Hi Everyone,
Recently, I've made a few posts recounting my travails in trying to
get GeoTools working as an Eclipse plug-in. Specifically, I was
having problems with the Java Image extensions (JAI). Thanks to
help from Andrea and Jody, I now have a working system.
While I am not yet to the point where I feel that I can write on the
subject with any authority, I feel that I should at least tell you
all what worked for me (so far) and share the information with the
community. I wasted so much time on this nonsense that I'd hate to
see somebody else have to go through the same thing. So I will write
down what I've discovered so far. If anyone has corrections or
clarifications, I encourage you to post them.
Basically, some of the GeoTiff related rendering in GeoTools depends
on the Java Image extensions. These extensions permit Java to speed
up certain image processing functions, but they need to be downloaded
and installed from Sun or an other source. There has been plenty of
debate about this use, and while I fall squarely on the side that
thinks the JAI extensions are a good idea (software can never be too
fast), they do present special problems. In the case of a standard
AWT/Swing based implementation, it's merely a matter of ensuring that
they are properly installed on your system. This is easy enough once
you know how it's done. And the subject has been discussed elsewhere
on this mailing list, so I won't go into it here.
Other the other hand, Eclipse plug-ins present their own problem.
This happens because the Eclipse framework depends on its own, custom
classloaders (actually, the OSGi classloader). Unless you make
special provision when building an eclipse plug-in project (which are
essentially the same things as OSGi bundles), the classloader can't
find the resources defined in the JAI extensions jar files.
In my earlier posts I mentioned two kinds of exceptions. First,
there were "NoClassDefFoundError" exceptions. These were thrown in
the case where the classloader could not find the JAI classes it
needed. I attempted to solve this problem by including the JAI jar
files in my plug-in... (Yes, it felt wrong at the time, but it was
all I could think of) . This approach got rid of the not-found
exception, but led to another, the "ClassCastException". I believe
that the exception occurred because Java attempted to load the same
JAI classes from two separate places due to the way I'd configured
the plug-in. So it turned out that the fix was to not include the JAI
jar files in the plug in. Since I posted an email a couple of weeks
ago where I described this incorrect approach, I thought I should
clarify this point.
It turns out that the solution that was suggested to me was to
include the following line
Eclipse-BuddyPolicy: ext
In the manifest.mf file associated with the plug-in I built to hold
the GeoTools jars. This seems to have solved the problem. I know that
there is more to it than I've described here, but I have not yet
sorted out all the details. Perhaps somebody else can add the missing
information.
Over the last couple of weeks, I've done a lot of looking into the
OSGi classloaders trying to understand this issue. And I have
concluded that the whole situation related to understanding how to
use OSGi/Eclipse bundles was best described by a line in a certain
well-known pirate movie: "it is an isle of the dead which cannot be
reached, except by those who already know where it is." Web articles
on the subject assume a certain amount of background knowledge on the
readers part, but completely ignores the fact that if the reader had
that background knowledge he wouldn't need to be reading the web
article. And, while I know that I haven't done a lot better than that
myself in writing this posting, I do hope it helps a little.
Again, my thanks for the help I've received on this mailing list.
Gary
Gary W. Lucas
Sonalysts, Inc.
215 Parkway North
Waterford CT 06320, USA
(860) 326-3682
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