public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
    unsafe.write(msg, promise);
}

public final void write(Object msg, ChannelPromise promise) {
    assertEventLoop();
    //負(fù)責(zé)緩沖寫進(jìn)來的byteBuf
    ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
    if (outboundBuffer == null) {
        safeSetFailure(promise, WRITE_CLOSED_CHANNEL_EXCEPTION);
        ReferenceCountUtil.release(msg);
        return;
    }
    int size;
    try {
        //非堆外內(nèi)存轉(zhuǎn)化為堆外內(nèi)存
        msg = filterOutboundMessage(msg);
        size = pipeline.estimatorHandle().size(msg);
        if (size < 0) {
            size = 0;
        }
    } catch (Throwable t) {
        safeSetFailure(promise, t);
        ReferenceCountUtil.release(msg);
        return;
    }
    //插入寫隊(duì)列
    outboundBuffer.addMessage(msg, size, promise);
}

protected final Object filterOutboundMessage(Object msg) {
    if (msg instanceof ByteBuf) {
        ByteBuf buf = (ByteBuf) msg;
        //是堆外內(nèi)存, 直接返回
        if (buf.isDirect()) {
            return msg;
        }
        return newDirectBuffer(buf);
    }
    if (msg instanceof FileRegion) {
        return msg;
    }
    throw new UnsupportedOperationException(
            "unsupported message type: " + StringUtil.simpleClassName(msg) + EXPECTED_TYPES);
}

public void addMessage(Object msg, int size, ChannelPromise promise) {
    Entry entry = Entry.newInstance(msg, size, total(msg), promise); 
    if (tailEntry == null) { 
        flushedEntry = null;
        tailEntry = entry;
    } else { 
        Entry tail = tailEntry;
        tail.next = entry;
        tailEntry = entry;
    } 
    if (unflushedEntry == null) { 
        unflushedEntry = entry;
    }
    incrementPendingOutboundBytes(size, false);
}

private void incrementPendingOutboundBytes(long size, boolean invokeLater) {
    if (size == 0) {
        return;
    }
    //TOTAL_PENDING_SIZE_UPDATER當(dāng)前緩沖區(qū)里面有多少待寫的字節(jié)
    long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, size);
    //getWriteBufferHighWaterMark() 最高不能超過64k
    if (newWriteBufferSize > channel.config().getWriteBufferHighWaterMark()) {
        setUnwritable(invokeLater);
    }
}

long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, size)

private void setUnwritable(boolean invokeLater) {
    for (;;) {
        final int oldValue = unwritable;
        final int newValue = oldValue | 1; 
        if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
            if (oldValue == 0 && newValue != 0) { 
                fireChannelWritabilityChanged(invokeLater);
            }
            break;
        }
    }
}