Hi,
Let me give a try. Please see my comments inline.
Thanks,
Raymond
----- Original Message -----
From: "Nicholas Williams" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Tuesday, May 15, 2007 8:36 AM
Subject: Ambiguity in SCA Common Annotations and APIs specification re.
conversations
Hi all,
I would like clarification on what I perceive to be an ambiguity in the SCA
Common Annotations and API specification regarding when conversations are
started.
Lines [475 - 478] reads:
475 Conversations start on the client side when one of the following occur:
476 * A @Reference to a conversational service is injected
477 * A call is made to CompositeContext.getServiceReference [sidenote:
should read ComponentContext]
478 and then a method of the service is called.
Which of the following behaviour would you expect if a client chooses to
call ComponentContext.getServiceReference more than once using the same
method arguments:
a) each returned ServiceReference will refer to separate conversations.
b) each returned ServiceReference will refer to the same conversation
<rfeng>
I think it depends. There are two factors:
1) What's the current conversation if any?
2) What's the scope of the target component?
If the getServiceReference() is invoked twice in the same conversation, I
assume it will get ServiceReference instances for the same conversation. If
the getServiceReference() is invoked from two different conversations (or no
conversations), then ServiceReference instances will refer to different
conversations.
</rfeng>
My interpretation (and preference) would be a). This would allow a client
to start multiple conversations to the same reference, which is a common and
useful pattern.
**Please could you confirm this is what the spec intends.**
There is however one ugly element to coding a client that wishes to start
multiple conversations to the same reference. Consider the following client
code:
@Scope("CONVRSATION")
@Service(Client.class)
public class ClientImpl implements Client
{
@Context
protected ComponentContext componentContext;
@Reference(name = "server")
protected Server dummyServer;
public void op(int count)
{
for (int i = 0; i < count; i++)
{
//create a separate conversation for each iteration
final ServiceReference<Server> serviceReference =
componentContext.getServiceReference(Server.class, "server");
Server server = serviceReference.getService;
server.setId("id" + i);
server.doStuff();
}
}
}
The ugly bit in the above code is that I've had to declare an @Reference
against a dummyServer field, but I have no intention of using the injected
service because I am programmatically using the ComponentContext instead.
My suggestion would be to allow the @Reference to target a type (it can
currently only target field, method, constructor). The code would become:
@Scope("CONVERSATION")
@Service(Client.class)
@Reference(name = "server", type=Server.class)
public class ClientImpl implements Client
{
@Context
protected ComponentContext componentContext;
public void op(int count)
{
for (int i = 0; i < count; i++)
{
//create a separate conversation for each iteration
final ServiceReference<Server> serviceReference =
componentContext.getServiceReference(Server.class, "server");
Server server = serviceReference.getService;
server.setId("id" + i);
server.doStuff();
}
}
}
A Type-targetted @Reference annotation would be useful in a number of
scenarios:
- the client requires zero to many conversations (as described above)
- the client requires zero or one conversation (why inject a reference if
it might not be used)
- the client needs fine control of when the conversation is started (the
developer may want lazy reference lookup and won't appreciate the service
being injected before the @init method)
<rfeng>I think what you're asking for is a way to declare references using
annotation without being injected automatically. Another thing is that the
injection of the reference shouldn't trigger the start of the
conversation.</rfeng>
What do you think?
Thank you
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