UDP 广播

面向连接的传输（如 TCP）管理两个网络端点之间的连接的建立，在连接的生命周期的有序和可靠的消息传输，以及最后，连接的有序终止。相比之下，类似 UDP 的无连接协议中则没有持久化连接的概念，此外，UDP 也没有 TCP 的纠错机制。但 UDP 的性能比 TCP 要好很多，适合那些能够处理或者忍受消息丢失的应用程序

目前为止，我们所有的例子都是采用一种叫作单播的传输模式，定义为发送消息给一个由唯一地址所标识的单一的网络目的地。面向连接的协议和无连接协议都支持这种模式

UDP 提供了向多个接收者发送消息的额外传输模式：

• 多播：传输到一个预定义的主机组
• 广播：传输到网络（子网）上的所有主机

本章示例将通过发送能够被同一个网络中的所有主机接收的消息来演示 UDP 广播的使用

UDP 示例应用程序

我们的程序将打开一个文件，随后通过 UDP 把每一行作为一个消息广播到一个指定的端口。而接收方只需简单地在指定端口上启动一个监听程序，便可以创建一个事件监视器来接受消息。本次示例以日志文件处理程序为例

1. 消息 POJO：LogEvent

在这个应用程序中，我们将会把消息作为事件处理，并且由于该数据来自于日志文件，所以我们将它称为 LogEvent

public class LogEvent {

    public static final byte SEPARATOR = (byte) ':';
    private final InetSocketAddress source;
    private final String logfile;
    private final String msg;
    private final long received;

    public LogEvent(String logfile, String msg) {
        this(null, logfile, msg, -1);
    }

    public LogEvent(InetSocketAddress source, String logfile, String msg, long received) {
        this.source = source;
        this.logfile = logfile;
        this.msg = msg;
        this.received = received;
    }

    public InetSocketAddress getSource() {
        return source;
    }

    public String getLogfile() {
        return logfile;
    }

    public String getMsg() {
        return msg;
    }

    public long getReceived() {
        return received;
    }
}

2. 编写广播者

Netty 的 DatagramPacket 是一个简单的消息容器，DatagramChannel 实现和远程节点的通信，要将 LogEvent 消息转换为 DatagramPacket，我们需要一个编码器

下述是编码器的代码实现

public class LogEventEncoder extends MessageToMessageEncoder<LogEvent> {

    private final InetSocketAddress remoteAddress;

    public LogEventEncoder(InetSocketAddress remoteAddress) {
        this.remoteAddress = remoteAddress;
    }

    @Override
    protected void encode(ChannelHandlerContext ctx, LogEvent logEvent, List<Object> out) throws Exception {
        byte[] file = logEvent.getLogfile().getBytes(StandardCharsets.UTF_8);
        byte[] msg = logEvent.getMsg().getBytes(StandardCharsets.UTF_8);
        ByteBuf buf = ctx.alloc().buffer(file.length + msg.length + 1);
        buf.writeBytes(file);
        buf.writeByte(LogEvent.SEPARATOR);
        buf.writeBytes(msg);
        out.add(new DatagramPacket(buf, remoteAddress));
    }
}

接下来准备引导该服务器，包括设置 ChannelOption，以及在 ChannelPipeline 中安装所需的 ChannelHandler，这部分通过主类 LogEventBroadcaster 完成

public class LogEventBroadcaster {

    private final EventLoopGroup group;
    private final Bootstrap bootstrap;
    private final File file;

    public LogEventBroadcaster(InetSocketAddress address, File file) {
        group = new NioEventLoopGroup();
        bootstrap = new Bootstrap();
        bootstrap.group(group).channel(NioDatagramChannel.class)
                .option(ChannelOption.SO_BROADCAST, true)
                .handler(new LogEventEncoder(address));
        this.file = file;
    }

