• 这里为什么只存 packetLength 的低 5 位到 body 中？

public SendHeaderFrame(short identifier, SendPacket packet) {
    super(PACKET_HEADER_FRAME_MIN_LENGTH,
            Frame.TYPE_PACKET_HEADER,
            Frame.FLAG_NONE,
            identifier,
            packet);

    final long packetLength = packet.length();
    final byte packetType = packet.type();
    final byte[] packetHeaderInfo = packet.headerInfo();

    // 头部对应的数据信息长度
    body = new byte[bodyRemaining];

    // 头5字节存储长度信息低5字节（40位）数据
    // 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
    body[0] = (byte) (packetLength >> 32);
    body[1] = (byte) (packetLength >> 24);
    body[2] = (byte) (packetLength >> 16);
    body[3] = (byte) (packetLength >> 8);
    body[4] = (byte) (packetLength);

    body[5] = packetType;

    if (packetHeaderInfo != null) {
        System.arraycopy(packetHeaderInfo, 0,
                body, PACKET_HEADER_FRAME_MIN_LENGTH, packetHeaderInfo.length);
    }
}