On Dec 1, 2005, at 8:39 PM, Trenton D. Adams wrote:
As a general rule, singleton methods should not be synchronized for
performance reasons. If this method is only called once in awhile,
no big deal. But, if it's called regularly, then it can be a
really big performance problem. Does the log4j project have coding
standards for such things?
This particular singleton method should check the
"_defaultLogMonitor" for null first. If it is not null, it should
return it. If it is null, it should synchronize on some object,
perhaps LF5Appender.class. Then it should check it for null
again. If it is not null, it should return it. If it is null, it
should construct it.
That idiom is not thread-safe but the explanation is too complicated
to go into here. See "The Double-Checked Locking is Broken"
Declaration (http://www.cs.umd.edu/~pugh/java/memoryModel/
DoubleCheckedLocking.html). Also covered on pg 193 of "Effective
Java" which I highly recommend.
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