RFR: 8329829: HttpClient: Add a BodyPublishers.ofFileChannel method

[Daniel Fuchs]

On Mon, 7 Jul 2025 08:29:40 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

>> Adds a new `ofFileChannel(FileChannel channel, long offset, long length)` method to `java.net.HttpRequest.BodyPublishers` to provide an `HttpClient` publisher to upload a certain region of a file. The new publisher does not modify the state of the passed `FileChannel`, streams the file channel bytes as it publishes (i.e., avoids reading the entire file into the memory), and can be leveraged to implement sliced uploads. As noted in the Javadoc:
>> 
>>> The file channel will not be closed upon completion. The caller is
>>> expected to manage the life cycle of the channel, and close it
>>> appropriately when not needed anymore.
>> 
>> ### Implementation notes
>> 
>> - `FileChannel` is preferred over `{Readable,Seekable}ByteChannel`, since the latter does not provide a positional read without modifying the state of the `FileChannel`, which is necessary to use a single `FileChannel` instance to implement sliced uploads.
>> - `ofFileChannel(FileChannel,long,long)` is preferred over `ofPath(Path,long,long)` to avoid overloading the maximum file descriptor limit of the platform.
>
> src/java.net.http/share/classes/java/net/http/HttpRequest.java line 739:
> 
>> 737:          * @param channel a file channel
>> 738:          * @param offset the offset of the first byte
>> 739:          * @param length the number of bytes to use
> 
> I'd suggest to say something like:
>> the number of bytes to send
> 
> or
>> the number of bytes to read from the file channel

It might be also useful to specify what happens if length is zero or negative. Either here or in the `@throws` below if an exception is going to be thrown.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26155#discussion_r2189360176