Studying services - OSGi in Action - page 130

£

£

€

LSE

LSE

DAX

d

e

f

$

NYSE

b

OSGi
service
registry

KRX

ServiceReference

c

ctx.getServiceReference ( )

Figure 4.12 Discovering an OSGi service

Here's how you can find all stock listing services using the GBP currency:

ServiceReference[] references =
bundleContext.getServiceReferences(StockListing.class.getName(),
"(currency=GBP)");
This returns references to the two LSE services ( service.id s 3 and 4 in figure 4.12).
You can also use the objectClass property, mentioned in table 4.2, to query for
services that provide specific additional interfaces. Here, you narrow the search to
those stock listing services that use a currency of GBP and also provide a chart service:

ServiceReference[] references =
bundleContext.getServiceReferences(StockListing.class.getName(),
"(&(currency=GBP)(objectClass=org.example.StockChart))");
This returns only one LSE service reference ( service.id 4 from figure 4.12) because
the other LSE service provides listings, but not charts.
You can look up all sorts of service references based on your needs, but how do you
use them? You need to dereference each service reference to get the actual service
object.

USING A SERVICE
Before you can use a service, you must bind to the actual implementation from the
registry, like this:

StockListing listing =
(StockListing) bundleContext.getService(reference);
The implementation returned is typically exactly the same POJO instance previously
registered with the registry, although the OSG i specification doesn't prohibit the use
of proxies or wrappers.