Java Reference
In-Depth Information
define the filter used for matching capabilities. The attributes for a capability
must
contain an entry whose key matches the capability's namespace.
A set of directives that are used to provide instructions to the matching algo-
rithm, or to the
OSG
i runtime.
■
For
OSG
i bundles, there are standard mappings for the specification-defined capabil-
ities and requirements. Headers such as
Import-Package
,
Require-Bundle
, and
Fragment-Host
correspond to requirements, whereas
Export-Package
and the exis-
tence of the bundle are capabilities.
The bundle capabilities
Aside from the packages it exposes, a bundle still has other capabilities, though you
probably wouldn't think of them that way normally. These are the capability to be
required
by another bundle using
Require-Bundle
, and the capability to host a frag-
ment when specified by
Fragment-Host
. These capabilities both loosely correspond
to the
Bundle-SymbolicName
and
Bundle-Version
headers, and are therefore sim-
ilar to the
osgi.identity
capability!
It may seem odd to model the bundle like this, but it makes a lot of sense.
Require-Bundle
and
Fragment-Host
are clearly dependencies that need to be
mapped as requirements—therefore, there must be matching capabilities to satisfy
them! The two capabilities are in the
osgi.wiring.bundle
and
osgi.wiring.host
namespaces for
Require-Bundle
and
Fragment-Host
, respectively.
We've discussed how requirements and capabilities are structured, and that a capabil-
ity must provide the right attributes to
satisfy
a requirement. Although this model has
an elegant simplicity, it isn't, unfortunately, the full story. In addition to attributes and
their namespaces, both requirements and capabilities can express directives that
affect the way they resolve.
7.1.3
Directives affecting the resolver
As we've said before, resolution is closely tied to provisioning. Nowhere is this more
clear than in the resolution directives that can be passed to the resolver through capa-
bilities and requirements. Directives aren't usually used directly to match require-
ments to capabilities (the mandatory directive is a notable exception), but instead
offer instructions to the resolver saying how they should be processed. If you find
yourself scratching—or banging!—your head about why a bundle won't resolve when
all the packages it imports are clearly available, check to see if there are any unsatis-
fied directives on the packages—there almost certainly will be!
Six standard directives can be applied to any namespace, which makes them partic-
ularly important to understand. These are the
resolution
directive, the
mandatory
directive, the
cardinality
directive, the
uses
directive, the
effective
directive, and
the
filter
directive.
