On Sun, 5 Nov 2023 13:32:20 GMT, ExE Boss <d...@openjdk.org> wrote:
>> Strings, after construction, are immutable but may be constructed from
>> mutable arrays of bytes, characters, or integers.
>> The string constructors should guard against the effects of mutating the
>> arrays during construction that might invalidate internal invariants for the
>> correct behavior of operations on the resulting strings. In particular, a
>> number of operations have optimizations for operations on pairs of latin1
>> strings and pairs of non-latin1 strings, while operations between latin1 and
>> non-latin1 strings use a more general implementation.
>>
>> The changes include:
>>
>> - Adding a warning to each constructor with an array as an argument to
>> indicate that the results are indeterminate
>> if the input array is modified before the constructor returns.
>> The resulting string may contain any combination of characters sampled
>> from the input array.
>>
>> - Ensure that strings that are represented as non-latin1 contain at least
>> one non-latin1 character.
>> For latin1 inputs, whether the arrays contain ASCII, ISO-8859-1, UTF8, or
>> another encoding decoded to latin1 the scanning and compression is unchanged.
>> If a non-latin1 character is found, the string is represented as
>> non-latin1 with the added verification that a non-latin1 character is
>> present at the same index.
>> If that character is found to be latin1, then the input array has been
>> modified and the result of the scan may be incorrect.
>> Though a ConcurrentModificationException could be thrown, the risk to an
>> existing application of an unexpected exception should be avoided.
>> Instead, the non-latin1 copy of the input is re-scanned and compressed;
>> that scan determines whether the latin1 or the non-latin1 representation is
>> returned.
>>
>> - The methods that scan for non-latin1 characters and their intrinsic
>> implementations are updated to return the index of the non-latin1 character.
>>
>> - String construction from StringBuilder and CharSequence must also be
>> guarded as their contents may be modified during construction.
>
> src/java.base/share/classes/java/lang/String.java line 566:
>
>> 564: }
>> 565: // Decode with a stable copy, to be the result if the
>> decoded length is the same
>> 566: byte[] latin1 = Arrays.copyOfRange(bytes, offset,
>> offset + length);
>
> This has to be moved before the `if (dp == length) { … }` check, as that also
> does a copy:
>
> // Decode with a stable copy, to be the result if the decoded
> length is the same
> byte[] latin1 = Arrays.copyOfRange(bytes, offset, offset +
> length);
> int dp = StringCoding.countPositives(latin1, offset, length);
> if (dp == length) {
> this.value = latin1;
> this.coder = LATIN1;
> return;
> }
That may look like an improvement, to share common code, but it results in a
performance hit in the normal case.
The best performing case is to copy and return immediately.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/16425#discussion_r1383539522
