Netty基础H1

请求响应H2

ChannelInboundHandlerAdapter 中调用ChannelHandlerContext 进行write、flush动作，然后经过一些列的OutboundHandler进行处理和转换，最终由ChannelOutboundBuffer 写到pipeline中。

writeH2

ctx.writeAndFlush 是往输出方向的下一个Handler写

ctx.channel().writeAndFlush 是从pipelined的尾部开始写

Netty中ctx.writeAndFlush与ctx.channel().writeAndFlush的区别 - 代码先锋网

Modbus基础H1

Modbus数据报文格式如下

java
/**
 * Modbus TCP Frame Description
 *  - max. 260 Byte (ADU = 7 Byte MBAP + 253 Byte PDU)
 *  - Length field includes Unit Identifier + PDU
 *
 * <----------------------------------------------- ADU -------------------------------------------------------->
 * <---------------------------- MBAP -----------------------------------------><------------- PDU ------------->
 * +------------------------+---------------------+----------+-----------------++---------------+---------------+
 * | Transaction Identifier | Protocol Identifier | Length   | Unit Identifier || Function Code | Data          |
 * | (2 Byte)               | (2 Byte)            | (2 Byte) | (1 Byte)        || (1 Byte)      | (1 - 252 Byte |
 * +------------------------+---------------------+----------+-----------------++---------------+---------------+
 * @throws Exception
 */

数据帧由MBAP和PDU构成

• MBAP 包含2个字节的事务编号、协议版本号、数据长度（包含后续的Slave 标识的长度以及PDU的长度）、Slave标识
• PDU 2个字节的Function Code和数据区，不同的Function Code对应的对应的数据区稍有不同。

实现H1

java
public class ProxyHandler extends ChannelInboundHandlerAdapter {

    private TcpMaster tcpMaster;

    public ProxyHandler(String targetHost, Integer targetPort) throws ModbusInitException {
        IpParameters ipParameters = new IpParameters();
        ipParameters.setHost(targetHost);
        ipParameters.setPort(targetPort);
        tcpMaster = (TcpMaster) new ModbusFactory().createTcpMaster(ipParameters, (short)1, true);
        tcpMaster.init();
    }
    
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        if (msg instanceof ModbusFrame) {
            ModbusFrame requestFrame = (ModbusFrame) msg;
            ModbusRequest request = (ModbusRequest) requestFrame.getMessage();
            ModbusFrame responseFrame = tcpMaster.callModbusMessageSync(request);
            ModbusHeader header = responseFrame.getHeader();
            ctx.writeAndFlush(new ModbusFrame(
                new ModbusHeader(requestFrame.getHeader().getTransactionIdentifier(),
                    header.getProtocolIdentifier(),
                    header.getLength(), 
                    header.getUnitIdentifier()), 
                responseFrame.getMessage()));
        }
    }
}