RFR: 8014814 (str) StringBuffer "null" is not appended

David Holmes david.holmes at oracle.com
Mon May 20 07:44:49 UTC 2013

On 20/05/2013 4:25 PM, Martin Buchholz wrote:
> Note that my pending change
> http://cr.openjdk.java.net/~martin/webrevs/openjdk8/getChars/getChars.patch
> does the same kind of thing, but without recursive lock acquisitions.

I will take a look.

> I'm curious why a recursive lock acquisition would be considered "very"
> cheap.  Is there some hotspot magic, or is it simply that we have
> another write to a cache line that is already probably owned by the cpu
> by virtue of the previous cas to acquire?

Yes "hotspot magic". Acquiring a lock you already own doesn't require a 
CAS; and if it is locked via biased-locking then it is an even shorter path.


> On Sun, May 19, 2013 at 10:48 PM, David Holmes <david.holmes at oracle.com
> <mailto:david.holmes at oracle.com>> wrote:
>     The change put through for JDK-8013395 (StringBuffer toString cache)
>     has exposed a previously unnoticed bug in the
>     StringBuffer.append(__CharSequence cs) method. That method claimed
>     to achieve synchronization (and then correct toStringCache
>     behaviour) by the super.append method calling other StringBuffer
>     methods after narrowing of cs to a specific type. But that is
>     incorrect if cs==null as in that case the
>     AbstractStringBuilder.__appendNull method is called directly, with
>     no calls to an overridden StringBuffer method. (I have verified that
>     none of the other methods claiming to not need sync suffer from a
>     similar flaw - this is an isolated case.)
>     Consequently we started failing some existing append(null) tests.
>     The fix is quite simple: append(CharSequence) behaves as for other
>     append methods and is declared synchronized and clears the cache
>     explicitly. The existing test is extended to check append(null).
>     webrev:
>     http://cr.openjdk.java.net/~__dholmes/8014814/webrev/
>     <http://cr.openjdk.java.net/~dholmes/8014814/webrev/>
>     This fix does mean that recursive synchronization will now be used
>     for append(CharSequence) but recursive synchronization is very
>     cheap. An alternative fix suggested by Alan Bateman in the bug
>     report is to override appendNull and add the synchronization there.
>     That requires a change to the accessibility of
>     AbstractStringBuilder.__appendNull so I chose the more constrained
>     fix. Alan's fix will also introduce nested synchronization, but only
>     for the append(null) case. As I said I don't think performance will
>     be a concern here.
>     Testing (in progress): JPRT -testset core, SQE test that caught this
>     Thanks,
>     David

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