Netty源代码之FrameDecoder

FrameDecoder说白了是解码器，可以将网络数据一帧一帧地解码出来

主要看起messageReceived

 public void messageReceived(            ChannelHandlerContext ctx, MessageEvent e) throws Exception {        Object m = e.getMessage();        if (!(m instanceof ChannelBuffer)) {            ctx.sendUpstream(e);            return;        }        ChannelBuffer input = (ChannelBuffer) m;        if (!input.readable()) {            return;        }        if (cumulation == null) {            try {                // the cumulation buffer is not created yet so just pass the input to callDecode(...) method                callDecode(ctx, e.getChannel(), input, e.getRemoteAddress());            } finally {                updateCumulation(ctx, input);            }        } else {            input = appendToCumulation(input);            try {                callDecode(ctx, e.getChannel(), input, e.getRemoteAddress());            } finally {                updateCumulation(ctx, input);            }        }    }

这里这个cumulation是关键，cumulation的是"累计的意思"，即将数据累计起来，首先判断cululation是不是null,是null的话直接调用callDecode方法，不然先将input添加到culutation

callDecode()

    private void callDecode(            ChannelHandlerContext context, Channel channel,            ChannelBuffer cumulation, SocketAddress remoteAddress) throws Exception {        while (cumulation.readable()) {            int oldReaderIndex = cumulation.readerIndex();            Object frame = decode(context, channel, cumulation);            if (frame == null) {                if (oldReaderIndex == cumulation.readerIndex()) {                    // Seems like more data is required.                    // Let us wait for the next notification.                    break;                } else {                    // Previous data has been discarded.                    // Probably it is reading on.                    continue;                }            } else if (oldReaderIndex == cumulation.readerIndex()) {                throw new IllegalStateException(                        "decode() method must read at least one byte " +                        "if it returned a frame (caused by: " + getClass() + ")");            }            unfoldAndFireMessageReceived(context, remoteAddress, frame);        }    }

在unfoldAndFireMessageReceived()里面，会调用Channels.fireMessageReceived（），最后调用ctx.sendUpstream，事件会交给下一个Handler来处理

            ChannelHandlerContext context, SocketAddress remoteAddress, Object result) {        if (unfold) {            if (result instanceof Object[]) {                for (Object r: (Object[]) result) {                    Channels.fireMessageReceived(context, r, remoteAddress);                }            } else if (result instanceof Iterable<?>) {                for (Object r: (Iterable<?>) result) {                    Channels.fireMessageReceived(context, r, remoteAddress);                }            } else {                Channels.fireMessageReceived(context, result, remoteAddress);            }        } else {            Channels.fireMessageReceived(context, result, remoteAddress);        }    }

appendToCumulation（）这里可以看出cumulation是CompositeChannelBuffer,当之前还有buffer没处理完时，与这次的buffer合并一起进行解码

    protected ChannelBuffer appendToCumulation(ChannelBuffer input) {        ChannelBuffer cumulation = this.cumulation;        assert cumulation.readable();        if (cumulation instanceof CompositeChannelBuffer) {            // Make sure the resulting cumulation buffer has no more than the configured components.            CompositeChannelBuffer composite = (CompositeChannelBuffer) cumulation;            if (composite.numComponents() >= maxCumulationBufferComponents) {                cumulation = composite.copy();            }        }        this.cumulation = input = ChannelBuffers.wrappedBuffer(cumulation, input);        return input;    }

另外一个函数时finally里面的updateCumulation（）

    protected ChannelBuffer updateCumulation(ChannelHandlerContext ctx, ChannelBuffer input) {        ChannelBuffer newCumulation;        int readableBytes = input.readableBytes();        if (readableBytes > 0) {            int inputCapacity = input.capacity();            // If input.readableBytes() == input.capacity() (i.e. input is full),            // there's nothing to save from creating a new cumulation buffer            // even if input.capacity() exceeds the threshold, because the new cumulation            // buffer will have the same capacity and content with input.            if (readableBytes < inputCapacity && inputCapacity > copyThreshold) {                // At least one byte was consumed by callDecode() and input.capacity()                // exceeded the threshold.                cumulation = newCumulation = newCumulationBuffer(ctx, input.readableBytes());                cumulation.writeBytes(input);            } else {                // Nothing was consumed by callDecode() or input.capacity() did not                // exceed the threshold.                if (input.readerIndex() != 0) {                    cumulation = newCumulation = input.slice();                } else {                    cumulation = newCumulation = input;                }            }        } else {            cumulation = newCumulation = null;        }        return newCumulation;    }

在这个函数里面会对cumulation进行操作修改。这个是在finally，也就是对剩余的数据进行暂时的存取，因为不管是这个buffer比一帧数据大或者小，都不能只是一帧数据，少于一帧不能解码出来，大于一帧势必会有一部分需要暂时存取