Re: Applets running in different thread groups under Plugin

From: rpadev (rpadev_at_nospam.optonline.net)
Date: 03/19/04 

Date: Fri, 19 Mar 2004 15:36:32 GMT

"John C. Bollinger" <jobollin@indiana.edu> wrote in
news:c3cnrs$g6k$1@hood.uits.indiana.edu:

> John Raggio wrote:
>
>> We have an application that uses multiple applets in frames. We
>> noticed that when we open the frameset that some of the applets run
>> in different thread groups. This would not be a big deal, but we
>> find that the applets in different groups can no longer communicate
>> via static methods the way they do when running in the same group.
>> We
>
> Being in different thread groups should not interfere with access to
> shared variable and static methods.
>
>> have done nothing in our code to place them in groups so why is this
>> happening?
>
> Evidently your browser / VM implementation is doing it. I would guess
> that the thread grouping is not really the core issue here.
>
>> Can it be avoided?
>
> It is unlikely that it can be avoided from the Java side, but it could
> potentially be avoided by changes to the page / frameset in which the
> applets are presented. It is conceivable that the VM or browser could
> be configured differently as well, but I wouldn't hold out too much
> hope for that.
>
>> The applets also use helper threads and we find that each of the
>> groups has instances of the helper threads even though we have tried
>> to ensure that these are singletons.
>>
>> The applets are behaving as if they are running in their own VM.
>
> Or as if they were loaded by different ClassLoaders.
>
>> If
>> there are multiple VMs would I see additional plugin icons (coffee
>> cups) in the task bar? I only see one.
>
> I typically do see multiple plugin icons in the task bar when multiple
> copies of the plugin are running, but I wouldn't count on that as a
> sure diagnostic. Nevertheless, chances are that your applets are
> being loaded by different ClassLoaders, one per frame in your
> frameset. Classes are scoped to the ClassLoader that loads them, so
> two copies of the same class loaded by different loaders are
> effectively different classes. It is arguably the desired behavior
> (in most cases) to isolate applets running in different frames from
> each other, so loading them with different loaders would make sense.
>
> You have relatively few options for inter-applet communication:
> (1) Use the server to establish a message queue
> (2) Put all the applets in the same page, displayed in only one frame
> of your frameset, and cross your fingers.
> (3) Make your applets trusted (by signing their jars) and communicate
> via local sockets or (less preferred) via the local filesystem.
>
> There may be other options, but the ones above are probably your best
> bets.
>
>
> John Bollinger
> jobollin@indiana.edu
>
>

John:

Thanks for your time and the lengthy resonse. You hit the nail on the
head. I was going to post a follow up myself this morning. One of the
applets was loaded into a different classloader which is why it was in
its own thread group. Apparently applets will be loaded into a different
classloader if they are from a different codebase. We thought ours were
in the same codebase all along, but after looking at the html we found
that the wayward applet had an extra space in its <Archive> tag. It had
the exact same set of jars (in the same order even), but the string had
an extra space. I can't belive that the plugin does a string compare to
determine if applets arein the same codebase, but alas that was the
cause.

Thanks,
John

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