The problems with StringBuilders...

Mon Oct 3 23:09:00 GMT 2005

We had an enlightening (for me anyhow) discussion on IRC about the 
problems with Sun's new StringBuilder class.
Summary:
StringBuilder is kind of useless. Because it is unsynchronized, the 
results of StringBuilder.toString() cannot share the underlying char 
array (a copy of the array must be made). This is because there are 
race conditions that would result in a String changing its value after 
it is created if a copy were not made. Having to copy the contents 
probably has a much overhead as all the synchronization required by the 
StringBuffer that it replaces (and that can share the array).
TODO:
If the compiler could do some sort of escape analysis on the 
StringBuilder objects, in cases where a instance of StringBuilder could 
not escape into another thread, it could rewrite 
StringBuilder.toString() into a call to a nearly equivalent method that 
does share the backing array. This could only be done when the 
capabilities of the runtime library were known (i.e. the runtime had the 
non-copying toString equivalent). The optimization could not be done 
when generating class files.
Once you had this optimization in place you could convert all 
StringBuffers that don't escape into StringBuilder as well. This way 
you could get all the advantages of StringBuilder without having to 
actually re-write any of your code.
Someone should get started on this right away. :)
David Daney