RFR: 8372353: API to compute the byte length of a String encoded in a given Charset

[ExE Boss]

On Fri, 21 Nov 2025 14:58:55 GMT, Liam Miller-Cushon <cushon at openjdk.org> wrote:

> This implements an API to return the byte length of a String encoded in a given charset. See [JDK-8372353](https://bugs.openjdk.org/browse/JDK-8372353) for background.
> 
> ---
> 
> 
> Benchmark                              (encoding)  (stringLength)   Mode  Cnt          Score          Error  Units
> StringLoopJmhBenchmark.getBytes             ASCII              10  thrpt    5  406782650.595 ± 16960032.852  ops/s
> StringLoopJmhBenchmark.getBytes             ASCII             100  thrpt    5  172936926.189 ±  4532029.201  ops/s
> StringLoopJmhBenchmark.getBytes             ASCII            1000  thrpt    5   38830681.232 ±  2413274.766  ops/s
> StringLoopJmhBenchmark.getBytes             ASCII          100000  thrpt    5     458881.155 ±    12818.317  ops/s
> StringLoopJmhBenchmark.getBytes            LATIN1              10  thrpt    5   37193762.990 ±  3962947.391  ops/s
> StringLoopJmhBenchmark.getBytes            LATIN1             100  thrpt    5   55400876.236 ±  1267331.434  ops/s
> StringLoopJmhBenchmark.getBytes            LATIN1            1000  thrpt    5   11104514.001 ±    41718.545  ops/s
> StringLoopJmhBenchmark.getBytes            LATIN1          100000  thrpt    5     182535.414 ±    10296.120  ops/s
> StringLoopJmhBenchmark.getBytes             UTF16              10  thrpt    5  113474681.457 ±  8326589.199  ops/s
> StringLoopJmhBenchmark.getBytes             UTF16             100  thrpt    5   37854103.127 ±  4808526.773  ops/s
> StringLoopJmhBenchmark.getBytes             UTF16            1000  thrpt    5    4139833.009 ±    70636.784  ops/s
> StringLoopJmhBenchmark.getBytes             UTF16          100000  thrpt    5      57644.637 ±     1887.112  ops/s
> StringLoopJmhBenchmark.getBytesLength       ASCII              10  thrpt    5  946701647.247 ± 76938927.141  ops/s
> StringLoopJmhBenchmark.getBytesLength       ASCII             100  thrpt    5  396615374.479 ± 15167234.884  ops/s
> StringLoopJmhBenchmark.getBytesLength       ASCII            1000  thrpt    5  100464784.979 ±   794027.897  ops/s
> StringLoopJmhBenchmark.getBytesLength       ASCII          100000  thrpt    5    1215487.689 ±     1916.468  ops/s
> StringLoopJmhBenchmark.getBytesLength      LATIN1              10  thrpt    5  221265102.323 ± 17013983.056  ops/s
> StringLoopJmhBenchmark.getBytesLength      LATIN1             100  thrpt    5  137617873.887 ±  5842185.781  ops/s
> StringLoopJmhBenchmark.getBytesLength      LATIN1            1000  thrpt    5   92540259.130 ±  3839233.582  ops/s
> StringLoopJmhBenchmark.ge...

src/java.base/share/classes/java/lang/String.java line 2127:

> 2125:      *          equivalent to this string, {@code false} otherwise
> 2126:      *
> 2127:      * @see  #compareTo(String)

For the **BOM**‑less **UTF‑16** charsets, this can simply return `value.length << (1 ‑ coder())`[^1]:

Suggestion:

        if (cs instanceof sun.nio.cs.UTF_16LE ||
            cs instanceof sun.nio.cs.UTF_16BE) {
            return value.length << (1 - coder());
        }
        return getBytes(cs).length;


[^1]: Lone surrogates get replaced with `U+FFFD` when encoding to **UTF‑16** by [`String​::getBytes​(Charset)`], and all of **LATIN1** can be encoded in **UTF‑16**.

[`String​::getBytes​(Charset)`]: https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/String.html#getBytes(java.nio.charset.Charset)

test/micro/org/openjdk/bench/java/lang/foreign/StringLoopJmhBenchmark.java line 74:

> 72:       stringData += (char) (Math.random() * 26) + 'a';
> 73:     }
> 74:     stringData += c;

Maybe avoid creating intermediate strings in a loop to avoid excess GC pressure?
Suggestion:

    var stringDataBuilder = new StringBuilder(stringLength + 1);
    while (stringDataBuilder.length() < stringLength) {
      stringDataBuilder.append((char) (Math.random() * 26) + 'a');
    }
    stringData = stringDataBuilder.append(c).toString();

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28454#discussion_r2552768341
PR Review Comment: https://git.openjdk.org/jdk/pull/28454#discussion_r2552801055