On 05/20/2013 07:48 AM, David Holmes 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/
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.
Hi David, Alan,
The third possibility could be the following
AbstractStringBuilder.append(CharSequence) method:
public AbstractStringBuilder append(CharSequence s) {
if (s == null || s instanceof String)
return this.append((String)s);
if (s instanceof AbstractStringBuilder)
return this.append((AbstractStringBuilder)s);
return this.append(s, 0, s.length());
}
... and no synchronization in StringBuffer.append(CharSequence)...
Regards, Peter
Testing (in progress): JPRT -testset core, SQE test that caught this
Thanks,
David
