Re: [sca-j] Comparing ServiceRefernces and CallbackReferences using equals() method

Mark,

I think this question is well worth
an issue.

Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.

Co Chair OASIS SCA Assembly TC.

IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.

Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  

Email:  

"Mark Combellack"
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20/05/2008 11:18

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Subject

[sca-j] 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.

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

           
        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:

 

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
{

    @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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