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https://issues.apache.org/jira/browse/SLING-2575?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13478685#comment-13478685
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Chetan Mehrotra commented on SLING-2575:
----------------------------------------
Interesting discussion. Did not knew about ServiceUtil. I have also struggled
with handling of ordered multiple references so far with Declarative Service.
This was one area where Bleuprint's sorted dynamic reference list [1] was
pretty useful. However we can get similar support by using
o.a.s.commons.osgi.ServiceUtil.getComparableForServiceRanking and JDK 6
ConcurrentSkipListMap. With this we need not require synchronized map.
@Reference(name = "AuthorizableAction",
policy = ReferencePolicy.DYNAMIC,
cardinality = ReferenceCardinality.OPTIONAL_MULTIPLE,
referenceInterface = AuthorizableAction.class
)
public class MultiplexingAuthorizableAction extends AbstractAuthorizableAction{
private Map<Comparable<Object>,AuthorizableAction> actionMap =
new ConcurrentSkipListMap<Comparable<Object>,
AuthorizableAction>();
...
private Collection<AuthorizableAction> getActions() {
return actionMap.values(); //Sorted list
}
private void bindAuthorizableAction(AuthorizableAction
action,Map<String,Object> config){
actionMap.put(ServiceUtil.getComparableForServiceRanking(config),
action);
}
private void unbindAuthorizableAction(AuthorizableAction
action,Map<String,Object> config){
actionMap.remove(ServiceUtil.getComparableForServiceRanking(config));
}
}
For Alex's requirement of having a list sorted in reverse order (which is
probably the more common usecase) we can modify the ServiceUtil to
- Provide a Compartor implementation which does the reverse and pass that on to
ConcurrentSkipListMap as keys
- OR Have a new ServiceUtil.getDescendingComparableForServiceRanking(config)
[1]
http://www.eclipse.org/gemini/blueprint/documentation/reference/1.0.1.RELEASE/html/service-registry.html#service-registry:refs:collection
> Utility for tracking a multi-cardinality OSGi service reference
> ---------------------------------------------------------------
>
> Key: SLING-2575
> URL: https://issues.apache.org/jira/browse/SLING-2575
> Project: Sling
> Issue Type: Improvement
> Components: Commons
> Reporter: Alexander Klimetschek
> Priority: Minor
> Attachments: SLING-2575-ServiceReferences.patch
>
>
> Managing a SCR @Reference that's basically a list is very difficult when
> compared to the simple unary, static reference. It seems a typical use case
> is 0..n cardinality, dynamic policy and ordered by service ranking with the
> higher ranked ones first. This supports the use case to ask a list of
> services and have the first responding one win.
> There is the ServiceTracker [0], but its getServiceReferences() method does
> not return the list sorted in any way, only gives your references and not the
> typed object(s) and it's a bit cumbersome to use.
> A typical manual approach can be seen in the SlingPostServlet [1] in the
> register*() methods. Important is to handle thread-safeness.
> [0]
> http://www.osgi.org/javadoc/r4v42/org/osgi/util/tracker/ServiceTracker.html
> [1]
> http://svn.apache.org/repos/asf/sling/trunk/bundles/servlets/post/src/main/java/org/apache/sling/servlets/post/impl/SlingPostServlet.java
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