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Re: [orbit-dev] Question on details of export, import, and requires
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Hi, David,
These are good questions. See some comments in-line, below. I'm not
an expert.
Cheers,
Christian
On 3-Nov-08, at 2:18 AM, David M Williams wrote:
I've been thinking of improving the constraints specified by our
xerces,
and javax.xml bundles, etc. and have a few questions.
1. If "import-packages" is specified, is there any reason to also
specify
"require-bundle"? I'd suspect not, but would it be wrong to? In
particular, I'm thinking of the "Require-Bundle: system.bundle"
technique
we've used. We do that so our bundle would "override" anything found
from
the java VM, so am wondering it there's an equivalent concept with
Import-package, or if it would not be required in that case.
I would expect that, as require-bundle indicates a specific *source*
from which we want to consume packages, that also specifying Import-
Package (with dependency ranges) would be redundant and, worse, would
increase the likelihood of resolution conflicts.
2. How to specify ranges? When we 'require bundles' we always
specify a
range, such as [1.3.4, 2.0.0). Is that still important when using
import
package? I'd assume so ... but, sure seems tedious. :) What about on
"uses" ... any ranges to specify there?
Yes, these ranges work in package imports much as they do in require-
bundles. At least, they did in my Batik bundles before I had to
resort to Require-Bundle to resolve grievous class-loading issues when
multiple qualifier versions of the same bundles were installed.
The "uses" directive doesn't specify version, because the imports do
that. The "uses" is only a constraint on the package-wiring algorithm
to help OSGi determine which dependency ranges to match with which
export versions.
3. I've heard, as I'm sure we all have, we should import what we
export.
See
http://www.osgi.org/blog/2007/04/importance-of-exporting-nd-importing.html
While counter-intuitive, I can see the rationale to be a "good osgi
citizen" ... but, what to do about ranges then? We are obviously only
exporting exactly one version ... I assume we'd import only that exact
same version ... even if ranges specified on other imports?
Hmmm ... I remember hearing this advice, trying it in Apache Batik,
and finding that it failed miserably, again in the multiple-qualifiers-
of-the-same-version scenario. Class-path inconsistency grief
abounded. Certain OSGi experts advised me to desist in importing my
exports, IIRC.
I wish I understood better what the issues were that I ran into.
Is this, then, the generally recommended practice? Should we make it
standard for Orbit bundles? If so, I'll have a lot of work on my
hands ...
Thanks for any insights.
Wish I had some to offer. :-(
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--
Christian W. Damus
Senior Software Developer, Zeligsoft Inc.
Component Lead, Eclipse MDT OCL and EMF-QTV
E-mail: cdamus@xxxxxxxxxxxxx
