Netty源码阅读入门实战(八)-解码(更新 ing)

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        //基于 ByteBuf 进行解码的,如果不是直接将当前对象向下传播
        if (msg instanceof ByteBuf) {
            CodecOutputList out = CodecOutputList.newInstance();
            try {
                ByteBuf data = (ByteBuf) msg;
                //若当前累加器为空,说明是第一次从 IO 流中读取数据
                first = cumulation == null;
                if (first) {
                    //第一次会将累加器赋值为刚读进来的 ByteBuf 对象数据
                    cumulation = data;
                } else {
                    //非第一次,则将当前累加器中的数据和读取进来的数据进行累加
                    cumulation = cumulator.cumulate(ctx.alloc(), cumulation, data);
                }
                //调用子类的解码方法去解析
                callDecode(ctx, cumulation, out);
            } catch (DecoderException e) {
                throw e;
            } catch (Throwable t) {
                throw new DecoderException(t);
            } finally {
                if (cumulation != null && !cumulation.isReadable()) {
                    numReads = 0;
                    cumulation.release();
                    cumulation = null;
                } else if (++ numReads >= discardAfterReads) {
                    // We did enough reads already try to discard some bytes so we not risk to see a OOME.
                    // See https://github.com/netty/netty/issues/4275
                    numReads = 0;
                    discardSomeReadBytes();
                }

                int size = out.size();
                decodeWasNull = !out.insertSinceRecycled();
                fireChannelRead(ctx, out, size);
                out.recycle();
            }
        } else {
            ctx.fireChannelRead(msg);
        }
    }

    public static final Cumulator MERGE_CUMULATOR = new Cumulator() {
        @Override
        public ByteBuf cumulate(ByteBufAllocator alloc, ByteBuf cumulation, ByteBuf in) {
            ByteBuf buffer;
            //当前的写指针后移一定字节,若超过最大容量,则进行扩容
            if (cumulation.writerIndex() > cumulation.maxCapacity() - in.readableBytes()
                    || cumulation.refCnt() > 1) {
                // Expand cumulation (by replace it) when either there is not more room in the buffer
                // or if the refCnt is greater then 1 which may happen when the user use slice().retain() or
                // duplicate().retain().
                //
                // See:
                // - https://github.com/netty/netty/issues/2327
                // - https://github.com/netty/netty/issues/1764
                buffer = expandCumulation(alloc, cumulation, in.readableBytes());
            } else {
                buffer = cumulation;
            }
            //将当前数据写到累加器中
            buffer.writeBytes(in);
            in.release();
            return buffer;
        }
    };