Remove usage of Applet/JAppelt and AppletInfo

[ptziegler]

The Applet/JApplet class has been marked as deprecated back in Java 9 and marked for removal in Java 17. Given that most browsers dropped their support for Java plugins, we should remove the corresponding code from WB.

See JEP 398 for reference.

From the original Oracle whitepaper:

With modern browser vendors working to restrict or reduce the support of plugins like Flash, Silverlight and Java in their products, developers of applications that rely on the Java browser plugin need to consider alternative options. Java developers currently relying on browser plugins should consider migrating from Java Applets to the plugin-free Java Web Start technology.

Supporting Java in browsers is only possible for as long as browser vendors are committed to supporting standards based plugins. By late 2015, many browser vendors had either removed or announced timelines for the removal of standards based plugin support, while some are introducing proprietary browser-specific extension APIs. Consequently, Oracle is planning to deprecate the Java browser plugin in JDK 9.

The deprecated plugin technology will be completely removed from the Oracle Java Development Kit (JDK) and Java Runtime Environment (JRE) in a future Java release TBD. Java Web Start applications do not rely on a browser plugin and will not be affected by these changes.

[vogella]

+1, please provide PR

[lcaron]

👍 for me too

[ForceRs]

I would not consider removing Applet support until at least December 2030. Java 8 supports applets natively and via JNLP. Applets can still be run in the IE 11 browser via Edge and IE compatibility mode. Java 8's extended support ends December 2030. See page https://www.oracle.com/java/technologies/java-se-support-roadmap.html for details. Although the browser Java Plugin is no longer formally supported, JNLP (aka Java web start) is until December 2030. Applets can be deployed through JNLP.
Webswing supports applets, too, which further bolsters their viability.

[ptziegler]

If part of the user base is still using Applets, then I'm content with keeping them for as long as long as they are functional.

But I don't know how realistic it is, to keep supporting them until 2030. The final decision will probably be made by Oracle, when they decide to remove those classes from the JDK.

They still seem to be contained in the upcoming Java 21, but once they are gone, I'm leaning towards keeping WB compatible with the latest Java version, rather than trying to keep Applets alive... But I suppose that's something that can be discussed again, when that time has come.