Java Reference
In-Depth Information
How do these
uses
relationships arise?
The example shows the typical way, which is
when the method signatures of classes in an
exported package expose classes from other
packages. This seems obvious, because the
used types are visible, but it isn't always the
case. You can also expose a type via a base
class that's downcast by the consumer. Because these types of
uses
relationships are
important, how do you capture them in the bundle metadata?
import
javax.servlet
version= "2.3.0"
HTTP
service
Uses
export
org.osgi.service.http
Figure 2.23
Bundle export
uses
import
USES DIRECTIVE
A directive attached to exported packages whose value is a
comma-delimited list of packages exposed by the associated exported package.
The sidebar “
JAR
file manifest syntax” in section 2.5 introduced the concept of a direc-
tive, but this is the first example of using one.
Directives
are additional metadata to alter
how the framework interprets the metadata to which the directives are attached. The syn-
tax for capturing directives is similar to arbitrary attributes. For example, the following
modified metadata for the
HTTP
service example shows how to use the
uses
directive:
Export-Package: org.osgi.service.http;
uses:="javax.servlet"; version="1.0.0"
Import-Package: javax.servlet; version="2.3.0"
Notice that directives use the
:=
assignment syntax, but the ordering of the directives
and the attributes isn't important. This particular example indicates that
org.osgi.
service.http
uses
javax.servlet
. How exactly does the framework use this informa-
tion?
uses
relationships among packages act like grouping constraints for the packages.
In this example, the framework ensures that importers of
org.osgi.service.http
also
use the same
javax.servlet
used by the
HTTP
service implementation.
This captures the previously missing intra-bundle package dependency. In this spe-
cific case, the exported package expresses a
uses
relationship with an imported pack-
age, but it could use other exported packages. These sorts of
uses
relationships
constrain which choices the framework can make when resolving dependencies, which
is why they're also referred to as
constraints
. Abstractly, if package
foo
uses package
bar
,
importers of
foo
are constrained to the same
bar
if they use
bar
at all. Figure 2.24
depicts how this would impact the original incremental dependency resolutions.
import
javax.servlet
version= "2.4.0"
import
javax.servlet
version= "2.3.0"
import
org.osgi.service.http
Tomcat
HTTP
client
HTTP
service
Servlet
API
export
javax.servlet
version= "2.4.0"
export
org.osgi.service.http
uses: = "javax.servlet"
export
javax.servlet
version= "2.3.0"
Figure 2.24
Uses
constraints detect class-space inconsistencies, so the framework can determine
that it isn't possible to resolve the HTTP client bundle.
