I don't know if the jar duplication problem is that compelling overall. Even
several megabytes of duplicated jar's seems like a drop in the bucket these
days. Certainly it would take a lot for any serious product vendor to be
willing to relinquish control over the libraries they depend on and risk their
dependencies not getting installed properly on the client. For years, OO.o was
shipping with its own whole JRE just in case. I think it's only recently that
it's become smart enough to recognize when the client already has Java
installed.
And if we don't mind duplicated jar's, then having your own modularization
supporting multiple versions of the same jar is trivial. I wrote this as part
of my own plug-in architecture for an app several years ago:
160 public Module loadModule(Properties modConfig) throws
ModuleLoadException
161 {
162 String enabled = modConfig.getProperty("mod.enabled");
163 if(enabled != null && "false".equalsIgnoreCase(enabled))
164 {
165 return null;
166 }
167 URL[] cpURLs = this.getCPURLs(modConfig);
168 Module module = this.loadModule(new URLClassLoader(cpURLs,
this.getClass().getClassLoader()), modConfig.getProperty("mod.impl.class"));
169 module.init(modConfig);
170 return module;
171 }
That's the crux of it and it allows each module/plug-in to initialize in the
context of its own class loader, which in turn allows me to stuff different
copies of the jar's possibly containing different versions of the same class
into different modules. No problem.
Where duplicate jars count seems to be the two opposite ends of deployment
spectrum -- mobile applications and app servers. In the case of mobile
deployments, right now we have two options: Java ME, which is as good as dead
in terms of forward momentum, and Android, which solves the modularity problem
at the core of its service-oriented architecture. And as far as app servers, I
suspect it's not a big deal for admins to keep control of shared apps and
employ whatever modularization they deem necessary -- if JVM comes with it,
they won't see a huge win over using an external modularization framework.
Alexey
2001 Honda CBR600F4i (CCS)
1992 Kawasaki EX500
http://azinger.blogspot.com
http://bsheet.sourceforge.net
http://wcollage.sourceforge.net
________________________________
From: mcculls <mccu...@gmail.com>
To: The Java Posse <javaposse@googlegroups.com>
Sent: Monday, June 29, 2009 3:21:16 AM
Subject: [The Java Posse] Re: more jigsaw vs osgi vs javaposse
On Jun 29, 12:27 pm, Augusto <augusto.sellh...@gmail.com> wrote:
> No I mean that exactly.
>
> I don't know, I mean the point of modularizing something for me is I
> may want to use your module but I don't care about its internals. Or
> at most, I don't want the internals of your module to affect me.
[disclaimer: I contribute to OSGi projects and I'm co-authoring a book
on OSGi]
exactly, that's why libraries often use tools to embed/repackage
dependencies:
http://maven.apache.org/plugins/maven-shade-plugin/
http://code.google.com/p/jarjar/
for example Guice has CGLIB and Google-Collections as internal
dependencies,
but I wouldn't want to be forced to use the same version of these
libraries when
using Guice - similarly the Guice team probably doesn't want to be
bothered with
problems caused by someone putting a different version of CGLIB before
Guice
on the classpath (the JVM will pick the first matching class when
scanning the
classpath, so ordering makes a big difference when there's overlapping
content)
so Guice repackages CGLIB and Google-Collections inside the jar -
unfortunately
this means anyone who already has those jars gets duplicate content
(~400k?)
now imagine if everyone does the same - you could end up with ten
copies of the
Google-Collection classes, embedded inside various libraries - you can
already
see this happening in applications today, and it's caused by a lack of
modularity
if there was a standard modularity system that supported multiple
versions then
the Guice team could distribute just their classes (plus the necessary
metadata)
safe in the knowledge that regardless of what jars were on the
'module' path, the
right version would be wired to Guice
that's one of the reasons why I find module systems compelling - now I
can totally
understand why you might need a special framework to modularize the
JVM, just
like the JVM has the "Unsafe" class for internal use - but I'm a
little bit wary about
using the same solution for applications, exactly because it might be
optimized
for the JVM (for example the "no multiple versions" requirement)
still hoping that JSR 294 will help bring both sides together in some
way... oh well,
time will tell - I'd hate for people to be put off the general idea of
modularity (and to
some extent programming to interfaces) as imho it does lead to better
apps
--
Cheers, Stuart
PS. many thanks to the JavaPosse for doing both of the interviews in
the first place
> So yeah, you can expect your 3rd party libraries to "keep up" with the
> latest and greatest, but that's kind of an unreasonable expectation
> with fast paced technology. What I want is to use your library, but
> not have it affect the same libraries it might be using internally but
> that I explicitly depend on.
>
> BTW, when people say "classpath hell" (or jar hell) this is one of the
> main scenarios they refer to.
>
> http://c2.com/cgi/wiki?ClasspathHell
>
> ----------
> One big need for OsGi / JavaModuleSystem? (JSR 277) functionality is
> to fix the ClasspathHell problem:
>
> * My application uses libraries "B" and "C", both of which use
> library "D".
> * But "B" and "C" require different versions of "D".
> * There's no version of "D" I can put on the CLASSPATH that will
> satisfy both "B" and "C".
> * Thus, I'm in "ClasspathHell" -- there's no "standard Java way"
> to fix the problem.
> ------------
>
> I assume that the whole "Application Context" in Jigsaw means that for
> webapps and apps running in an EJB container you can overcome this but
> no I meant it more in a regular application outside of any of these
> containers.
>
> Augusto
>
> On Jun 28, 11:21 pm, Jess Holle <je...@ptc.com> wrote:
>
> > Augusto wrote:
> > > On Jun 28, 6:38 pm, Steve <stephen.a.lind...@gmail.com> wrote:
>
> > >> If an alternative modularity platform for app developers was more
> > >> compelling than OSGi I certainly would jump ship, but it would need to
> > >> at least provide what the OSGi core does now (proper component
> > >> encapsulation, supporting multiple versions of the same 3rd party jar,
> > >> runtime dynamism, etc.).
>
> > > Multiple versions of the same jar is one thing I described in my blog
> > > post incorrectly. Well, I said that was a core problem solved by a
> > > module system but in fact Jigsaw doesn't seem to support it. It is not
> > > needed for modularizing the JDK, but it is essential for modularizing
> > > applications.
>
> > It is essential for /some/ applications.
>
> > Personally I generally prefer to make all the parties involved work
> > /really/ hard at allowing for and arriving at a single version of any
> > given library (ideally the latest stable version) to be used at runtime
> > rather than allowing multiple versions within an application. Using
> > multiple library versions in one application is pretty much a worst case
> > scenario to me -- and is generally a strong indication that someone is
> > not keeping their software up-to-date (i.e. so that it can use the
> > latest stable versions of the libraries it depends on). If that someone
> > is a vendor or 3rd-party component then that's generally a sign to go
> > shopping for another one -- unless, of course, you're the one who has
> > been foolish enough to stay on an old version of that component instead
> > of moving to the new version, in which case it is time to upgrade.
>
> > --
> > Jess Holle
>
> > P.S. If you mean multiple versions just for things like a web app
> > reload, that's a different matter entirely, of course.
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