Pasi Salminen created MYFACES-3705:
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Summary: Concurrency "feature" in FaceletCacheImpl
Key: MYFACES-3705
URL: https://issues.apache.org/jira/browse/MYFACES-3705
Project: MyFaces Core
Issue Type: Bug
Components: General
Affects Versions: 2.1.0
Environment: All
Reporter: Pasi Salminen
Priority: Trivial
I'm implementing my own FaceletCache which is decorating
org.apache.myfaces.view.facelets.impl.FaceletCacheImpl by adding my own caching
policy. When I was studying the code I'm decorating, I noticed that scrictly
speaking it was not behaving. The problem lies in this code snippet (and the
same for metadata facelets):
if (_refreshPeriod != NO_CACHE_DELAY)
{
Map<String, DefaultFacelet> newLoc = new HashMap<String,
DefaultFacelet>(_facelets);
newLoc.put(key, f);
_facelets = newLoc;
}
First of all, multiple concurrent modifications of _facelets map may cause lost
updates. Think what happens when thread one copies the hashmap, updates local
copy but before it sets the reference, thread b does the same. One update is
now lost. In reality, the number of concurrent threads and number of lost
updates may be much larger. The second thing is that the new reference set to
_facelets is not quaranteed to be visible to other threads due to missing
synchronization. To prove my concerns, I created a small test bench which
proved my doubts and was able to show both lost updates and visibility problem.
When I modified the map to be ConcurrentHashMap and just used put-method all
problems vanished. So instead of
Map<String, DefaultFacelet> newLoc = new HashMap<String,
DefaultFacelet>(_facelets);
newLoc.put(key, f);
_facelets = newLoc;
I used
_facelets.put( key,f );
I know it may not be a problem, possibly just causing multiple loads of same
resource, but still I don't feel comfortable with the code behaving
concurrency-wise incorrectly.
BR, Paci
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