Hi tsuna,
Changing this class to use ConcurrentHashMap, and completely removing
synchronization, won't work, as it's written. The lazy-loading in
regionCachePreFetchEnabled() (get - if null, put) won't work correctly with
a ConcurrentHashMap without external synchronization.
I'm not sure to understand why, can you explain?
No problem. With that change, there would be a potential race condition:
Thread 1 Thread 2
---------- -------------
enter regionCachePrefetchEnabled
call ConcurrentMap.get() - get null
enter disableRegionCachePrefetch
ConcurrentMap.put(false);
exit disableRegionCachePrefetch
ConcurrentMap.put(true)
exit regionCachePrefetchEnabled
which means that after one (nominally 'read-only') call to
regionCachePrefetchEnabled and one call to disableRegionCachePrefetch
have completed, the value in the cache for the table is true, when it
really should be false.
The get-nullcheck-put triad really needs to be atomic, which requires
either external synchronization or use of some of the other features of
ConcurrentHashMap like below.
Slide 31 of the aforementioned presentation reads "Calls putIfAbsent
every time it needs to read a value" "Unfortunately, this usage is
very common" so I think this is not a good idea.
I'm still fairly new to Java so I tend to trust what people like
Joshua Bloch write, but I can be convinced otherwise :)
That's purely for performance reasons -- you're right, I should have
suggested
get, if (result == null) {putIfAbsent, get}
rather than
putIfAbsent, get
Both are correct but as Josh talks about on that slide (and the
subsequent one) it's needless contention. I would be almost certain that
it's still faster than a synchronized block, but not as much as other
options. Should have mentioned that.
Of the 4 options I mentioned (putIfAbsent(), get() and default return
value if missing, CopyOnWriteArraySet, ConcurrentHashSet), the
putIfAbsent() method is likely the slowest (given the usage pattern
anyway), and (more importantly, in my opinion) the least clear anyway. I
think the ConcurrentSet solution is much cleaner.
Hope that makes thing clear.
Cheers,
Paul
