In article <maj766$qjf$1@dont-email.me>,
Knute Johnson <nospam@rabbitbrush.frazmtn.com> wrote:
So I have a Map<Integer,String> but it could be a List or an Array,
shouldn't make any difference. Access to the Map is controlled by a
ReadWriteLock. Several threads read values from the Map and one thread
can change values or write to or remove key/value pairs from the Map.
The thread that writes new values to the Map is the EDT, in a
DocumentListener. One of the reading threads is time consuming and if
that thread has the read lock the EDT blocks until it can obtain the
write lock. That can generate noticeable delays in data entry through
the GUI.
Since data entry is slow it doesn't matter much if the data entry
changes are available during the time period that the time consuming
thread is reading the Map. However to ensure that I have current data
at some point any read access of the Map must be controlled by a read lock.
I needed a way to make the time consuming thread not need to hold the
read lock for the entire time. My solution was to acquire the read lock
and create a List of values from the Map and return the read lock, this
is very quick. Then I iterate through the List to perform the time
consuming task.
I know that it is possible that the GUI could change the Map while the
time consuming thread is running. That isn't critical as the data would
be current the next time the time consuming thread ran. The reference
to the List goes out of scope when the time consuming thread completes.
Any values that were in the List that were no longer in the Map should
be available for garbage collection.
That's the usual way to do it. Perform a snapshot then use the snapshot
at a leisurely pace without worry.
Eric's recommendation of batch updates is an excellent solution for
large data sets.
I don't recommend ConcurrentHashMap EVER unless you need atomic
conditional updates. The implementation is expensive and it's still not
entirely lock free. You need a lot of concurrency from a lot CPU cores
before you'll see it outperform the old Hashtable class.
That is not my experience. CHM seems to be as fast or faster than Ht