--- robert burrell donkin wrote:
>
> On 11 Nov 2004, at 07:40, Brian Stansberry wrote:>
> > A couple things occurred to me as I looked.
> >
> > 1) The instances of Referenced are not cleared
> from the underlying
> > table if their underlying references are cleared.
> > 2) Passing null keys/values to put() does not
> result in a NPE.
> >
> > One thought on #1 is to make Referenced a subclass
> of WeakReference
> > instead of a wrapper. You can then keep a
> ReferenceQueue and poll it
> > to remove cleared references from the hashtable
> whenever get() is
> > called. This is similar to what WeakHashMap does.
>
> i had a bit of a think about the best way to do
> this. i think the
> approach outlined would be best if this were a
> general implementation.
> in this case, though, the majority of calls are
> going to be to get with
> somewhat less going to put and very few to any
> others. i can't think of
> any occasions when the symantics of put and get are
> influenced by the
> presence of extra entries. so i've opted for code
> that simply purges
> entries that have lost their referants which is
> called at the start of
> other interrogative methods. the data returned will
> be more stale than
> using a reference queue but i think that liveliness
> for put and get should be improved.
>
Yep, slowing down the critical get() just to sweep up
some dust in the corners makes no sense.
> i'd be grateful if people would take a look at the
> code and try to find
> any holes in this approach or reasons why using a
> ReferenceQueue might
> improve liveliness (preferably with patches)...
I was thinking about this and concluded that the
approach of iterating the Hashtable.entrySet() would
be faster since you're checking if either the key or
the value has been cleared. Using a ReferenceQueue
for values would force you to use a reverse lookup
map, which seems inefficient.
But then I thought, wait, should the values be held in
WeakReferences? In a typical case where the
application just calls LogFactory.getLog(), won't the
only reference to the LogFactory instance be the value
in the map? In this case a lot of calls to getLog()
will end up going through the getFactory() discovery
mechanism as the GC keeps clearing the values from the
hashtable.
Brian
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