RFR: 8329829: HttpClient: Add a BodyPublishers.ofFileChannel method [v8]

[Jaikiran Pai]

On Mon, 11 Aug 2025 07:47:03 GMT, Volkan Yazici <vyazici 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.
>
> Volkan Yazici has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve style for declaring multiple consecutive fields of the same type

The implementation and test changes look good to me.

I had some suggestions for the javadoc text. Here's what I had in mind, if Daniel and Michael suggest that the current form in this PR is OK, then it's OK to not change it:


/**
 * {@return a request body publisher whose body is the {@code length}
 * content bytes read from the provided file {@code channel} starting
 * from the specified {@code offset}}
 * <p>
 * This method does not modify the {@code channel}'s position.
 * <p>
 * This method does not close the {@code channel}. The caller is
 * expected to close the {@code channel} when no longer needed.
 *
 * @apiNote
 * This method can be used to either publish just a portion of a
 * file's content as the request body or to publish different portions
 * of the file's content concurrently.
 * The typical approach to concurrently publish different portions of a
 * file's content is to create an instance of {@link FileChannel}
 * and then create multiple {@code HttpRequest}s each of which use
 * a {@code ofFileChannel BodyPublisher} with a different non-overlapping
 * offset and length.
 *
 * @param channel a file channel
 * @param offset the offset of the first byte
 * @param length the number of bytes to read from the file channel
 *
 * @throws IndexOutOfBoundsException if the specified byte range is
 * found to be {@linkplain Objects#checkFromIndexSize(long, long, long)
 * out of bounds} compared with the size of the file referred by the
 * channel
 *
 * @throws IOException if the size of the {@code channel} cannot be
 *                      determined or the {@code channel} is closed
 *
 * @throws NullPointerException if {@code channel} is null
 *
 * @since 26
 */



(Apart from the text changes, do note that the `@throws IOException` specification in that text is intentionally changed to note that it will be thrown if the given channel is closed, in addition to if the size() cannot be determined).

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PR Comment: https://git.openjdk.org/jdk/pull/26155#issuecomment-3199431572