Interpretation of the use of threads with Sessions in the JMS Specification

Hello,
I’ve been interacting with two JMS Providers which do a different interpretation of the JMS Specification with the use of different threads for accessing to a session and session’s related objects.
The question is:
When a client is consuming messages asynchronously, is it possible to access session’s related objects from a thread different to the one dedicated to the asynchronous consumption, on (thread of control)?
- Never?
- Yes, but the client should provide explicit synchronization.?
Our execution scenario is:
1.- Messages are consumed asynchronously from a dedicated thread.
2.- From other thread, messages consumed are acknowledged.
In the JMS specification there is, at least, two parts where this issue is treated:
1.- The first one (4.4.1 - page 60):
+“There are no restrictions on the number of threads that can use a Session object or those it creates. The restriction is that the resources of a Session should not be used concurrently by multiple threads. It is up to the user to insure that this concurrency restriction is met. The simplest way to do this is to use one thread. In the case of asynchronous delivery, use one thread for setup in stopped mode and then start asynchronous delivery. In more complex cases the user must provide explicit synchronization.”+
This extract, makes me think that it is possible the access from different threads, but If I do that, I should provide explicit synchronization.
2.- The second one (4.4.6 – page 62,63)
+"Once a connection has been started, any session with a registered message listener(s) is dedicated to the thread of control that delivers messages to it. It is erroneous for client code to use this session or any of its constituent objects from another thread of control. The only exception to this is the use of the session or connection close method"?+
This extract, could be interpreted as that the concurrent access is never allowed in the described situation.

I think the key words are in 4.4.6 (and also the javadoc for Session): "Once a connection has been started, any session with a registered message listener(s) is dedicated to the thread of control that delivers messages to it".
My interpretation of this is that you when you register a message listener and call connection.start(), the thread which delivers messages to the listener should be thought of as "using the resources of the session" (as mentioned in 4.4.1) continuously until the connection is stopped (or until the session or connection is closed). it doesn't matter whether the session is executing onMessage() or not.
Given this, both 4.4.1 and 4.4.6 say the same thing: that once you have registered and started a message listener, the only thing you can do to the session from another thread is to close it.
I agree the spec could be clearer. In particular the words "In more complex cases the user must provide explicit synchronization" in 4.4.6 should be clarified to state what this actually means rather than give a hint.
Please note that work has just started on updating the JMS specification, so now is a great time to report areas of the JMS spec which would benefit from clarification. If you'd like this to be considered for clarification, please report it in the official JSR 343 issue tracker at http://jms-spec.java.net
Nigel

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1. See above. There is no one correct answer It all depends on what you intend to do with the image once it's in FCP. If you only plan to push in about 10%, you won't need much higher resolution than the equivalent of your FCP Sequence resolution. If you plan to push in further, you'll need a higher resolution to support that move.
2. I've been doing these for many years (SD Sequences only) and I've never loaded them into Photoshop first. I import the entire folder into FCP and drop it on the Timeline so I can see what the images look like on TV; unless your program is for CD-ROM or web delivery, that's where it counts. When you place them on the Timeline, FCP will automatically scale to your Sequence settings and pixel aspect ratio. If I see that some of them need adjustment using an external editor such as Photoshop, then and only then do I use that program. And by the way, if you have a vertical image and a 16 x 9 aspect ratio in FCP, how do you propose to crop it to fit? Perhaps you meant another term instead of "crop."
3. If it's a print, it won't matter what size they supply because you'll control the image size when scanning. If it's a file ... once again, the size (pixel resolution) depends on what you intend to do with the image once in FCP.
I really don't want to know WHY any of this is the way it is. I just want someone to say: "Do this; do this; and then do this."<<</div>
With that attitude, how did you ever earn a PhD from an Ivy League university? But if that is truly all you want, you might consider hiring an experienced editor to do it for you.
-DH

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