Yup, ordering is a pain when it is not based on service dependencies. The
actual problem you point out was noticed during the design phase but it was
deemed very important by some EEG member that the WAR file acted as a WAR
according to the Java EE Servlet Spec, which prohibited that it played nice in
the OSGi world. This impedance mismatch between Blueprint and the Web Extender
was therefore accepted.
After working with these kind of problems for a long time (even long before
OSGi) I've come to the conclusion that only services work well in all cases.
You therefore have to translate the problem to a service problem. iPOJO, DS,
Blueprint are all experts in handling service dependencies and ordering is a
dependency problem. In this particular case the problem is that the WAR
extender is oblivious of any dependencies and assumes the world is ready when
it is ready, ok and the extendee. In this case, the WAR extender will create a
servlet. The simplest solution is to create a ServletDepender servlet. This
dummy servlet must be parameterized with a filter. Once it starts, it gets the
service if it exists, or waits until it is registered.
In your Blueprint code you just register the servlet you want as a service.
This model uses services to handle dependencies and thus ordering. Almost all
of the OSGi machinery is geared to make service dependencies work reliable and
actually quite nicely.
Kind regards,
Peter Kriens
On 30 apr 2010, at 17:49, Richard S. Hall wrote:
>
>
> On 4/30/10 2:30, ext2 wrote:
>> Hi:
>> In osgi, extender is a very useful pattern. But sometimes if we use
>> several extenders to extending the same bundle's function. They may cause
>> un-desirable underlying sequential coupling;
>>
>> For example: WEB Container in OSGI use a extender to recognize a
>> War Bundle , and also Spring-DM use a extender to recognize a Spring bundle.
>> So if I write a bundle which is a war-bundle and also a spring-dm bundle.
>> The servlet (in bundle) use a osgi-service injected by spring-dm; for this
>> example , I wish the spring-extender works before the Web-Extender, so the
>> servlet created by Web-Extender can safely call the osgi-service injected by
>> the spring-extender;
>> But in osgi framework, how could I control the extenders work order
>> while they extending the sample bundle?
>>
>> Thanks any suggestion;
>>
>>
>>
>>
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