    public void run() throws Exception {
        // 绑定 Channel
        Channel ch = bootstrap.bind(0).sync().channel();
        long pointer = 0;
        for (; ; ) {
            long len = file.length();
            if (len < pointer) {
                // 将文件指针指向文件的最后一个字节
                pointer = len;
            } else if (len > pointer) {
                RandomAccessFile raf = new RandomAccessFile(file, "r");
                // 设置当前文件指针
                raf.seek(pointer);
                String line;
                while ((line = raf.readLine()) != null) {
                    ch.writeAndFlush(new LogEvent(null, line, file.getAbsolutePath(), -1));
                }
                pointer = raf.getFilePointer();
                raf.close();
            }
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                Thread.interrupted();
                break;
            }
        }
    }

    public void stop() {
        group.shutdownGracefully();
    }

    public static void main(String[] args) throws Exception {
        if (args.length != 2) {
            throw new InterruptedException();
        }
        LogEventBroadcaster broadcaster = new LogEventBroadcaster(new InetSocketAddress("255.255.255.255",
                Integer.parseInt(args[0])), new File(args[1]));
        try {
            broadcaster.run();
        }
        finally {
            broadcaster.stop();
        }
    }
}

3. 编写监视器

编写一个称为 LogEventMonitor 的消费者程序，它的作用包括：

• 接收由 LogEventBroadcaster 广播的 UDP DatagramPacket
• 解码为 LogEvent 消息
• 处理 LogEvent 消息

和之前一样，解码器 LogEventDecoder 负责将传入的 DatagramPacket 解码为 LogEvent 消息

public class LogEventDecoder extends MessageToMessageDecoder<DatagramPacket> {

    @Override
    protected void decode(ChannelHandlerContext ctx, DatagramPacket datagramPacket, List<Object> out) throws Exception {
        ByteBuf data = datagramPacket.content();
        int idx = data.indexOf(0, data.readableBytes(), LogEvent.SEPARATOR);
        String filename = data.slice(0, idx).toString(CharsetUtil.UTF_8);
        String logMsg = data.slice(idx + 1, data.readableBytes()).toString(CharsetUtil.UTF_8);
        LogEvent event = new LogEvent(datagramPacket.sender(), logMsg, filename, System.currentTimeMillis());
        out.add(event);
    }
}

创建一个处理 LogEvent 的 ChannelHandler

public class LogEventHandler extends SimpleChannelInboundHandler<LogEvent> {

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }

    @Override
    protected void messageReceived(ChannelHandlerContext ctx, LogEvent event) throws Exception {
        StringBuilder builder = new StringBuilder();
        builder.append(event.getReceived());
        builder.append("[");
        builder.append(event.getSource().toString());
        builder.append("] [");
        builder.append(event.getLogfile());
        builder.append("] : ");
        builder.append(event.getMsg());
        System.out.println(builder.toString());
    }
}

现在需要将 LogEventDecoder 和 LogEventHandler 安装到 ChannelPipeline 中，下述代码展示了如何通过 LogEventMonitor 主类来做到这一点

public class LogEventMonitor {

    private final EventLoopGroup group;
    private final Bootstrap bootstrap;

    public LogEventMonitor(InetSocketAddress address) {
        group = new NioEventLoopGroup();
        bootstrap = new Bootstrap();
        bootstrap.group(group)
                .channel(NioDatagramChannel.class)
                .option(ChannelOption.SO_BROADCAST, true)
                .handler(new ChannelInitializer<Channel>() {

                    @Override
                    protected void initChannel(Channel ch) throws Exception {
                        ChannelPipeline pipeline = ch.pipeline();
                        pipeline.addLast(new LogEventDecoder());
                        pipeline.addLast(new LogEventHandler());
                    }
                })
                .localAddress(address);
    }

    public Channel bind() {
        return bootstrap.bind().syncUninterruptibly().channel();
    }

    public void stop() {
        group.shutdownGracefully();
    }

    public static void main(String[] args) throws Exception {
        if (args.length != 1) {
            throw new IllegalArgumentException("Usage: LogEventMonitor <port>");
        }
        LogEventMonitor monitor = new LogEventMonitor(new InetSocketAddress(Integer.parseInt(args[0])));
        try {
            Channel channel = monitor.bind();
            channel.closeFuture().sync();
        }
        finally {
            monitor.stop();
        }
    }
}