本实例主要参考的是官网的examples：点击这里

使用场景：客户端向Netty请求一个文件，Netty服务端下载指定位置文件到客户端。

本实例使用的是Http协议，当然，可以通过简单的修改即可换成TCP协议。

需要注意本实例的关键点是，为了更高效的传输大数据，实例中用到了ChunkedWriteHandler编码器，它提供了以zero-memory-copy方式写文件。

第一步：先写一个HttpFileServer

package NettyDemo.file.server;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.http.HttpObjectAggregator;
import io.netty.handler.codec.http.HttpServerCodec;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
import io.netty.handler.stream.ChunkedWriteHandler;

/*******************************************************************************
 * Reserved. BidPlanStructForm.java Created on 2014-8-19 Author: <a
 * href=mailto:wanghouda@126.com>wanghouda</a>
 * @Title: HttpFileServer.java
 * @Package NettyDemo.file.server Description: Version: 1.0
 ******************************************************************************/
public class HttpFileServer {
	static final int PORT = 8080;
	public static void main(String[] args) throws Exception {
		EventLoopGroup bossGroup = new NioEventLoopGroup(1);
		EventLoopGroup workerGroup = new NioEventLoopGroup();
		try {
			ServerBootstrap b = new ServerBootstrap();
			b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).handler(new LoggingHandler(LogLevel.INFO))
					.childHandler(new ChannelInitializer<SocketChannel>() {// 有连接到达时会创建一个channel
								@Override
								protected void initChannel(SocketChannel ch) throws Exception {
									ChannelPipeline pipeline = ch.pipeline();
									pipeline.addLast(new HttpServerCodec());
									pipeline.addLast(new HttpObjectAggregator(65536));
									pipeline.addLast(new ChunkedWriteHandler());
									pipeline.addLast(new FileServerHandler());
								}
							});

			Channel ch = b.bind(PORT).sync().channel();
			System.err.println("打开浏览器，输入： " + ("http") + "://127.0.0.1:" + PORT + '/');
			ch.closeFuture().sync();
		} finally {
			bossGroup.shutdownGracefully();
			workerGroup.shutdownGracefully();
		}
	}
}

</pre><pre>

package NettyDemo.file.server;

import static io.netty.handler.codec.http.HttpHeaders.Names.CACHE_CONTROL;
import static io.netty.handler.codec.http.HttpHeaders.Names.CONTENT_TYPE;
import static io.netty.handler.codec.http.HttpHeaders.Names.DATE;
import static io.netty.handler.codec.http.HttpHeaders.Names.EXPIRES;
import static io.netty.handler.codec.http.HttpHeaders.Names.IF_MODIFIED_SINCE;
import static io.netty.handler.codec.http.HttpHeaders.Names.LAST_MODIFIED;
import static io.netty.handler.codec.http.HttpHeaders.Names.LOCATION;
import static io.netty.handler.codec.http.HttpResponseStatus.BAD_REQUEST;
import static io.netty.handler.codec.http.HttpResponseStatus.FORBIDDEN;
import static io.netty.handler.codec.http.HttpResponseStatus.FOUND;
import static io.netty.handler.codec.http.HttpResponseStatus.INTERNAL_SERVER_ERROR;
import static io.netty.handler.codec.http.HttpResponseStatus.NOT_FOUND;
import static io.netty.handler.codec.http.HttpResponseStatus.NOT_MODIFIED;
import static io.netty.handler.codec.http.HttpResponseStatus.OK;
import static io.netty.handler.codec.http.HttpVersion.HTTP_1_1;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelProgressiveFuture;
import io.netty.channel.ChannelProgressiveFutureListener;
import io.netty.channel.DefaultFileRegion;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.handler.codec.http.DefaultFullHttpResponse;
import io.netty.handler.codec.http.DefaultHttpResponse;
import io.netty.handler.codec.http.FullHttpRequest;
import io.netty.handler.codec.http.FullHttpResponse;
import io.netty.handler.codec.http.HttpChunkedInput;
import io.netty.handler.codec.http.HttpHeaders;
import io.netty.handler.codec.http.HttpResponse;
import io.netty.handler.codec.http.HttpResponseStatus;
import io.netty.handler.codec.http.HttpVersion;
import io.netty.handler.codec.http.LastHttpContent;
import io.netty.handler.ssl.SslHandler;
import io.netty.handler.stream.ChunkedFile;
import io.netty.util.CharsetUtil;
import io.netty.util.internal.SystemPropertyUtil;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.RandomAccessFile;
import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Locale;
import java.util.TimeZone;
import java.util.regex.Pattern;

import javax.activation.MimetypesFileTypeMap;

