caigy commented on issue #8017:
URL: https://github.com/apache/rocketmq/issues/8017#issuecomment-2056368726
> > `ByteStreams.toByteArray()` creates large array in memory and is
unnecessary. It seems `FileChannel.transferFrom()` with NIO channel would be
more efficient for writing streams to a file.
>
> Thx for reply, I tried `FileChannel.transferFrom()` but find out some
unittest failed:
>
> ```
> @Test
> public void consumeQueueTest() throws ClassNotFoundException,
NoSuchMethodException {
> ...
> ByteBuffer cqItem1 = fileSegment.read(initPosition, unitSize);
> Assert.assertEquals(baseOffset, cqItem1.getLong()); // failed
> ...
> }
> ```
>
> > java.lang.AssertionError:
> > Expected :1000
> > Actual :0
>
> It turns out the cq data was not flush correctly by debugging, I wonder if
the way I use `FileChannel.transferFrom()` is wrong: Before:
>
> ```
> byte[] byteArray = ByteStreams.toByteArray(inputStream);
> writeFileChannel.position(position);
> ByteBuffer buffer = ByteBuffer.wrap(byteArray);
> while (buffer.hasRemaining()) {
> writeFileChannel.write(buffer);
> }
> writeFileChannel.force(true);
> ```
>
> After:
>
> ```
> ReadableByteChannel readableByteChannel =
Channels.newChannel(inputStream);
> writeFileChannel.transferFrom(readableByteChannel,
position, length);
> writeFileChannel.force(true);
> ```
It seems the modification is not equivalent.
`writeFileChannel.position(position)` has side effect, making
`writeFileChannel` written from `position`. But
`writeFileChannel.transferFrom()` exits when `position` is larger than the size
of `writeFileChannel`.
You might extend the file of `writeFileChannel` to `position` and also check
if the transferred bytes is equal to `length`.
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