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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).
This is not a theoretical problem... we are observing threads
overwriting each other's caches in the session.
The good news is that I only have to deal with a single container
instance on a single server/JVM, so there is no concern about session
replication or anything else like that. I also don't have to worry
about the session being created/destroyed around the same time.
The servlet API says that HttpServletRequest.getSession() returns an
HttpSession object (well, an object implementing HttpSession), but
makes no guarantees about its thread-safety (e.g. with respect to
getAttribute/setAttribute calls), so I cannot guarantee anything about
whether getAttribute/setAttribute calls will be safe, though they
would really have to be otherwise the server would encounter all kinds
of problems.
That doesn't really matter because I need to do something like this:
Cache myCache = null;
synchronized (something) {
myCache = session.getAttribute("cacheKey");
if(null == myCache) {
myCache = new Cache();
session.setAttribute("cacheKey", cache);
}
}
If I don't synchronize on a consistent monitor, there is a race
condition between the "get" and the "set" between threads using the
session.
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?
That is, will Tomcat always give an identical object to two different
threads whose requests are associated with the same session? Or are
there cases where the two threads might get different objects?
Thanks,
- -chris
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