Consider the following:
```
        try (FileChannel fc = FileChannel.open(file, CREATE, WRITE)) {
            OutputStream out = Channels.newOutputStream(fc);
            byte[] b = new byte[10*1024*1024];
            out.write(b);
        }
```
This will invoke FileChannel.write with a BB that is backed by the byte array, this needs to be copied into a direct buffer to do the actual I/O. In this case, it will allocate a temporary direct buffer of 10Mb.

There is an argument that the channel implementations should limit their direct memory usage but these are low-level classes. It may be better to limit the writes to the channels instead, e.g. ChannelOutputStream.writeFully could be work like this:

```
    private void writeFully(ByteBuffer bb) throws IOException {
        int pos = bb.position();
        int rem = bb.limit() - pos;
        while (rem > 0) {
            bb.limit(pos + Integer.min(rem, DEFAULT_BUFFER_SIZE));
            int n = ch.write(bb);
            if (n <= 0)
                throw new RuntimeException("no bytes written");
            pos += n;
            rem -=n;
        }
    }
```