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 <[EMAIL PROTECTED]> 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 > >> > >> > >>>>-----Ursprüngliche Nachricht----- > >>>>Von: [EMAIL PROTECTED] > >>>>[mailto:commons-dev-return-64857- > >> > >>[EMAIL PROTECTED] > >> > >>>>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 > >>>> > >>>> > >>>>--------------------------------------------------------------------- > >>>>To unsubscribe, e-mail: [EMAIL PROTECTED] > >>>>For additional commands, e-mail: [EMAIL PROTECTED] > >> > >> > >>--------------------------------------------------------------------- > >>To unsubscribe, e-mail: [EMAIL PROTECTED] > >>For additional commands, e-mail: [EMAIL PROTECTED] > > --------------------------------------------------------------------- > To unsubscribe, e-mail: [EMAIL PROTECTED] > For additional commands, e-mail: [EMAIL PROTECTED] > > --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
