Hello. Consider an API like ImageIO. To write an image in the PNG format, one calls:
ImageIO.write(image, "PNG", output);
The implementation of the write() method gets a list of the available
formats via ServiceLoader and picks the one named "PNG". The "PNG"
value is the sole criteria used to pick an image format provider.
In OSGi, we'd use the whiteboard pattern (probably with declarative
services) to implement this. The implementation might look something
like:
interface ImageFormat
{
String name();
...
}
@Component
public final class ImageFormats
{
private final ConcurrentHashMap<String, ImageFormat> formats =
new ConcurrentHashMap<String, ImageFormat>();
@Reference(
cardinality = ReferenceCardinality.MULTIPLE,
policy = ReferencePolicy.DYNAMIC,
unbind = "onFormatUnregister")
public void onFormatRegister(
final ImageFormat format)
{
this.formats.put(format.name(), format);
}
public void onFormatUnregister(
final ImageFormat format)
{
this.formats.remove(format.name(), format);
}
...
}
This would work perfectly well, but what happens if two bundles
try to register a format with the same name? We could check for
and reject registrations with overlapping names, but that might
leave developers/users stuck with a format implementation that
they don't like. What if two different versions of the same bundle
are installed, and both try to register themselves?
This is a problem that seems to come up in various guises each
time I work with OSGi, and I've not really seen a satisfactory
solution to it. Many solutions involve bubbling up the entirely
internal concern of there being multiple versions of a format
provider present to the API, and although this can work, it does
mean that users are then forced to intelligently pick providers
("I just want PNG, why do I have to care which of the providers
I end up with?! Isn't the ImageFormats class supposed to pick
for me?!"). Part of this problem is caused by the fact that the
ImageFormats class exists at all; users could, for example,
search for services providing ImageFormat themselves via DS or
the OSGi APIs. Sometimes, though, a class analogous to ImageFormats
is necessary, particularly when you expect callers to be OSGi-unaware
and/or you only have very simple selection criteria (like a "PNG"
string).
How do people that know more about OSGi than I do usually handle
this?
--
Mark Raynsford | http://www.io7m.com
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