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From Craig McClanahan <craig...@gmail.com>
Subject Re: AW: AW: AW: AW: [proposal] avoiding jar version nightmares
Date Tue, 21 Dec 2004 20:28:03 GMT
Don't forget to do a little light reading first :-)

Mechanism to declare dependencies in a MANIFEST.MF File:

  http://java.sun.com/j2se/1.4.2/docs/guide/extensions/versioning.html

ClassLoader implementation for representing an "Assembly":

  http://java.sun.com/j2se/1.4.2/docs/api/java/net/URLClassLoader.html

Craig



On Tue, 21 Dec 2004 14:34:56 -0500, Matt Sgarlata
<sgarlatm@users.sourceforge.net> wrote:
> Count me in!  I'm planning on taking some time off for the holidays, so
> that should give me time to play around with this :)
> 
> Matt
> 
> Daniel Florey wrote:
> > <snip/>
> >
> >>>an application using 1.x.b will work with component 1.x.a
> >>>Does this sound reasonable? Missed something?
> >>
> >>Have you seen the guidelines in use by the Apache APR project?  It looks
> >>to me like you're basically advocating the same system they have in
> >>place.  It might save us hassle to just adopt their version numbering
> >>system whole-sale (as the Spring Acegi Security subs(ystem does)
> >>
> >>http://apr.apache.org/versioning.html
> >
> >
> > Very funny - looks pretty identical to me ;-) So, yes, let's adopt this.
> > I've not played around with classloading a lot, so I'm very curious if this
> > will work. I'll try to start working on this if I'll find the time for it.
> > If you are interested I could setup an account for you at my personal root
> > server (I've installed Subversion). Or should I create a subproject in the
> > commons-sandbox?
> > As it's in stage of brainstorming I'd prefer to do it in my personal space
> > first.
> > Are you interested?
> >
> > Cheers,
> > Daniel
> >
> >
> >>>I'd prefer to keep the "jar" naming as introducing "assembly" would
> >>
> >>cause
> >>
> >>>some confusion.
> >>>If anyone would be interested I could put a simple proposal to the
> >>
> >>sandbox.
> >>
> >>Good point, JAR may be a better name.  I see two benefits to using
> >>"assembly" or "assembler" as the name:
> >>- Clearly indicates that you aren't dealing with plain-old-JAR files
> >>anymore
> >>- Parallels name used in .NET so that the analogy is directly obvious
> >>
> >>
> >>>This approach will not address the trouble that may be caused by
> >>>applications not using this package. So finally I think that it is
> >>
> >>required
> >>
> >>>that this feature (or something comparable) will make it into Java 1.6.
> >>>Up to then I still think it's a very simple but easy way to add the
> >>
> >>version
> >>
> >>>number to the package names to avoid at least the very big problems
> >>>concerning incompatible jars in the same classloader.
> >>
> >>I understand your reasoning behind putting this code in Java 1.6, but I
> >>think we can do this without a new release of the Java language (see
> >>below).  If our ideas are successful, this new Commons component could
> >>always migrate later to a JSR proposal, as Doug Lea's concurrent package
> >>did.
> >>
> >>With regards to problems caused by components that aren't using this new
> >>package, I'm thinking that as long as the component does not make any
> >>Class.forName calls, we should be OK.  If there are Class.forName calls,
> >>the component may still be able to work, but we would strongly encourage
> >>a migration to using Assembly.getType or whatever.  This entails the
> >>component introducing a dependency on Assembler, which means the
> >>Assembler API will need to maintain backwards compatability as much as
> >>possible (e.g. - imagine the nightmare that would ensue if
> >>java.util.Vector were to change its semantics!)
> >>
> >>
> >>>Regards,
> >>>Daniel
> >>
> >>Matt
> >>
> >>
> >>>>-----Urspr√ľngliche Nachricht-----
> >>>>Von: commons-dev-return-64857-daniel.florey=web.de@jakarta.apache.org
> >>>>[mailto:commons-dev-return-64857-
> >>
> >>daniel.florey=web.de@jakarta.apache.org]
> >>
> >>>>Im Auftrag von Matt Sgarlata
> >>>>Gesendet: Dienstag, 21. Dezember 2004 13:04
> >>>>An: commons-dev@jakarta.apache.org
> >>>>Betreff: Re: AW: AW: [proposal] avoiding jar version nightmares
> >>>>
> >>>>Chris Lambrou wrote:
> >>>>
> >>>>
> >>>>>Matt Sgarlata wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>>>Does this mean .NET doesn't have reflection?  That's such a killer
> >>>>>>feature of Java; I can't believe they wouldn't have ported it
to .NET.
> >>>>>>Any .NET developers out there that can tell us how .NET deals
with
> >>>>>>reflection when you have multiple versions of the same class?
> >>>>>
> >>>>>
> >>>>>Since the class name alone is insufficient to fully identify a specific
> >>>>>version of a class, to my knowledge there is no equivalent to
> >>>>>Class.forName(String classname) in .NET. Instead, .NET has the Assembly
> >>>>>class. An Assembly is roughly akin to a java jar file, and is typically
> >>>>>a single DLL that contains one or more classes. Assembly has a
> >>>>>non-static getType(String typeName) method, that performs the same
job
> >>>>>as the static Class.forName(String classname) method in java, but
for a
> >>>>>specific Assembly instance. There is never any ambiguity over which
> >>>>>version of the named Type that is returned, since an Assembly can
only
> >>>>>contain one version of any given class. Support for multiple versions
> >>
> >>of
> >>
> >>>>>a class at runtime is achieved by storing those multiple class versions
> >>>>>in separate Assemblies.
> >>>>
> >>>>Thanks for the info, Chris!  This definitely sounds like a good
> >>>>approach.  Now my question is, can we simulate this in a new commons
> >>>>component? :)
> >>>>
> >>>>Here are the steps I would imagine to be involved:
> >>>>1) Define our own JAR sub-type to mirror the .NET assembly notion.
> >>>>Include some type of a plain-text file that describes the versions of
> >>>>the software required to perform certain tasks.  It would be nice to
do
> >>>>this in an existing structure like MANIFEST.MF, but I don't know... are
> >>>>you allowed to add arbitrary information to that file?  In any case,
we
> >>>>wouldn't use the existing dependency descriptors because that would
> >>>>prevent multiple versions of the same class from being loaded.
> >>>>2) Call org.apache.commons.assembler.Assembler.getType(String
> >>>>assembledPackage, String className).  The Assembler would then go to
the
> >>>>assemblyPackage path on the classpath and search the plain-text file
> >>>
> >>>>from step #1 which would list the versions of classes that are required
> >>>
> >>>>by the given assembledPackage.  For example, if assembledPackage was
the
> >>>>Digester, which required collections 3, the assembledPackage would be
> >>>>org.apache.commons.digester.  A dynamic proxy or generated bytecode
> >>>>would be loaded that fulfilled the given contract and that would be
> >>>>returned to the client.  Any existing code that is just calling
> >>>>Class.forName would have classes looked up in the normal way, so we
> >>>>would need to make sure that this dynamic proxy doesn't get loaded into
> >>>>the JVM in the same way as Class.forName (this is where the dynamic
> >>>>proxy and/or bytecode generation comes in)
> >>>>
> >>>>What do you guys think?  Does this sound feasible?  I'd rather spin this
> >>>>as a commons component than a J2SE 1.6 enhancement request, because the
> >>>>later will take years to come to fruition.
> >>>>
> >>>>
> >>>>
> >>>>>Chris
> >>>>
> >>>>Matt
> >>>>
> >>>>
> >>>>---------------------------------------------------------------------
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> >>
> >>
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