Hey All -
I'm working on a data service based on Abdera (working with Chris
Berry, who's a regular on these lists...) When we were running our
first battery of serious load testing on our system, we encountered
memory-leaky behavior, and a profiler showed us that we were indeed
leaking hundreds of megabytes a minute, all traceable back to the
wrappers field on org.apache.abdera.factory.ExtensionFactoryMap. This
field is a map from elements to their wrappers, if any. At first, I
was puzzled by the memory leak, as the field is initialized thusly:
this.wrappers = Collections.synchronizedMap( new
WeakHashMap<Element,Element>());
clearly, the implementor took care to make sure that this cache would
not leak by making it a WeakHashMap, which generally guarantees that
the map itself will not keep a key and its corresponding entry from
being garbage collected. I dug throughout our application code to
find if we were actually holding other references to these objects,
and I googled for anyone having problems with esoteric interactions
between Collections.synchronizedMap and WeakHashMaps - found nothing
there. Then I went back to square one and re-read the WeakHashMap
javadoc very carefully. Here's the relevant section:
Implementation note: The value objects in a WeakHashMap are held by
ordinary strong references. Thus care should be taken to ensure that
value objects do not strongly refer to their own keys, either directly
or indirectly, since that will prevent the keys from being discarded.
Note that a value object may refer indirectly to its key via the
WeakHashMap itself; that is, a value object may strongly refer to some
other key object whose associated value object, in turn, strongly
refers to the key of the first value object. One way to deal with this
is to wrap values themselves within WeakReferences before inserting,
as in: m.put(key, new WeakReference(value)), and then unwrapping upon
each get.
This is why there is a memory leak - the map is a mapping from
elements to their wrappers - by the very nature of the object being a
wrapper of the element, it will usually have a strong reference to the
element itself, which is the key! You can verify that Abdera wrappers,
in general, will do this by looking at
org.apache.abdera.model.ElementWrapper, which takes the element being
wrapped as its constructor argument, and holds a strong reference to
it as an instance variable.
This map is an optimization to memoize the calls to
getElementWrapper() and not reconstruct them more than is necessary -
it is not needed for abdera to function properly, so we have
temporarily worked around the problem in our own application like so -
we used to acquire our FOMFactory by calling abdera.getFactory() on
our org.apache.abdera.Abdera instance, and re-using that singleton
throughout our application. Now we construct a new FOMFactory with
new FOMFactory(abdera) once per request to the server, and since the
only appreciable state on the factory is this map itself, this is a
valid work-around.
I'd initially planned to really fix this issue and submit a patch
along with this message, but digging a little deeper, I'm not sure
that the correct fix is crystal clear... We could do as the javadoc
above suggests, and wrap the values with WeakReferences to plug the
leak, or we could use a LinkedHashMap configured as an LRU cache to
just bound the cache, so it can't grow out of control - but right now,
I don't think that either of those solutions would be correct, because
it seems to me that none of the objects in the hierarchy rooted at
FOMElement define equals() and/or hashCode() methods, so all of the
objects are cached based on their actual object identity. It seems
that in the all likely use cases, instances of FOMElement and its
descendants are re-parsed on every request to a server running abdera,
and so what we will see is cache misses virtually 100% of the time, so
even though we'd have plugged the leak, strictly speaking, we would
have ignored the underlying issue that we're caching data on every
request that will be fundamentally unable to be retrieved on
subsequent requests. This is based only on my reading over the code
for a few hours, so I could be missing something, and I also might be
forgetting about a use case that demands and makes proper use of this
memoization, but as it stands right now, my recommended fix would
probably be to just cut out the cache altogether, and allow for
wrappers to get constructed fresh every time they are requested. One
more possibility is that the cache is actually a useful optimization,
but only during the scope of one request - in which case the "work-
around" we are using now is actually the best practice, and the fix
would be to remove the factory instance on the Abdera class...
I'd like to hear from the Abdera developers what their thoughts are on
this issue, and what the best resolution is likely to be. This is no
longer a pressing issue for our team, but it is potentially a time
bomb waiting to blow up for any project dependent on Abdera.
thanks! (and thanks for Abdera, generally - we're easily a year ahead
of where we'd be on this project without it!)
-Bryon (long-time listener, first-time caller)
