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From Matt Sgarlata <>
Subject Re: AW: AW: [proposal] avoiding jar version nightmares
Date Sun, 19 Dec 2004 23:37:43 GMT
Craig McClanahan wrote:
> On Fri, 17 Dec 2004 19:10:32 -0500, Matt Sgarlata
> <> wrote:
>>How do we go about petitioning Sun for something like this?
> A while back now (while the details for Tiger were being planned), I
> happened to be in a meeting with Graham Hamilton (who basically owns
> the direction that J2SE is going from a Sun perspective), talking
> about the very issue of class loaders and the contortions that you
> have to go through in order to implement things like webapp reloading.
>  I asked him for a Christmas present to all Java developers -- add
> something like ClassLoader.unloadClass() or ClassLoader.replaceClass()
> to deal with things like this.  He said "hmm ... that's a hard
> problem" and proceeded to describe several of the places where
> implementing this would be extremely difficult (and/or would have
> nasty performance impacts) in the current architecture of JVMs.

Well what about introducing the versioned library approach that is done 
in .NET?  I'm not familiar with the details myself, but Chris Lambrou 
wrote earlier:

The .NET equivalent of a jar file is called an assembly. For libraries, 
this is basically a DLL. Every time the code is compiled, the DLL is 
automatically allocated a unique version number. When you compile your 
code that refers to code in a library assembly, your assembly has an 
explicit dependency on that library assembly. At runtime, when your code 
tries to invoke the library code, an exception will be raised if the 
exact version of the library assembly is not available.

It would appear that if there are bug fixes or other improvements to the 
library, and a recompiled assembly DLL is substituted for the one you 
originally compiled against, then your code will break. At runtime, your 
code will fail to link to the library code, since the version number no 
longer matches. Obviously, a maintenance release of a library component 
shouldn't require a recompilation and redeployment of all software that 
uses the library, so .NET provides a mechanism for you to explicitly 
define a version number. This allows you to provide updated library 
components to users without requiring them to recompile. However, this 
only works if you don't break backwards compatibility.

If you break backwards compatibility in a library, then you have to 
change the version number. However, .NET still allows you to deploy 
different, incompatible versions of the same DLL. When you deploy the 
application, your installer has to register both versions of the DLL 
with the GAC - the Global Assembly Cache. In this way, if you have a 
complex application that contains two components that rely on 
incompatible versions of the same library DLL, the VM instantiates two 
separate versions of the library DLL, and links the two components to 
the appropriate instance.

One possible Java analogy to this would be to bundle all code inside jar 
archives. Each jar contains dependency information, perhaps stored in 
the manifest, or some other meta-file, that describes the jar's own name 
and version number, and a list of the names and version numbers of its 
dependencies. A suitable class loader can then use this meta information 
to stitch the classes together appropriately. Actually, my knowledge of 
java class loaders isn't  sufficient for me to assert that this solution 
would definitely work, but it's a start, and I hope all of this serves 
to illustrate how .NET allows multiple versions of the same library to 

> Regarding the original use case in this thread (an app that wants to
> use two modules that have conflicting versions of common
> dependencies), about the best you can do right now is to have your
> application create its own class loaders for the modules involved, and
> set up their classpaths to pick up their own versions of the
> dependencies.  That is essentially what a servlet container does
> (creates a class loader for each webapp) to maintain separation, and
> allows each webapp to load its own version of a common dependency JAR
> from its own "class path" ... the /WEB-INF/classes and /WEB-INF/lib
> directories of that app.
> Craig

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