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Subject: Comparing ServiceRefernces and CallbackReferences using equals() method



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.

  • There is a DownloadService which you can use to asynchronously download files.
  • The client of the DownloadService can optionally register for download progress notifications
  • The client of the DownloadService can unregister for download progress notifications.
  • The DownloadService should ignore multiple requests from the same client to register for Download Service progress notifications
  • More than one client may register for download progress notifications for a particular download



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





            // Not registered so add








I believe what the user wants the equals() method to do is the following. Two ServiceReferences/CallbackReferences are equal if:


  • For Stateless, they refer to the same target
  • For Conversational, they refer to the same target and Conversation.






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 {


    protected ComponentContext componentContext;



    protected DownloadService downloadService;


    public void download(URL url) {

        final String downloadID = downloadService.download(url);


        // wait 10 seconds to see if the download is completed



        if (downloadService.isDownloadCompleted(downloadID)) {

            // Download completed




        // Download not complete so register a listener and handle asynchronously

        final ServiceReference<DownloadServiceNotification> self

            = componentContext.createSelfReference(DownloadServiceNotification.class);

        downloadService.addListener(downloadID, self);











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