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From "Arun Thomas" <>
Subject RE: TimerMap
Date Mon, 24 Nov 2003 18:24:18 GMT
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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