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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