Java Applet not caching

Question

I'm having a problem with a Java applet I've deployed that is refusing to be cached in the jvm's "sticky" cache (or by the browser). For some reason every time a user loads the page this applet is on, the jvm re-downloads the jar file from the server which causes a long delay.

The webpage containing the applet is being accessed via the internet, so according to Sun's Java applet documentation I'm using an <applet> tag rather than an <object> or <embed> tag.

Any help debugging or identifying the problem would be much appreciated.

Below is the full applet tag I'm using:

<applet alt="Scanning Applet failed to load" archive="scanning.jar"
        code="scanning.scanlet.class" codebase="/java/" codetype="application/java"
        height="30" mayscript="True" name="scanlet" width="200">
    <param name="domain" value="192.168.12.23" />
    <param name="publishName" value="scan_attachment" />
    <param name="publishURL" value="http://192.168.12.23/draft/update/52" />
    <param name="curURL" value="http://192.168.12.23/draft/edit/52" />

Your browser is unable to process the Java &lt;APPLET&gt; tag needed to display this applet
<br />
One solution would be to download a better web browser like
<a href="http://www.mozilla.com/firefox">Mozilla's Firefox</a>

</applet>

Answer 1

I had the very same problem and found a trick that need to be applied server side. It seems that jars aren't cached if their mime type is incorrect. Please check your server reply using the application/java-archive mime type.

Another problem I found is related to a specific jar. My archive parameter was listing many jars, but only a few were cached. I found that all jars upto a specific one where cached, all remaining jars weren't cached at all. While loading applet, I pressed "5" in its java console and found this message just after the last cached jar: cache: signed entry missing from jar . I still don't know what is wrong with this file, but I moved that jar at the end of the archive parameter list. This "fixed" the problem.

Answer 2

Personally I solved the caching issues by using cache_archive and cache_version parameters docummented at http://java.sun.com/javase/6/docs/technotes/guides/plugin/developer_guide/applet_caching.html

My cache_version is based on the actual file modification date, e.g.

val archive = libs.map("/" + _.getPath).mkString (", ")
val version = libs.map(_.getUpdated / 1000 / 20 - 59281420).mkString (", ")

and to work with MSIE it looks as:

if (msie) {
  cms write <OBJECT
    classid="clsid:CAFEEFAC-0016-0000-0000-ABCDEFFEDCBA"
    codebase="http://java.sun.com/update/1.6.0/jinstall-6-windows-i586.cab"
    width="100%" height="100%">
    ...
    <PARAM name="cache_archive" value={archive}/>
    <PARAM name="cache_version" value={version}/>
  </OBJECT>
} else {
  cms write <applet ...
    cache_archive={archive} cache_version={version}></applet>
}