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From Chris Hostetter <>
Subject Re: Possible thread safety problem in CachingWrapperFilter
Date Tue, 04 Sep 2007 19:03:56 GMT

:     if (cache == null) {
:       cache = new WeakHashMap();
:     }
: I think the initial snippet is not thread safe and might result
: in two threads initializing this cache to different objects,
: possibly conflicting with the cache accesses after that:

i believe you are write ... if Thread A evaluates the "if (cache == null)" 
op and then Thread B is given priority and executes the entire method, 
when Thread A resumes it will blow away the old cache instance (and all of 
B's hard work)

I suspect there wasn't a synchro block arround that bit of code because 
synchronizing on an expression that evaluates to null returns an NPE 

: Would this be safe to initialize the cache:
: synchronized(this) {
:     if (cache == null) {
:       cache = new WeakHashMap();
:     }
: }

for the life of me i can't imaging why "cache = new WeakHashMap();" isn't 
just in the constructor.  then it's garunteed to only execute once.

: and should the cache accesses also use synchronized(this) ?

I can't see a need for that ... then the same instance couldn't be used on 
two distinct readers in parallel threads ... but that should be fine.


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