RFR: 8358533: Improve performance of java.io.Reader.readAllLines [v7]

[Roger Riggs]

On Fri, 27 Jun 2025 23:00:04 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> src/java.base/share/classes/java/io/Reader.java line 504:
>> 
>>> 502:                     }
>>> 503:                     if (fragLen >= cb.length/2) {
>>> 504:                         // allocate larger buffer and copy chars to beginning
>> 
>> It might be designed to use a more conservative resizing approach, for example, **cb.length + TRANSFER_BUFFER_SIZE**, rather than exponential doubling, more in line with how resizing is handled in string builder.  not a very strong opinion though :-)
>
>> [...] more in line with how resizing is handled in string builder
> 
> Looks like `StringBuilder` eventually calls `ArraysSupport.newLength(int,int.int)`. This is probably worth checking out.

On the call to ArraysSupport.newLength(curr, min, preferred).
This will double the size (preferred) until it runs past SOFT_MAX_ARRAY_LENGTH.
Then it starts growing by minGrowth, it should probably be smaller; something that will allow it to get pretty close to SOFT_MAX_ARRAY_LENGTH before throwing OOME.  But still large enough to allow efficient reads.
Maybe 4k, or 8k, or 16k.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25863#discussion_r2180106662