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https://issues.apache.org/jira/browse/DERBY-4497?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12796287#action_12796287
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Dag H. Wanvik commented on DERBY-4497:
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I agree with Knut's analysis of the intention of the synchronization block
here. I'm not quite sure how this method is supposed to be used, though..As
Knut says, it's in not in use, nor is it's corresponding getSharedState method.
Interestingly, in FromVTI.java, the state is not shared (saved in
getPreparedStatement().getSavedObjects()), cf. FromVTI#setSharedState. This
does not seem to fulfill the interface contract: "Saves an object associated
with a key that will be maintained for the lifetime of the statement plan." But
this may not be meaningful for FromVTI.
Also, in VTIResultSet#setSharedState, a null value removes the key from the
map, but this behavior is not replicated in FromVTI's version.
> Incorrect double checked locking idiom used in VTIResultSet
> -----------------------------------------------------------
>
> Key: DERBY-4497
> URL: https://issues.apache.org/jira/browse/DERBY-4497
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.5.3.0
> Environment: OS: Redhat-5 Linux 2.6.18-92.el5
> JDK: java version "1.6.0"
> Java(TM) SE Runtime Environment (build pxi3260sr4-20090219_01(SR4))
> IBM J9 VM (build 2.4, J2RE 1.6.0 IBM J9 2.4 Linux x86-32
> jvmxi3260-20090215_29883 (JIT enabled, AOT enabled)
> J9VM - 20090215_029883_lHdSMr
> JIT - r9_20090213_2028
> GC - 20090213_AA)
> JCL - 20090218_01
> Reporter: Daniel Luo
> Original Estimate: 0.08h
> Remaining Estimate: 0.08h
>
> In method setSharedState of class VTIResultSet, double checked locking idiom
> is used. But the field compileTimeConstants involved in the idiom is not
> declared with volatile modifier which is incorrect. Simply add volatile
> modifier in field compileTimeConstants declaration can quickly fix the
> problem. Below link and description explain the details.
> http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#dcl
> "The double-checked locking idiom (also called the multithreaded singleton
> pattern) is a trick designed to support lazy initialization while avoiding
> the overhead of synchronization. Sometimes it doesn't work correctly since
> the writes initializing the object and the write to the field storing the
> object instance can be reordered by the compiler or the cache, which would
> have the effect of returning what appears to be a partially constructed
> object instance. The result would be that we read an uninitialized object. In
> JVMs 1.5 or above, the use of the volatile keyword in field declaration would
> eliminate the problems."
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