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 1341:

> 1339:      * @param length  length in bytes of the string to read
> 1340:      * @return a Java string constructed from the bytes read from the given starting
> 1341:      *         address reading the given length of bytes

Maybe:


a Java string constructed from the bytes read from the given starting address up to the given length


(that seems to match the existing `getString` a bit more, and avoids the reading/read repetition)

src/java.base/share/classes/java/lang/foreign/SegmentAllocator.java line 157:

> 155: 
> 156:     /**
> 157:      * Converts a Java string into a C string using the provided charset,

I think using the term `C string` here is misleading. 

I suggest:


Encodes a Java string using the provided charset and stores the resulting byte array into a memory segment.

src/java.base/share/classes/java/lang/foreign/SegmentAllocator.java line 170:

> 168:      * will appear truncated when read again.
> 169:      *
> 170:      * @param str      the Java string to be converted into a C string

Suggestion:

     * @param str      the Java string to be encoded

src/java.base/share/classes/java/lang/foreign/SegmentAllocator.java line 175:

> 173:      * @param srcIndex the starting index of the source string
> 174:      * @param numChars the number of characters to be copied
> 175:      * @return a new native segment containing the converted C string

Watch out for C string again

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549976060
PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549982704
PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549985304
PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549990113