定長(zhǎng)協(xié)議

1 FixedLengthFrameDecoder簡(jiǎn)介

FixedLengthFrameDecoder采用的是定長(zhǎng)協(xié)議：即把固定的長(zhǎng)度的字節(jié)數(shù)當(dāng)做一個(gè)完整的消息

例如，我們規(guī)定每3個(gè)字節(jié)，表示一個(gè)有效報(bào)文，如果我們分4次總共發(fā)送以下9個(gè)字節(jié)：

   +---+----+------+----+
   | A | BC | DEFG | HI |
   +---+----+------+----+

那么通過(guò)FixedLengthFrameDecoder解碼后，實(shí)際上只會(huì)解析出來(lái)3個(gè)有效報(bào)文

   +-----+-----+-----+
   | ABC | DEF | GHI |
   +-----+-----+-----+

FixedLengthFrameDecodert提供了以下構(gòu)造方法

public FixedLengthFrameDecoder(int frameLength) {
    if (frameLength <= 0) {
        throw new IllegalArgumentException(
                "frameLength must be a positive integer: " + frameLength);
    }
    this.frameLength = frameLength;
}

其中：frameLength就是我們指定的長(zhǎng)度。

需要注意的是FixedLengthFrameDecoder并沒(méi)有提供一個(gè)對(duì)應(yīng)的編碼器，因?yàn)榻邮辗街恍枰鶕?jù)字節(jié)數(shù)進(jìn)行判斷即可，發(fā)送方無(wú)需編碼

2 FixedLengthFrameDecoder使用案例

server端：FixedLengthFrameDecoderServer

public class FixedLengthFrameDecoderServer {
    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup(); // (1)
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap(); // (2)
            b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class) // (3)
                    .childHandler(new ChannelInitializer<SocketChannel>() { // (4)
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline().addLast(new FixedLengthFrameDecoder(3));
                            // 自定義這個(gè)ChannelInboundHandler打印拆包后的結(jié)果
                            ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
                                @Override
                                public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
                                    if (msg instanceof ByteBuf) {
                                        ByteBuf packet = (ByteBuf) msg;
                                        System.out.println(
                                                new Date().toLocaleString() + ":" + packet.toString(Charset.defaultCharset()));
                                    }
                                }
                            });
                        }
                    });
            // Bind and start to accept incoming connections.
            ChannelFuture f = b.bind(8080).sync(); // (7)
            System.out.println("FixedLengthFrameDecoderServer Started on 8080...");
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }
}

Client端：FixedLengthFrameDecoderClient

public class FixedLengthFrameDecoderClient {
   public static void main(String[] args) throws Exception {
      EventLoopGroup workerGroup = new NioEventLoopGroup();
      try {
         Bootstrap b = new Bootstrap(); // (1)
         b.group(workerGroup); // (2)
         b.channel(NioSocketChannel.class); // (3)
         b.option(ChannelOption.SO_KEEPALIVE, true); // (4)
         b.handler(new ChannelInitializer<SocketChannel>() {
            @Override
            public void initChannel(SocketChannel ch) throws Exception {
               ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
                   //在于server建立連接后，即發(fā)送請(qǐng)求報(bào)文
                  public void channelActive(ChannelHandlerContext ctx) {
                     ByteBuf A = Unpooled.buffer().writeBytes("A".getBytes());
                     ByteBuf BC = Unpooled.buffer().writeBytes("BC".getBytes());
                     ByteBuf DEFG = Unpooled.buffer().writeBytes("DEFG".getBytes());
                     ByteBuf HI = Unpooled.buffer().writeBytes("HI".getBytes());
                     ctx.writeAndFlush(A);
                     ctx.writeAndFlush(BC);
                     ctx.writeAndFlush(DEFG);
                     ctx.writeAndFlush(HI);
                    }
               });
            }
         });
         // Start the client.
         ChannelFuture f = b.connect("127.0.0.1",8080).sync(); // (5)
         // Wait until the connection is closed.
         f.channel().closeFuture().sync();
      } finally {
         workerGroup.shutdownGracefully();
      }
   }
}

先后運(yùn)行server端與client端后，server端控制臺(tái)輸出

LineBasedFrameDecoderServer Started on 8080...
2018-9-8 15:20:20:ABC
2018-9-8 15:20:20:DEF
2018-9-8 15:20:20:GHI

可以看到FixedLengthFrameDecoder的確將請(qǐng)求的數(shù)據(jù)，按照每3個(gè)字節(jié)當(dāng)做一個(gè)完整的請(qǐng)求報(bào)文。

通常情況下，很少有client與server交互時(shí)，直接使用定長(zhǎng)協(xié)議，可能會(huì)造成浪費(fèi)。例如你實(shí)際要發(fā)送的實(shí)際只有3個(gè)字節(jié)，但是定長(zhǎng)協(xié)議設(shè)置的1024，那么可能你就要為這3個(gè)字節(jié)基礎(chǔ)上，在加1021個(gè)空格，以便server端可以解析這個(gè)請(qǐng)求。在下一節(jié)我們將要介紹的LengthFieldBasedFrameDecoder，支持動(dòng)態(tài)指定報(bào)文的長(zhǎng)度。