Re: sync on local variable

Daniel Pitts <>
Thu, 25 Mar 2010 12:28:45 -0700
On 3/25/2010 11:06 AM, Eric Sosman wrote:

On 3/25/2010 1:29 PM, Roedy Green wrote:

On Wed, 24 Mar 2010 16:21:29 -0700, Patricia Shanahan<>
wrote, quoted or indirectly quoted someone who said :

It's difficult to know if the message was valid or not without an SSCCE.

you can see the complete code at

it is a bit fat for a SSCCE.

Would have been nice of you to point out which of the twelve
source files elicited the complaint ... For others who may want
a look, seems to be the culprit.

Roedy, I still don't know what's going on, but there's at least
one synchronization problem apparent in the code:

for ( int rowIndex = 0; rowIndex < ALL_ROWS.size(); rowIndex++ )
synchronized ( ALL_ROWS )
row = ALL_ROWS.get( rowIndex );

As the comment just before this loop says, rows might be deleted
from ALL_ROWS while this is running, meaning that rowIndex might
be out of range by the time you call get().

This doesn't appear closely related to a complaint about
synchronizing on row -- but I've often found, when confronted
by a mysterious problem, that it pays to clean up *all* the
other issues, even those that are "obviously unrelated."

By the way, if ALL_ROWS can be perturbed while the loop is
in progress, the iteration may miss rows (visit row 4, other
thread deletes row 2 and slides everything down one slot, visit
row 5 which used to be row 6, never visit the old row 5), or may
visit a row more than once (visit row 4, other thread inserts a
row after row 2 and slides everything up, visit row 5 which is
the same row just visited as 4). The comment indicates you don't
want to lock ALL_ROWS for the entire loop, but to keep the
iteration self-consistent you might want to do something like
lock ALL_ROWS, grab a snapshot with toArray(), unlock, and run
the iteration on the (private, stable) snapshot.

I agree except, don't use toArray, use new ArrayList<Row>();
Or, instead of all the sync, check, blah-de-blah, use the standard EDT
mechanisms for modifying/accessing this table. That way, you're actions
are all serial, and you don't have to worry so much.

Threading isn't that hard to get right, but it isn't that hard to get
wrong either. The only truly difficult part of writing MT code is that
people aren't taught how to look at it and determine whether it is right
or not.

Daniel Pitts' Tech Blog: <>

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