Eliminate unnecessary buffering and encoding conversion in BufferedWriter

[Chen Liang]

Brainstorming time:
I think Brett's suggestion makes sense: BufferedWriter now can handle a mix
of String and char[] inputs. So if the input is all LATIN1 Strings, we can
allow them to stay LATIN1 in the StringBuilder and let OutputStreamWriter
decide if the OSW has a fast path for LATIN1 when it is passed the SB
through append(CharSequence).

At my first glance, this proposal might be broken into a few actions:
1. BufferedWriter now buffers to a StringBuilder (which can consider
resetting the coder when length is reset to 0)
2. OutputStreamWriter now has a fast path for append(CharSequence csq) when
this method receives a StringBuilder - if the LATIN1 or UTF16 bytes are
compatible, we can trivially perform a write.
3. (Optional) StringBuilder can reset its coder to LATIN1 if it is called
setLength(0)
4. (Alternative) StreamEncoder can define append(CharSequence) itself to
handle StringBuilder, or even make use of the latest addition of
CharSequence.getChars (but we need to defend against untrusted arrays). We
have the ArrayEncoder interface that we might use.

However, this implies potential incompatibilities as in:
1. BufferedWriter previously only calls write(char[], int, int) to the
delegated writer. Calling other methods may introduce incompatibilities.
2. Performing checks on mutable structures, such as StringBuilder and/or
its array, is inherently risky to TOCTOU.

These are just the opportunities and the risks I have discovered on a first
quick glance. I believe the whole situation is much more than what I have
here, but we might start from some small, uncontroversial, and low-risk
gains. For example, I would consider resetting StringBuilder coder to
LATIN1 upon setLength(0) as a small gain with lower risk, though it does
not directly relate to this effort of speeding up the write buffering.

Chen

On Sun, Jun 29, 2025 at 11:51 PM wenshao <shaojin.wensj at alibaba-inc.com>
wrote:

>
> Both Writer and CharsetEncoder are designed for char[]. Converting
> BufferedWriter to use byte[] value + byte coder like StringBuilder will
> also require redundant encoding conversion when using LATIN1 String, and
> the performance will not be good.
>
> ------------------------------------------------------------------
> 发件人：Brett Okken <brett.okken.os at gmail.com>
> 发送时间：2025年6月30日(周一) 11:39
> 收件人："温绍锦(高铁)"<shaojin.wensj at alibaba-inc.com>
> 抄 送："core-libs-dev"<core-libs-dev at openjdk.org>
> 主 题：Re: Eliminate unnecessary buffering and encoding conversion in
> BufferedWriter
>
> Maybe another option would be to implement BufferedWriter with a
> StringBuilder rather than a char[]. This would remove the force to utf-16
>
> On Sun, Jun 29, 2025 at 10:36 PM Brett Okken <brett.okken.os at gmail.com>
> wrote:
> Is StreamEncoder buffering content to only write to the underlying
> OutputStream when some threshold is hit? While the layers of conversions
> are unfortunate, it seems there could be negative performance implications
> of having many extremely small writes (such as 1 character/byte) at a time
> to the underlying OutputStream.
>
> Presumably this is a common pattern, as it is recommended:
>
> https://github.com/openjdk/jdk/blob/4dd1b3a6100f9e379c7cee3c699d63d0d01144a7/src/java.base/share/classes/java/io/OutputStreamWriter.java#L45
>
>
> On Sun, Jun 29, 2025 at 11:04 AM wenshao <shaojin.wensj at alibaba-inc.com>
> wrote:
> BufferedWriter -> OutputStreamWriter -> StreamEncoder
>
> In this call chain, BufferedWriter has a char[] buffer, and StreamEncoder
> has a ByteBuffer. There are two layers of cache here, or the BufferedWriter
> layer can be removed.
>
> LATIN1 (byte[]) -> UTF16 (char[]) -> UTF8 (byte[])
>
> And when charset is UTF8, if the content of write(String) is LATIN1, a
> conversion from LATIN1 to UTF16 and then to LATIN1 will occur here.
>
> We can improve BufferedWriter. When the parameter Writer instanceof
> OutputStreamWriter is passed in, remove the cache and call it directly. In
> addition, improve write(String) in StreamEncoder to avoid unnecessary
> encoding conversion.
>
> -
> Shaojin Wen
>
>
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