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本文主要研究一下rocketmq的NettyEncoder及NettyDecoder

NettyEncoder

org/apache/rocketmq/remoting/netty/NettyEncoder.java

public class NettyEncoder extends MessageToByteEncoder<RemotingCommand> {
    private static final Logger log = LoggerFactory.getLogger(RemotingHelper.ROCKETMQ_REMOTING);

    @Override
    public void encode(ChannelHandlerContext ctx, RemotingCommand remotingCommand, ByteBuf out)
        throws Exception {
        try {
            ByteBuffer header = remotingCommand.encodeHeader();
            out.writeBytes(header);
            byte[] body = remotingCommand.getBody();
            if (body != null) {
                out.writeBytes(body);
            }
        } catch (Exception e) {
            log.error("encode exception, " + RemotingHelper.parseChannelRemoteAddr(ctx.channel()), e);
            if (remotingCommand != null) {
                log.error(remotingCommand.toString());
            }
            RemotingUtil.closeChannel(ctx.channel());
        }
    }
}

• 这里继承MessageToByteEncoder，类型是RemotingCommand，先写入header再写入body

RemotingCommand.encodeHeader

org/apache/rocketmq/remoting/protocol/RemotingCommand.java

    public ByteBuffer encodeHeader() {
        return encodeHeader(this.body != null ? this.body.length : 0);
    }

    public ByteBuffer encodeHeader(final int bodyLength) {
        // 1> header length size
        int length = 4;

        // 2> header data length
        byte[] headerData;
        headerData = this.headerEncode();

        length += headerData.length;

        // 3> body data length
        length += bodyLength;

        ByteBuffer result = ByteBuffer.allocate(4 + length - bodyLength);

        // length
        result.putInt(length);

        // header length
        result.put(markProtocolType(headerData.length, serializeTypeCurrentRPC));

        // header data
        result.put(headerData);

        result.flip();

        return result;
    }

• 这里先写数据总长度，再写入header长度，最后写入header的数据，在写入body数据
• 整体结构为：Length | Header length | Header data | Body

NettyDecoder

org/apache/rocketmq/remoting/netty/NettyDecoder.java

public class NettyDecoder extends LengthFieldBasedFrameDecoder {
    private static final Logger log = LoggerFactory.getLogger(RemotingHelper.ROCKETMQ_REMOTING);

    private static final int FRAME_MAX_LENGTH =
        Integer.parseInt(System.getProperty("com.rocketmq.remoting.frameMaxLength", "16777216"));

    public NettyDecoder() {
        super(FRAME_MAX_LENGTH, 0, 4, 0, 4);
    }

    @Override
    public Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
        ByteBuf frame = null;
        try {
            frame = (ByteBuf) super.decode(ctx, in);
            if (null == frame) {
                return null;
            }

            ByteBuffer byteBuffer = frame.nioBuffer();

            return RemotingCommand.decode(byteBuffer);
        } catch (Exception e) {
            log.error("decode exception, " + RemotingHelper.parseChannelRemoteAddr(ctx.channel()), e);
            RemotingUtil.closeChannel(ctx.channel());
        } finally {
            if (null != frame) {
                frame.release();
            }
        }

        return null;
    }
}

• 这里继承LengthFieldBasedFrameDecoder，maxFrameLength为FRAME_MAX_LENGTH，lengthFieldOffset为0，lengthFieldLength为4，lengthAdjustment为0，initialBytesToStrip为4
• 获取ByteBuffer之后调用RemotingCommand.decode(byteBuffer)

RemotingCommand.decode

org/apache/rocketmq/remoting/protocol/RemotingCommand.java

    public static RemotingCommand decode(final ByteBuffer byteBuffer) {
        int length = byteBuffer.limit();
        int oriHeaderLen = byteBuffer.getInt();
        int headerLength = getHeaderLength(oriHeaderLen);

        byte[] headerData = new byte[headerLength];
        byteBuffer.get(headerData);

        RemotingCommand cmd = headerDecode(headerData, getProtocolType(oriHeaderLen));

        int bodyLength = length - 4 - headerLength;
        byte[] bodyData = null;
        if (bodyLength > 0) {
            bodyData = new byte[bodyLength];
            byteBuffer.get(bodyData);
        }
        cmd.body = bodyData;

        return cmd;
    }

    public static int getHeaderLength(int length) {
        return length & 0xFFFFFF;
    }

    private static RemotingCommand headerDecode(byte[] headerData, SerializeType type) {
        switch (type) {
            case JSON:
                RemotingCommand resultJson = RemotingSerializable.decode(headerData, RemotingCommand.class);
                resultJson.setSerializeTypeCurrentRPC(type);
                return resultJson;
            case ROCKETMQ:
                RemotingCommand resultRMQ = RocketMQSerializable.rocketMQProtocolDecode(headerData);
                resultRMQ.setSerializeTypeCurrentRPC(type);
                return resultRMQ;
            default:
                break;
        }

        return null;
    }

• 这里先获取header长度，获取header的数据，decode出来RemotingCommand，之后获取body

小结

rocketmq的netty编码解码，使用的是RemotingCommand对象，编码继承MessageToByteEncoder，解码采用的是LengthFieldBasedFrameDecoder

doc

• NettyEncoder
• NettyDecoder