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Subject: Comparing ServiceRefernces and CallbackReferences using equals() method
Hi, One of our SCA users has just asked whether it is OK to call
equals() on ServiceReferences and CallbackReferences to compare whether two
references refer to the same target. I had a quick skim through the Java Specifications
but I did not find anything. I’m interested as to whether the equals() method on
ServiceReferences has been previously discussed and whether the TC feels that
it is worth opening an issue for it. I’ve distilled the main points of what the user wanted
to do and illustrated it with a small sample as described below.
The problem is to make sure that a client does not register
more than once to receive progress notifications, the DownloadService implementation
needs to be able to compare ServiceReferences using the standard Java equals() method as
shown by the code below: public class DownloadServiceImpl implements
DownloadService { private final Map<String,
List<ServiceReference<DownloadServiceNotification>>> listeners = new
HashMap<String,
List<ServiceReference<DownloadServiceNotification>>>(); public synchronized void
addListener(
String downloadID, ServiceReference<DownloadServiceNotification>
newListener) { final
List<ServiceReference<DownloadServiceNotification>>
downloadListeners
= listeners.get(downloadID); if
(downloadListeners == null) {
// listener for a download that has no listeners so add one.
// Code not shown..... } else {
// Is the listener already registered
for (ServiceReference<DownloadServiceNotification> l : downloadListeners)
{ // ********* // PROBLEM IS HERE. Can I
call equals() on a ServiceReference? // *********
if (l.equals(newListener)) {
// Already registered so ignore request
return;
}
}
// Not registered so add
downloadListeners.add(newListener); } } I believe what the user wants the equals() method to do is
the following. Two ServiceReferences/CallbackReferences are equal if:
Below this point, I have included some sample code fragments
that may or may not be useful in illustrating what the user wants to do. Feel
free to skip the rest of the email if you feel the problem is already well
defined. The Download Service interface would end up being something
like: public interface DownloadService { // Returns a downloadID String download(URL url); void addListener(String
downloadID, ServiceReference<DownloadServiceNotification> listener); void removeListener(String
downloadID, ServiceReference<DownloadServiceNotification> listener); boolean
isDownloadCompleted(String downloadID); } The DownloadServiceNotification interface would be something
like: public interface DownloadServiceNotification { // Download is waiting in queue
to be downloaded void queued(String id); // Download has started void started(String id); // Download progress notification void progress(String id, int
percentageComplete); // Download completed void completed(String id); } The client code would want to do something similar to: public class Client implements
DownloadServiceNotification { @Context protected ComponentContext
componentContext; @Reference protected DownloadService
downloadService; public void download(URL url) { final
String downloadID = downloadService.download(url); // wait
10 seconds to see if the download is completed
sleep(10); if
(downloadService.isDownloadCompleted(downloadID)) {
// Download completed
return; } // Download
not complete so register a listener and handle asynchronously final
ServiceReference<DownloadServiceNotification> self
= componentContext.createSelfReference(DownloadServiceNotification.class);
downloadService.addListener(downloadID, self); } } Thanks, Mark |
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