2018-01-17 20:05 GMT+03:00 Christopher Schultz <ch...@christopherschultz.net>:
>
> All,
>
> I have a use-case related to caching where I need to make sure that an
> operation only happens one time with respect to an object in the
> session. Basically, I want to build a cache and put it into the
> session, but it needs to be thread-safe enough that two threads can't
> see the object isn't there, build such an object, and then put it into
> the session (thereby overwriting each other).
>
[...]
>
> So the question is "what should I use as the monitor?"
>
> My first thought was that I should use the session object itself:
>
> Cache myCache = null;
> synchronized (session) {
> myCache = session.getAttribute("cacheKey");
> if(null == myCache) {
> myCache = new Cache();
> session.setAttribute("cacheKey", cache);
> }
> }
>
> That would be the best option, since it's the object I actually care
> about. However, I believe Tomcat sometimes (always) provides wrapper
> objects around servlet-spec-defined objects and I'm wondering if there
> are any guarantees about the HttpSession object being consistent
> across threads?
I think that the best way is to implement a listener
https://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionListener.html
There is "sessionCreated" event that can be used to create the cache
or any kind of a lock object for the cache (e.g. an AtomicReference),
See also other HttpSession* listeners in that package,
https://docs.oracle.com/javaee/7/api/javax/servlet/http/package-summary.html
Best regards,
Konstantin Kolinko
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