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custom classloader in applet

Mark Deneen

Joined: Jan 31, 2005
Posts: 3
I have a signed applet, which uses a custom class loader. Here is a brief explanation of what I am doing:

Applet loads data files from the server, and decompresses them using my custom class loader.

Applet launches main class.

This allows me to use a better compression algorithm than Zip. It works great until one of the loaded classes calls getClass()getResource("blah"); I have implemented findResource(String) for my class loader, which uses a custom URLStreamHandler to provide the InputStream for the URL. It returns a ByteArrayInputStream containing a copy of the data read from the data file.

At first this did not work, as the URL constructor would throw a security exception because it did not want to allow any custom URLStreamHandlers. I got around this by using AccessController.doPrivileged(...); I'm not sure why this is needed, since my applet is signed.

Now, this works and I get a working URL... but it throws a security exception down in Swing. :-(

Here is a trace:

java.security.AccessControlException: access denied (java.security.AllPermission <all permissions> <all actions>
at java.security.AccessControlContext.checkPermission(Unknown Source)
at java.security.AccessController.checkPermission(Unknown Source)
at java.lang.SecurityManager.checkPermission(Unknown Source)
at sun.awt.SunToolkit.getImageFromHash(Unknown Source)
at sun.awt.SunToolkit.getImage(Unknown Source)
at javax.swing.ImageIcon.<init>(Unknown Source)
at javax.swing.ImageIcon.<init>(Unknown Source)
at <snip>.NavigationToolbar$CompassRose.<init>(Unknown Source)
at <snip>.NavigationToolbar.<init>(Unknown Source)
at <snip>.MapFrame.intialize(Unknown Source)
at <snip>.MapViewer.initWindowMode(Unknown Source)
at <snip>.MapViewer.init(Unknown Source)
at launcher.AppletLauncher.start(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

I really don't want to muck around with all of the getResource(String) calls. Isn't it odd that I have already loaded classes from an external source, but I can't display an image which is already in memory??

I am signing my applet with an RSA style certificate, so I should have all privileges.

Mark Deneen

Joined: Jan 31, 2005
Posts: 3
i fixed my problem by doing the following:

Policy.setPolicy(new Policy()
public PermissionCollection getPermissions(CodeSource source)
Permissions result = new Permissions();
result.add(new AllPermission());

return result;

public void refresh()

Works like a charm. By using the LZMA algorithm & proguard, I have been able to make my applet jar (now dat) files 1/3 the size. The only downside is that your applet must be signed.
wood burning stoves
subject: custom classloader in applet
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