Ok, got it.
Thanks Stuart
/pierre
Stuart McCulloch wrote:
2009/3/6 Pierre De Rop <pierre.de_...@alcatel-lucent.fr>
Hello everyone;
I have read the Richard's osgi presentation from
http://cwiki.apache.org/FELIX/presentations.data/osgi-berlin-20070321.pdf
and there is something in the slide 36 ("OSGI R4 modularity details 4/7,
sophisticated class space consistency model")
which I don't understand clearly.
This slide is about the "uses" clause. The whole point of the slide 36 is
to demonstrate that bundle* C* must not be wired to bundle *D*** (because of
a potential linkage exception ...)
This is clear for me.
What is not clear is the following:
we see in the slide that:
bundle A exports package "foo"
bundle D exports package "foo"
so, in the osgi core specification, chapter 3.7, the following rules are
applied, when the framework calculates bundle resolution for bundle C:
1. A resolved exporter must be preferred over an unresolved exporter.
2. An exporter with a higher version is preferred over an
exporter with lower version.
3. An exporter with a lower bundle ID is preferred over a bundle
with higher ID.
Now:
* if A (or D) is not RESOLVED, then rules 1 applies and we are sure
that both B/C will be wired to the same exporter for package foo
(the RESOLVED exporter will win)
* if A and D are both RESOLVED, then the rules 3 applies and we are
also sure that both B/C will be wired to same exporter for package
foo (with lower bundle ID)
what if A and B are installed and resolved - then later on D is installed
and resolved, and say D has a slightly higher version than A for "foo"
(perhaps only in the version qualifier)
then when we install C it would normally wire "foo" to D because it has
a higher version (depending of course on the import ranges used in C)
UNLESS there's a "uses" constraint to force it to choose A
here are some other blog entries about "uses" constraints:
http://underlap.blogspot.com/2007/10/osgi-type-safety-and-uses-directive.html
http://blog.springsource.com/2008/11/22/diagnosing-osgi-uses-conflicts/
HTH
So, regarding the two cases above: what is the benefit of using the "uses"
clause ?
Could you please give me a use case which justify the usage of the "uses"
clause ?
Many thanks;
/pierre
