文件传输通常采用FTP或者HTTP附件的方式，事实上通过TCP Socket+File的方式进行传输也有一定的应用场景，尽管不是主流，但是掌握文件传输的方式还是很重要的，特别是针对两个跨主机的JVM进程之间进行持久化数据的相互交换。

下面使用netty实现文件传输：

文件服务器

public class FileServer {

public void run(int port) throws InterruptedException {

    EventLoopGroup bossGroup = new NioEventLoopGroup();
    EventLoopGroup workerGroup = new NioEventLoopGroup();

    try {
        ServerBootstrap boot = new ServerBootstrap();
        boot.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, 100);
        boot.childHandler(new ChannelInitializer<SocketChannel>() {

            @Override
            protected void initChannel(SocketChannel ch) throws Exception {
                ch.pipeline().addLast(new StringEncoder(CharsetUtil.UTF_8));
                ch.pipeline().addLast(new LineBasedFrameDecoder(1024));
                ch.pipeline().addLast(new StringDecoder(CharsetUtil.UTF_8));
                ch.pipeline().addLast(new FileServerHandler());
            }

        });
        ChannelFuture cf = boot.bind(port).sync();
        cf.channel().closeFuture().sync();

    } finally {
        bossGroup.shutdownGracefully();
        workerGroup.shutdownGracefully();
    }
}

public static void main(String[] args) throws InterruptedException {

    new FileServer().run(8080);

}
}

文件服务器端处理类

public class FileServerHandler extends SimpleChannelInboundHandler<String> {

private static final String CR = System.getProperty("line.separator");

@Override
protected void messageReceived(ChannelHandlerContext ctx, String msg) throws Exception {

    File file = new File(msg);
    if (file.exists()) {
        if (!file.isFile()) {
            ctx.writeAndFlush("not a file :" + file + CR);
            return;
        }
        ctx.write(file + " " + file.length() + CR);

        RandomAccessFile raf = new RandomAccessFile(file, "r");
        FileRegion fileRegion = new DefaultFileRegion(raf.getChannel(), 0, raf.length());

        ctx.write(fileRegion);
        ctx.writeAndFlush(CR);
        raf.close();

    } else {
        ctx.writeAndFlush("file not found: " + file + CR);
    }
}

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

}