[weld-dev] BeanManagerImpl permits multiple Contexts per scope type: why?

[Matej Novotny]

Hi Laird,

I just wonder, what is it you are ultimately trying to achieve with contexts since you are diving this deep?
That is, if it's not classified ;)

Comments inline...

M

----- Original Message -----
> From: "Laird Nelson" <ljnelson at gmail.com>
> To: "Martin Kouba" <mkouba at redhat.com>
> Cc: weld-dev at lists.jboss.org
> Sent: Friday, November 2, 2018 10:07:07 PM
> Subject: Re: [weld-dev] BeanManagerImpl permits multiple Contexts per scope type: why?
> 
> On Fri, Nov 2, 2018 at 1:25 AM Martin Kouba < mkouba at redhat.com > wrote:
> 
> 
> The spec is very clear that this should be possible. See for example
> 6.5.1. The active context object for a scope:
> "The container must search for an active instance of Context associated
> with the scope type."
> 
> Not so sure about the "very clear" part :-) but yes, you can read it this way
> and clearly that's the intent so that's good enough for me!
> 
> Follow-up question: a Context 's "activeness" is defined per-thread ( section
> 6.2 ):
> 
> "At a particular point in the execution of the program a context object may
> be active with respect to the current thread."
> 
> So when I ask the BeanManager for the (singular) Context
> -that-is-active-with-respect-to-the-current-thread by giving it a scope
> annotation, I get back a Context whose isActive() method returned true at
> some point during the implementation of the BeanManager#getContext(Class)
> method . And certainly I'd hope that once I receive this Context its
> isActive() method would return true .
> 
> But this is just a "best effort" area of the specification, right?

I would say it is defined well, but yes, it behaves pretty much like you say.
Take for instance session scope (and its context). While you query the context, it may well be active, but right after that, there could be session timeout
and that obviously kills the context as well. It is not something you can control, but you need to account for that.
Contexts have quite well defined lifecycle in a way that you know when they are supposed to be active and what ends them (request, session, conversation, your custom scope...).

> 
> Consider a (hypothetical, thought-experiment) stupid Context that tracks
> whether it's active or not for a given Thread (using a Map of Thread s-to-
> boolean s or something similar). Then pretend solely for discussion purposes
> that it has a daemon Thread in it that randomly and periodically sets its
> activeness to true or false for a given Thread . So sometimes a caller of
> its isActive() method gets true back, and sometimes it gets false based on
> the random activeness-toggling actions of the internal daemon thread. Again,
> this is a thought experiment, not proposed or useful code. :-)
> 
> All the BeanManager can do in this (stupid but legal?) case is make a best
> effort : at the time it selects the Context for returning from its
> getContext(Class) method, the Context must have returned true from its
> isActive() method (otherwise it would not be a candidate for returning), but
> the caller of BeanManager#getContext(Class) receiving this stupid Context
> may invoke isActive() and discover that suddenly it is returning false
> because of the actions of its stupid embedded daemon thread. Is this
> thought-experiment scenario correct and permitted by the laws of the
> specification?
> 
> Another way to ask this: is the following statement true or false? I believe
> it is true:
> 
> There is no requirement that the activeness of a BeanManager -vended Context
> be immutable with respect to the calling thread.

If I understand you correctly, then yes, this is true.
If you retrieve an active context and store it for later use, by the time you reach for it, it may no longer be active.
For instance request context in SE can be activated on a per-method scope by annotation (interceptor in fact). In such case you may get to activate/deactivate request context many time over in a single thread.
Hence retrieving the context within the method which activates it will work, but using it right after you exit that method will fail.
The immutability of context activeness wouldn't make sense at all.

> 
> (It seems this statement must be true or the fact that a Context can throw
> ContextNotActiveException from its methods would seemingly make no sense.)
> 
> I'm just trying to understand this area very deeply, particularly with
> respect to threading. Thanks as always for your time.
> 
> Best,
> Laird
> 
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