Possible thread safety problem in CachingWrapperFilter

[Paul Elschot]

I'm trying to change the CachingWrapperFilter class to cache a
SortedVIntList for LUCENE-584. That is progressing nicely, but
I found this snippet at the beginning of the current
CachingWrapperFilter.bits() method:

    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:

synchronized (cache) { ... cache.get(...); } 
...
synchronized (cache) { cache.put(...); } 

Would this be safe to initialize the cache:

synchronized(this) {
    if (cache == null) {
      cache = new WeakHashMap();
    }
}

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

Regards,
Paul Elschot

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