RFR: 8341566: Adding factory for non-synchronized CharSequence Reader [v4]

[Jaikiran Pai]

On Tue, 8 Oct 2024 12:30:03 GMT, Chen Liang <liach at openjdk.org> wrote:

>> As the anonymous class MUST NOT be used with multiple threads, I always have seen the `CharSequence` as *fixed/static* text in the moment the `Reader` is getting used. But indeed, technically one could interleave `Reader::read()` invocations by `CharSequence.append()` (or even worse, `CharSequence.delete()`) invocations. The question is: Would that make *any* sense in the end? I mean, what happens if one has `read()` text that in the next step gets `delete()`'d? I cannot image *any* scenario where such a program would result in *useful* outcome.
>> 
>> <fun>The fact that nobody so far (before you) brought up this question seems to proof that nobody (besides you) would write such a program. 😄 </fun>
>> 
>> So I would plea for clearly saying in the JavaDocs that `cs` MUST NOT be modified before `close()` is called. Every other solution implies strange side effects and slower and error-prone implementation of both, anoynous reader *and* test.
>> 
>> @AlanBateman WDYT?
>
> I would treat this specific scenario as one of the "no concurrent usage" examples. Note that by this principle, mutable objects like `StringBuilder` should not override object comparison methods as these states can change, but they do :(

> The question is: Would that make any sense in the end?

Consider the example of `StringBuffer`, which is a `CharSequence`. Wouldn't something like the following be a logical use of `Reader.of(CharSequence)`, where you create a `Reader` for the underlying sequence (which is not yet populated) and then keep reading through the `Reader` until the underlying sequence's data is finished?


final StringBuffer sb = new StringBuffer();
try (final Reader reader = Reader.of(sb)) {
    final ContentGenerator cg = new ContentGenerator(sb);
    Thread.ofPlatform().start(cg);
    int numRead = 0;
    while (numRead != -1) {
        final char[] content = new char[1024];
        numRead = reader.read(content); // wait for content
    }
}

private class ContentGenerator {
    final StringBuffer sb;
    ContentGenerator(StringBuffer sb) {
        this.sb = sb;
    }

    @Override
    run() {
        while (true) {
            String foo = getSomeContent(); // maybe blocking
            sb.append(foo);
        }
    }
}

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PR Review Comment: https://git.openjdk.org/jdk/pull/21371#discussion_r1791789988