shared "value" in java.lang.StringBuilder

[Alan Bateman]

On 06/11/2013 21:54, Robert Stupp wrote:
> Hi,
>
> I was wondering why the mostly allocated class in nearly all applications is char[]. A deeper inspection showed that a lot of these char[] allocations are "caused" by the code from java.lang.StringBuilder.toString(), which created a copy of its internal char array. Most StringBuilder instances are no longer used after the call to StringBuilder.toString(). Many of these instances have been created by javac caused by "plain" string concatenation in source code.
>
> Wouldn't it worth to try whether passing the (Abstract)StringBuilder's value+count values to String results in less temporary object creations and therefore reduce pressure on new generation (and reduce GC effort)? My idea is to add a field 'shared' to AbstractStringBuilder, which is set when StringBuilder.toString() is called. If the StringBuilder is really modified after calling toString(), the StringBuilder creates a new copy of the value array and resets the 'shared' field. Since the value array might be longer than the current count, String class would need a "re-invention" of the count field.
>
> Another think I noticed is that the StringBuilder instances transiently created by javac seem to use the default constructor. But a huge amount of string concatenations in Java code result in (much) longer Strings, which means that each append creates a new, larger copy of the value array in AbstractStringBuilder. Is it possible to add some "guessing" for the initial capacity - this would eliminate a lot of temporary objects and reduce GC effort. Is it worth to check this out? Are the two places in com.sun.tools.javac.jvm.Gen#visitAssignop/visitBinary the only places where these StringBuilder instances are created?
>
> -
> Robert
I can't speak to the details but HotSpot has an optimization that 
recognizes some cases like 
new-StringBuilder(...).append(...).append(...).toString() where it can 
avoid the copy. This may be something to follow-up on the hotspot list 
if you want to get into the details.

-Alan.