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Re: [equinox-dev] support for component API refactoring


I am not sure whether BCEL or ASM are the only tools to do such kind of client migration to evolved APIs. I definitely think that you would need some kind of abstraction layer above those bytecode libraries to let API producers define those "API migrators". This abstraction layer could be a specialized API evolution DSL or a framework or something similar...

In addition to that: I think that the implementation of such a migrator framework would need to do a lot of analysis of the client code in order not to damage its implementation. For example: renaming a class would cause the migrator to adapt references to that class. What happens if a class with that name already exists in the client? And I think there are a lot of more difficult situations where a "stupid" modification of the client code would break the implementation. Any thoughts on that?


Hmm, we don't really want to support the old version of the API. It leads to all manner of surplus code, wrappering and strange coding practices. Ideally developers would be able to just move on and leave behind some aspects (whatever we want to call them) that would dynamically modify old syntax/references to new. Obviously this simplistic approach would only capture some of the cases. Even so, it would be interesting as those cases are pretty common.

So is the only way to do this with tools such as BCEL or ASM?


Matthew Webster <matthew_webster@xxxxxxxxxx> Sent by: equinox-dev-bounces@xxxxxxxxxxx
11/21/2005 04:03 AM
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Re: [equinox-dev] support for component API refactoring


I think Martin is right. While it is possible to (mis)use around advice to coerce a method call from one signature (type, name or parameters) to another you cannot change declared field types whether members or local. Are there fundamental problems with the old-fashioned way of dealing with API evolution i.e. support the old version of an API but only load it when needed?

Matthew Webster
AOSD Project
Java Technology Centre, MP146
IBM Hursley Park, Winchester, SO21 2JN, England
Telephone: +44 196 2816139 (external) 246139 (internal) Email: Matthew Webster/UK/IBM @ IBMGB, matthew_webster@xxxxxxxxxx http://w3.hursley.ibm.com/~websterm/ Please respond to Equinox development mailing list <equinox-dev@xxxxxxxxxxx> Sent by: equinox-dev-bounces@xxxxxxxxxxx To: Equinox development mailing list <equinox-dev@xxxxxxxxxxx> cc: Stefan Roock <stefan@xxxxxxxxxxxxxx> Subject: Re: [equinox-dev] support for component API refactoring

Hi Jeff,

I think this is a really interesting topic and it would be great if some
kind of aspect runtime could help reducing the problem of API evolution
somehow. I thought about it since you mentioned it during out short
phone conference and these are my first thoughts about it:

My colleque Stefan Roock is writing his PhD thesis about a quite related
topic (supporting framework API evolution). He analyzed in great detail
for what kinds of API changes it is possible to adapt clients
automatically and what dangerous situations might appear. I think we
could really benefit from his work to discuss the challenges and
especially the restrictions behind this idea. Therefore I would like to
invite Stefan to join the discussion. Hope this is okay with you.

I also thought about using aspects to adapting clients to class name
changes, for example. I didn't come up with an idea how to solve this. A
problem seems to be that the current AspectJ language is not able to
replace a type reference to a different type, I think. And the current
AspectJ weaver needs the type information that are used within the class
to be woven (would be a problem if you rename a class). But Matthew
might know more about the AspectJ requirements...

I could think of some kind of specialized weaving mechanism that does
not use the AspectJ implementation. But a specialized weaving would need
a specialized aspect language or API evolution language to describe the
API changes to adapt. Could be quite interesting...

Just to let you know about my first thoughts...


Jeff McAffer wrote:
To date we have operated under a 100% binary compatibility rule with an
underlying assumption that the original code itself could not be
 Over time our API has become ladden with old methods, old package and
class names, old extension points, ...  The challenge is keeping the API
fresh, clean and clear while not breaking existing code which, according
to the assumption, cannot be changed.

The importance of API maintenance should not be underestimated. It is
vital both for the smooth consumption of components as they evolve but
its effect on producer/consumer trust.

With that in mind, it may be interesting to challenge the assumption the
consumer's code cannot change and instead assume that the consumer does
not change the code -- allow the producer change it for them. This
direction was discussed amongst a number of us at the OSGi World
a few weeks ago.  There are several flavours but here is the basic idea.

- The producer of an API, for example, changes the name of a
package/class/method from A to B
- the producer also creates an aspect definition that "co-opts" all
consumers and modifies references to the old package/class/method to
reference the new package/class/method.
- this can capture package, class and mehod changes
- there are corner cases involving reflection and OSGi service lookup
that may be more challenging

Similarly, a registry munging mechanism would be required to update
extension/extension point usage in the face of renamed or refactored

Is there any interest in looking at this problem (I'm thinking
specifically from the aspect folks).  Thisis a pretty mundane use of
aspects but it would be hugely useful.



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