RFR: 8369564: Provide a MemorySegment API to read strings with known lengths [v11]

[Maurizio Cimadamore]

On Fri, 21 Nov 2025 14:26:00 GMT, Liam Miller-Cushon <cushon at openjdk.org> wrote:

>> This PR proposes adding a new overload to `MemorySegment::getString` that takes a known byte length of the content.
>> 
>> This was previously proposed in https://github.com/openjdk/jdk/pull/20725, but the outcome of [JDK-8333843](https://bugs.openjdk.org/browse/JDK-8333843) was to update `MemorySegment#getString` to suggest
>> 
>> 
>>     byte[] bytes = new byte[length];
>>     MemorySegment.copy(segment, JAVA_BYTE, offset, bytes, 0, length);
>>     return new String(bytes, charset);
>> 
>> 
>> However this is less efficient than what the implementation of getString does after [JDK-8362893](https://bugs.openjdk.org/browse/JDK-8362893), it now uses `JavaLangAccess::uncheckedNewStringNoRepl` to avoid the copy.
>> 
>> See also discussion in [this panama-dev@ thread](https://mail.openjdk.org/pipermail/panama-dev/2025-November/021193.html), and mcimadamore's document [Pulling the (foreign) string](https://cr.openjdk.org/~mcimadamore/panama/strings_ffm.html)
>> 
>> Benchmark results:
>> 
>> 
>> Benchmark                                 (size)  Mode  Cnt    Score   Error  Units
>> ToJavaStringTest.jni_readString                5  avgt   30   55.339 ± 0.401  ns/op
>> ToJavaStringTest.jni_readString               20  avgt   30   59.887 ± 0.295  ns/op
>> ToJavaStringTest.jni_readString              100  avgt   30   84.288 ± 0.419  ns/op
>> ToJavaStringTest.jni_readString              200  avgt   30  119.275 ± 0.496  ns/op
>> ToJavaStringTest.jni_readString              451  avgt   30  193.106 ± 1.528  ns/op
>> ToJavaStringTest.panama_copyLength             5  avgt   30    7.348 ± 0.048  ns/op
>> ToJavaStringTest.panama_copyLength            20  avgt   30    7.440 ± 0.125  ns/op
>> ToJavaStringTest.panama_copyLength           100  avgt   30   11.766 ± 0.058  ns/op
>> ToJavaStringTest.panama_copyLength           200  avgt   30   16.096 ± 0.089  ns/op
>> ToJavaStringTest.panama_copyLength           451  avgt   30   25.844 ± 0.054  ns/op
>> ToJavaStringTest.panama_readString             5  avgt   30    5.857 ± 0.046  ns/op
>> ToJavaStringTest.panama_readString            20  avgt   30    7.750 ± 0.046  ns/op
>> ToJavaStringTest.panama_readString           100  avgt   30   14.109 ± 0.187  ns/op
>> ToJavaStringTest.panama_readString           200  avgt   30   18.035 ± 0.130  ns/op
>> ToJavaStringTest.panama_readString           451  avgt   30   35.896 ± 0.227  ns/op
>> ToJavaStringTest.panama_readStringLength       5  avgt   30    4.565 ± 0.038  ns/op
>> ToJavaStringTest.panama_readStringLength      20...
>
> Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use Utils.checkNonNegativeArgument

src/java.base/share/classes/java/lang/foreign/MemorySegment.java line 1333:

> 1331:      * sequences with this charset's default replacement string. The {@link
> 1332:      * java.nio.charset.CharsetDecoder} class should be used when more control
> 1333:      * over the decoding process is required.

Should we say here, as you did for `copy` that this method ignores `\0` ?

src/java.base/share/classes/java/lang/foreign/MemorySegment.java line 2646:

> 2644:      * will appear truncated when read again.
> 2645:      *
> 2646:      * @param src      the Java string to be written into this segment

Suggestion:

     * @param src      the Java string to be written into the destination segment

src/java.base/share/classes/java/lang/foreign/MemorySegment.java line 2649:

> 2647:      * @param dstEncoding the charset used to {@linkplain Charset#newEncoder() encode}
> 2648:      *                 the string bytes.
> 2649:      * @param srcIndex the starting index of the source string

Do we feel like saying words like "index" is good enough to say what we mean? E.g. by index we mean something that is compatible with `length` and `charAt`. I wonder if using a qualifier e.g. `character index` might help? (open to suggestions here, I'm not 100% sure)

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549950855
PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549953662
PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549945644