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From "James Carman" <>
Subject RE: TimerMap
Date Mon, 24 Nov 2003 18:36:03 GMT
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 [] 
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.  


-----Original Message-----
From: Joseph Rosenblum [] 
Sent: Sunday, November 23, 2003 11:04 PM
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 

I'd love to donate this code to the commons, please let me know if you 
find it useful.


Joseph Rosenblum | 25th Street Networks
Easy, Reliable Web Hosting @

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