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Continue download of Applet and WebStart in background

3 replies [Last post]
Joined: 2003-10-04

One of the new feature of the consumer JRE will be the possibility to run an application even if you don't have at the start-up time all the classes available.

Wouldn't it be nice to have this feature for applets and webstart applications?

What does that means:

  • You need to specify multiple jar in the archive argument of the applet.
  • The code is executed as soon as the Jar containing classes is available.


  • Much smaller download -> faster start-up
  • Possibility to include more libraries
  • No new API or changes to the language needed

What can it be used for:

  • Have a small applet displaying a custom wait for loading and then execute the application
  • Use libraries that most of the time are not needed at start-up time such as JavaHelp, look and feels,
    Ant, commons-http, commons-network, BeanShell, SVG libraries, Flying saucer
  • Show the game options and choose level while downloading Java 3D or Java FX.
  • Have options, print preview, codec, plug-ins downloaded in background

All these libraries are quite likely excluded from applets because of their download size.

What are the challenges:

  • Class.getResource() and Class.getResourceAsStream() especially with ResourceBundle when the resource is not found.
  • Change the Applet/WebStart classloader to use this new feature
  • Get it in the Consumer JRE
  • Signed applet suddenly calling an unsigned library

Would you like me to write an RFE for this?

Best regards,

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Joined: 2003-06-16

Agreed; this is basically already supported.

See the Java Web Start Developer's Guide:

and the JNLP specification:

in particular denoting parts as "lazy".

The new Java Plug-In is going to support launching applets from JNLP files in a forthcoming build, which means that you will be able to mark various jars and extensions as lazily downloaded. This is a more powerful mechanism than those available in the archive parameter in the applet tag (which, by the way, also supports a form of lazy downloading). I think we will be able to cover all of your use cases using the JNLP support in the new Java Plug-In combined with the JNLP DownloadService, which will be supported for applets as well.

Joined: 2004-01-07

The only drawback for the current implementation is that the update-download/check is not lazy:

If you have an application with 1 eager jars and 9 lazy and all 9 lazy jars have been downloaded lazily before, if you update all 9, at startup Webstart will check for updates and will update all of them.
I already signed a bug-report because in general it makes the whole lazy-downloading thing a bit obsolete as soon as you start deploying updates.

lg Clemens

Joined: 2004-01-07

This is already possible, since java 1.3 ^^
Search for "indexed jars" should help.

lg Clemens