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From __matthewHawthorne <ma...@phreaker.net>
Subject Re: TimerMap
Date Mon, 24 Nov 2003 19:12:59 GMT
But, in a sense, how is this different than any other Map which may have 
other threads accessing it?  Care needs to be taken to avoid problems.

I think that the nature of this Map's behavior would require the creator 
to call Collections.synchronizedMap before any use, to avoid the 
concurrent modifications.  However, that doesn't mean that every piece 
of an application which accesses the Map needs to know this.  From their 
perspective, it's just another Map.

Question: What happens if you use a WeakHashMap, and one of the keys are 
garbage collected while you're iterating through it?  (Is this even 
possible?)

I think it's a good idea, could be used for caching.  It's a special 
Map, but it's still a Map.




Arun Thomas wrote:
> IMHO, if the intention is that this class should be used only in contexts (the specific
cases) that are _always_ aware of the implementation, then I would be -1 to implementing the
Map interface - even if the methods are the same.  I would also then change the interface
to represent only those methods that ought to be used in this context - put/get, removing
values() and keySet() in particular.  
> 
> On another point - any reason why TimerMapKey is public?
> 
> I still think it's something very useful!
> 
> Cheers, 
> -AMT  
> 
> -----Original Message-----
> From: James Carman [mailto:james@carmanconsulting.com] 
> Sent: Monday, November 24, 2003 10:36 AM
> To: 'Jakarta Commons Developers List'
> Subject: RE: TimerMap
> 
> 
> I wouldn't worry so much about the iterator issue.  Remember, this map implementation
is to be used in specific cases.  And, when somebody uses it, they should probably understand
that there are multiple threads modifying the map.  I would say that you can get by with a
warning in the JavaDoc on this one.  No need to get overly complicated here IMHO.  
> 
> -----Original Message-----
> From: Arun Thomas [mailto:arun.thomas@paybytouch.com] 
> Sent: Monday, November 24, 2003 1:24 PM
> To: Jakarta Commons Developers List
> Subject: RE: TimerMap
> 
> 
> Seems like a cool idea....  However, I'm concerned about the following points in the
implementation:
> 
> If the timer expires after an iterator on the keys or values is obtained, the underlying
map is modified directly - this means that the next access to the iterator (see HashMap javadoc)
will throw a ConcurrentModificationException.  To my mind, this makes iteration over this
map fairly difficult, particularly as many algorithms which use maps probably do use iterators
to access the data in the map.  A possible solution - provide wrapping implementations of
Set/Collection which return FilterIterators that filter out expired items during iteration
- only deleting those items after all existing iterators have completed iteration or have
been destroyed.  This gets complicated quickly, however.  
> 
> It seems (perhaps I'm missing something) to be overkill to synchronize all accesses to
the TimerMap.  I'm not quite sure why this is necessary - I don't think the implementation
can guarantee that an object will be expired at the instant the timer runs out - I don't believe
TimerTask provides this guarantee.  Therefore, the synchronization seems to be unneeded -
the expired object will be removed "soon" after the expiry time is reached.  
> 
> Cheers, 
> -AMT
> 
> -----Original Message-----
> From: Joseph Rosenblum [mailto:joey@25thstreet.net] 
> Sent: Sunday, November 23, 2003 11:04 PM
> To: commons-dev@jakarta.apache.org
> Subject: TimerMap
> 
> 
> Commons Developers,
> 
> The attached class is a very simple implementation of the java.util.Map 
> interface that gives each key a TTL (time to live) in the map. I've 
> found this extremely useful as a backing store for certain types of 
> caches (where you want to expire items based on time in cache, as 
> opposed to LRU, etc..) and for tracking services that I want check the 
> availability of. It's called TimerMap and takes a long TTL in the 
> constructor.
> 
> I'd love to donate this code to the commons, please let me know if you 
> find it useful.
> 
> -Joe
> 
> Joseph Rosenblum | 25th Street Networks
> Easy, Reliable Web Hosting @ http://www.25thstreet.net/
> 
> 
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> 
> 
> 
> 
> 
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