[Codenarc-user] CodeNarc rule for double-checked locking

[Chris M. <chr...@ea...>]

I am considering working on a CodeNarc rule for detecting the Double-Checked
Locking pattern. It would probably have to be pretty narrow/specific, along
the lines of the DCL rules in both PMD
<http://pmd.sourceforge.net/rules/basic.html#DoubleCheckedLocking>  and
Checkstyle, essentially only flagging code that follows the pattern:
 
        private Helper helper = null;

        public Helper getHelper() {

            if (helper == null) {

                synchronized(this) {

                    if (helper == null)

                        helper = new Helper();

                }

            }

            return helper;

        }

 
I realize that the memory model in Java 1.5 means that if you declare helper
as volatile, that this idiom will actually work, as discussed in
http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html and other
places. So, I may be able to craft the rule so that it only flags DCL if the
variable is NOT volatile. Then again, some have argued that even so, that
idiom is usually unnecessary and confusing, preferring the Initialization On
Demand Holder idiom or just plain synchronization, so there might be value
in a rule that always flags DCL (or makes that configurable).
 
I was just wondering whether anyone had any opinion on that, one way or the
other.
 
Thanks.
Chris