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From Gregg Wonderly <gr...@wonderly.org>
Subject Re: [jira] Updated: (RIVER-336) Jini should support platforms other than those with RMIClassLoader as the classloading control point. IDEs inparticular need help.
Date Fri, 02 Apr 2010 05:30:47 GMT
There are two primary issues that I needed to deal with for netbeans.

First, because netbeans is an application with a native launcher, and all kinds 
of things about it, as a platform, are private, I had problems getting the 
RMIClassLoaderSPI in River, PerferredClassProvider, to even activate.  There was 
an issue of having to copy the jars into a private directory.  That wasn't 
something that felt like a production step, and it would not make it possible to 
install and use a plugin supporting river without restarting the IDE.  This 
seemed a bit fragile if not just ugly.

Second, the simple implementation of PreferredClassLoader creation in 
net.jini.loader just uses the system classloader as the "parent" for the created 
class loader.  In netbeans, the class loader hierachy that a module sees, is 
just the module and its dependents, with the parent of that loader being the 
system class loader.  The netbeans system class loader is nearly empty.  It just 
has the bootstrap of netbeans visible, and then netbeans creates 
compartmentalized views of various modules as it loads things.  So, when I 
finally got PreferredClassProvider activated, downloaded jars could not find 
stuff that was "platform" in the since that the module had this stuff in its 
classloader.  That module classloader was not visible because the 
PreferredClassLoader's parent was just the netbeans startup jars and the JRE.

Thus, I really had a hard time making net.jini.loader work as it existed.  I 
looked around and saw that there are actually very few places that 
RMIClassLoaderSPI et.al. is used, and that all of those places where of interest 
to me in how I could make Jini work inside of netbeans (at least I hoped).

So, I decided to just put a layer of indirection (what programmers do to solve 
most problems :-) between net.jini.loader and RMIClassLoaderSPI that would allow 
the mechanism itself to be supplanted.

So, these patches allow one to override how all of the RMIClassLoaderSPI 
mechanisms are implemented, plus adding some twists specifically about how the 
"parent" classloader is chosen for PreferredClassLoader instances.

And, to top it off, some of you may recall that I modified preferred class 
loading to actually have the facility to "never prefer" some classes.  What this 
meant was that jar files in the codebase would never be downloaded and thus in 
large, distributed systems, with lots of jars visible across slow networks, a 
ServiceUI client application could startup, and show all visible services, 
without downloading anything.

Additionally, I also make changes to Reggie to return marshalled data values so 
that unmarshalling would not occur, which would trigger downloads.

By making these changes, I was able to move the "never prefer" implementation 
into my CodebaseClassAccess implementation so that it becomes a feature for me, 
not a platform implementation detail.

Please ask questions if I've not provided enough details somewhere.

Gregg Wonderly

Christopher Dolan wrote:
> Gregg,
> This patch is interesting to me.
> The implementation is clear: create a new plugin point to override the
> default RMIClassLoader. Could you explain the motivation a little more?
> It seems like the intention is to make River play nice in a large
> process by not insisting that the global RMIClassLoader be configured to
> suit River. And it looks like it is fully backward compatible for River
> deployments that are already overriding RMIClassLoader by defaulting to
> RMIClassLoaderCodebaseAccess.
> If nothing else, I would love to use something like this just to reduce
> the huge number of places where I have to include
> "-Djava.rmi.server.RMIClassLoaderSpi=...".  That alone would be a win.
> Or am I reading too much into this?
> Chris
> -----Original Message-----
> From: Peter Firmstone [mailto:jini@zeus.net.au] 
> Sent: Thursday, April 01, 2010 4:16 AM
> To: river-dev@incubator.apache.org
> Subject: Re: [jira] Updated: (RIVER-336) Jini should support platforms
> other than those with RMIClassLoader as the classloading control point.
> IDEs inparticular need help.
> Should I create a branch on svn for this?
> Regards,
> Peter.
> Gregg Wonderly (JIRA) wrote:
>>      [
> https://issues.apache.org/jira/browse/RIVER-336?page=com.atlassian.jira.
> plugin.system.issuetabpanels:all-tabpanel ]
>> Gregg Wonderly updated RIVER-336:
>> ---------------------------------
>>     Attachment: rmicl.diff.txt
>> This is a diff out of my perforce node for the affected classes that
> I've been using for some time.   The changes shown here are preliminary
> and should be considered experimental.
>>> Jini should support platforms other than those with RMIClassLoader as
> the classloading control point.  IDEs inparticular need help.
> ------------------------------------------------------------------------
> -----------------------------------------------------------
>>>                 Key: RIVER-336
>>>                 URL: https://issues.apache.org/jira/browse/RIVER-336
>>>             Project: River
>>>          Issue Type: New Feature
>>>          Components: net_jini_loader
>>>    Affects Versions: AR3
>>>            Reporter: Gregg Wonderly
>>>         Attachments: rmicl.diff.txt
>>> The RMIClassLoader class and RMIClassLoaderSPI is currently the
> control point for managing the "platform" view of how classes are
> loaded.  In IDEs and other different environments, the "parent"
> classloader view, is not always the "system class loader".  There are
> some other variations on class loading that seem to indicate that while
> RMIClassLoaderSPI can be plugged into, it doesn't always provide quite
> the right facilities because even plugging into the system class loader
> to override it might not be possible.
>>> The diffs included here show some preliminary work that I did
> investigating this issue to try and make it possible to discover and
> load Jini servers within the netbeans IDE.
>>> Refinement and some rework will be needed, and some other
> investigation into other platforms such as JEE and other IDEs would be
> helpful in making sure we understand what is really needed.  Even OSGi
> would be something to look at.

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