Carlo,
you don't have to challenge it. ThreadLocal is just a mostly working hack :-) I see it a lot in projects.
I will have to write a post about:
http://java.sun.com/javaee/6/docs/api/javax/ejb/EJBContext.html#getContextData() as well.
ThreadLocal works in the majority of all cases (mostly everything is execute in the same thread).
However: it is underspecified behavior. In most application server you can associate a bean with a certain thread pool. Then the context will get lost.
But: is a transaction always entirely executed by the same thread locally? IMHO this should be clarified,
thanks!,
adam
On 21.09.2011, at 15:01, Carlo de Wolf wrote:
> I wish to challenge the assumptions made in http://www.adam-bien.com/roller/abien/entry/how_to_pass_context_with.
> Not because it looks like a bad pattern, but more because it relies on un(der)specified behavior.
>
> Summary of the article: Context between EE layers can be passed via ThreadLocal.
>
> The one piece I can find in the spec only implies this behavior:
> EJB 3.1 FR 4.8.5:
> "That is, where an outbound call from a Singleton method results in a loopback call to the Singleton on the same thread."
>
> Given an invocation on a local view the assumption is pretty solid.
> Assuming that there is no ThreadLocal possible in a remote invocation coming from another VM is very solid.
>
> The weakness comes when two EARs are deployed and one calls the other via a remote view.
> Would this be a same thread invocation or not?
> If so, deploying the EARs on different VMs would change the semantics of the remote view invocation (while still being a valid assembly).
>
> So I would like to see the answers to the following be put into spec:
> - does a local view invocation imply a call on the same thread?
> - does a remote view invocation imply a call on the same thread?
>
> My answers would be: yes, no.
>
> In short: A container may or may not process a remote view invocation on the same thread. A portable application *must not* rely on the thread context of such an invocation. Local view invocation are always processed on the same thread.
>
> Carlo