/*******************************************************************************
 * Created on 2014-8-19 Author:
 * <href=mailto:wanghouda@126.com>wanghouda</a>
 * 
 * @Title: HttpFileServerHandler.java
 * @Package NettyDemo.file.server Description: Version: 1.0
 ******************************************************************************/
public class FileServerHandler extends SimpleChannelInboundHandler<FullHttpRequest> {
	public static final String HTTP_DATE_FORMAT = "EEE, dd MMM yyyy HH:mm:ss zzz";
	public static final String HTTP_DATE_GMT_TIMEZONE = "GMT";
	public static final int HTTP_CACHE_SECONDS = 60;

	@Override
	protected void channelRead0(ChannelHandlerContext ctx, FullHttpRequest request) throws Exception {
		// 监测解码情况
		if (!request.getDecoderResult().isSuccess()) {
			sendError(ctx, BAD_REQUEST);
			return;
		}
		final String uri = request.getUri();
		final String path = sanitizeUri(uri);
		if (path == null) {
			sendError(ctx, FORBIDDEN);
			return;
		}
		//读取要下载的文件
		File file = new File(path);
		if (file.isHidden() || !file.exists()) {
			sendError(ctx, NOT_FOUND);
			return;
		}
		if (file.isDirectory()) {
			if (uri.endsWith("/")) {
				sendListing(ctx, file);
			} else {
				sendRedirect(ctx, uri + '/');
			}
			return;
		}
		if (!file.isFile()) {
			sendError(ctx, FORBIDDEN);
			return;
		}
		// Cache Validation
		String ifModifiedSince = request.headers().get(IF_MODIFIED_SINCE);
		if (ifModifiedSince != null && !ifModifiedSince.isEmpty()) {
			SimpleDateFormat dateFormatter = new SimpleDateFormat(HTTP_DATE_FORMAT, Locale.US);
			Date ifModifiedSinceDate = dateFormatter.parse(ifModifiedSince);
			// Only compare up to the second because the datetime format we send
			// to the client
			// does not have milliseconds
			long ifModifiedSinceDateSeconds = ifModifiedSinceDate.getTime() / 1000;
			long fileLastModifiedSeconds = file.lastModified() / 1000;
			if (ifModifiedSinceDateSeconds == fileLastModifiedSeconds) {
				sendNotModified(ctx);
				return;
			}
		}
		RandomAccessFile raf;
		try {
			raf = new RandomAccessFile(file, "r");
		} catch (FileNotFoundException ignore) {
			sendError(ctx, NOT_FOUND);
			return;
		}
		long fileLength = raf.length();
		HttpResponse response = new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
		HttpHeaders.setContentLength(response, fileLength);
		setContentTypeHeader(response, file);
		setDateAndCacheHeaders(response, file);
		if (HttpHeaders.isKeepAlive(request)) {
			response.headers().set("CONNECTION", HttpHeaders.Values.KEEP_ALIVE);
		}

		// Write the initial line and the header.
		ctx.write(response);

		// Write the content.
		ChannelFuture sendFileFuture;
		if (ctx.pipeline().get(SslHandler.class) == null) {
			sendFileFuture = ctx.write(new DefaultFileRegion(raf.getChannel(), 0, fileLength), ctx.newProgressivePromise());
		} else {
			sendFileFuture = ctx.write(new HttpChunkedInput(new ChunkedFile(raf, 0, fileLength, 8192)), ctx.newProgressivePromise());
		}
		sendFileFuture.addListener(new ChannelProgressiveFutureListener() {
			@Override
			public void operationProgressed(ChannelProgressiveFuture future, long progress, long total) {
				if (total < 0) { // total unknown
					System.err.println(future.channel() + " Transfer progress: " + progress);
				} else {
					System.err.println(future.channel() + " Transfer progress: " + progress + " / " + total);
				}
			}

			@Override
			public void operationComplete(ChannelProgressiveFuture future) {
				System.err.println(future.channel() + " Transfer complete.");
			}
		});

		// Write the end marker
		ChannelFuture lastContentFuture = ctx.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT);

		// Decide whether to close the connection or not.
		if (!HttpHeaders.isKeepAlive(request)) {
			// Close the connection when the whole content is written out.
			lastContentFuture.addListener(ChannelFutureListener.CLOSE);
		}
	}

	@Override
	public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
		cause.printStackTrace();
		if (ctx.channel().isActive()) {
			sendError(ctx, INTERNAL_SERVER_ERROR);
		}
	}

	private static final Pattern INSECURE_URI = Pattern.compile(".*[<>&\"].*");

	private static String sanitizeUri(String uri) {
		// Decode the path.
		try {
			uri = URLDecoder.decode(uri, "UTF-8");
		} catch (UnsupportedEncodingException e) {
			throw new Error(e);
		}

		if (!uri.startsWith("/")) {
			return null;
		}

		// Convert file separators.
		uri = uri.replace('/', File.separatorChar);

		// Simplistic dumb security check.
		// You will have to do something serious in the production environment.
		if (uri.contains(File.separator + '.') || uri.contains('.' + File.separator) || uri.startsWith(".") || uri.endsWith(".")
				|| INSECURE_URI.matcher(uri).matches()) {
			return null;
		}

