringBuffer( filename );
buf.setCharAt( 2, ":" );
filename = buf.toString();
}
instream = new FileInputStream( filename );
}
}
/*
* return the open stream to the local file (open if necessary).
*/
public InputStream getInputStream() throws IOException {
if ( ! connected ) {
connect();
}
if ( instream == null ) {
throw new IOException();
}
return instream;
}
}
</source>
