RFR: 8369564: Provide a MemorySegment API to read strings with known lengths [v16]

[Hannes Greule]

On Sat, 10 Jan 2026 18:51:06 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 21 additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8369564
>  - Rename the parameter of getString from length to byteLength
>  - Merge branch 'master' into JDK-8369564
>  - Review feedback
>  - Update discussion of truncated reads of strings containing \0
>  - Return the number of copied bytes
>  - More javadoc updates
>  - Use Utils.checkNonNegativeArgument
>  - Review feedback
>    
>    * handle numChars + srcIndex overflow, and add tests
>    * replace yen with a character that round trips
>  - Improve test coverage, and more fixes
>  - ... and 11 more: https://git.openjdk.org/jdk/compare/87751392...e2cc6f0b

src/java.base/share/classes/java/lang/foreign/MemorySegment.java line 1351:

> 1349:      *         largest string supported by the platform
> 1350:      * @throws IndexOutOfBoundsException if {@code offset < 0}
> 1351:      * @throws IndexOutOfBoundsException if {@code offset > byteSize() - length}

Suggestion:

     * @throws IndexOutOfBoundsException if {@code offset > byteSize() - byteLength}

I assume this was missed?

src/java.base/share/classes/java/lang/foreign/MemorySegment.java line 1356:

> 1354:      * @throws WrongThreadException if this method is called from a thread {@code T},
> 1355:      *         such that {@code isAccessibleBy(T) == false}
> 1356:      * @throws IllegalArgumentException if {@code length < 0}

Suggestion:

     * @throws IllegalArgumentException if {@code byteLength < 0}

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2679311350
PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2679311392