		// Convert to absolute path.
		return SystemPropertyUtil.get("user.dir") + File.separator + uri;
	}

	private static final Pattern ALLOWED_FILE_NAME = Pattern.compile("[A-Za-z0-9][-_A-Za-z0-9\\.]*");

	private static void sendListing(ChannelHandlerContext ctx, File dir) {
		FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, OK);
		response.headers().set(CONTENT_TYPE, "text/html; charset=UTF-8");

		StringBuilder buf = new StringBuilder();
		String dirPath = dir.getPath();

		buf.append("<!DOCTYPE html>\r\n");
		buf.append("<html><head><title>");
		buf.append("Listing of: ");
		buf.append(dirPath);
		buf.append("</title></head><body>\r\n");

		buf.append("<h3>Listing of: ");
		buf.append(dirPath);
		buf.append("</h3>\r\n");

		buf.append("<ul>");
		buf.append("<li><a href=\"../\">..</a></li>\r\n");

		for (File f : dir.listFiles()) {
			if (f.isHidden() || !f.canRead()) {
				continue;
			}

			String name = f.getName();
			if (!ALLOWED_FILE_NAME.matcher(name).matches()) {
				continue;
			}

			buf.append("<li><a href=\"");
			buf.append(name);
			buf.append("\">");
			buf.append(name);
			buf.append("</a></li>\r\n");
		}

		buf.append("</ul></body></html>\r\n");
		ByteBuf buffer = Unpooled.copiedBuffer(buf, CharsetUtil.UTF_8);
		response.content().writeBytes(buffer);
		buffer.release();

		// Close the connection as soon as the error message is sent.
		ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
	}

	private static void sendRedirect(ChannelHandlerContext ctx, String newUri) {
		FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, FOUND);
		response.headers().set(LOCATION, newUri);

		// Close the connection as soon as the error message is sent.
		ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
	}

	private static void sendError(ChannelHandlerContext ctx, HttpResponseStatus status) {
		FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, status, Unpooled.copiedBuffer("Failure: " + status + "\r\n", CharsetUtil.UTF_8));
		response.headers().set(CONTENT_TYPE, "text/plain; charset=UTF-8");

		// Close the connection as soon as the error message is sent.
		ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
	}

	/**
	 * When file timestamp is the same as what the browser is sending up, send a
	 * "304 Not Modified"
	 * 
	 * @param ctx
	 *            Context
	 */
	private static void sendNotModified(ChannelHandlerContext ctx) {
		FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, NOT_MODIFIED);
		setDateHeader(response);

		// Close the connection as soon as the error message is sent.
		ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
	}

	/**
	 * Sets the Date header for the HTTP response
	 * 
	 * @param response
	 *            HTTP response
	 */
	private static void setDateHeader(FullHttpResponse response) {
		SimpleDateFormat dateFormatter = new SimpleDateFormat(HTTP_DATE_FORMAT, Locale.US);
		dateFormatter.setTimeZone(TimeZone.getTimeZone(HTTP_DATE_GMT_TIMEZONE));

		Calendar time = new GregorianCalendar();
		response.headers().set(DATE, dateFormatter.format(time.getTime()));
	}

	/**
	 * Sets the Date and Cache headers for the HTTP Response
	 * 
	 * @param response
	 *            HTTP response
	 * @param fileToCache
	 *            file to extract content type
	 */
	private static void setDateAndCacheHeaders(HttpResponse response, File fileToCache) {
		SimpleDateFormat dateFormatter = new SimpleDateFormat(HTTP_DATE_FORMAT, Locale.US);
		dateFormatter.setTimeZone(TimeZone.getTimeZone(HTTP_DATE_GMT_TIMEZONE));

		// Date header
		Calendar time = new GregorianCalendar();
		response.headers().set(DATE, dateFormatter.format(time.getTime()));

		// Add cache headers
		time.add(Calendar.SECOND, HTTP_CACHE_SECONDS);
		response.headers().set(EXPIRES, dateFormatter.format(time.getTime()));
		response.headers().set(CACHE_CONTROL, "private, max-age=" + HTTP_CACHE_SECONDS);
		response.headers().set(LAST_MODIFIED, dateFormatter.format(new Date(fileToCache.lastModified())));
	}

	/**
	 * Sets the content type header for the HTTP Response
	 * 
	 * @param response
	 *            HTTP response
	 * @param file
	 *            file to extract content type
	 */
	private static void setContentTypeHeader(HttpResponse response, File file) {
		MimetypesFileTypeMap mimeTypesMap = new MimetypesFileTypeMap();
		response.headers().set(CONTENT_TYPE, mimeTypesMap.getContentType(file.getPath()));
	}

}

     如图所示：即可在浏览器中看到工程目录下所有文件，点击即可下载

ps:通过对本例进行简单修改可实现各种方式的文件下载