netty的粘包解包问题

1、简介Java1.4提供了NIO使开发者可以使用Java编写高性能的服务端程序，但使用原生的NIO API就像Linux C中网络编程一样，还是需要做IO处理、协议处理等低层次工作。所以，就像C服务端程序大量使用libevent作为网络应用框架一样，Java社区也不断涌现出基于NIO的网络应用框架。在这其中，Jboss出品的Netty就是个中翘楚。Netty是个异步的事件驱动网络应用框架，具有高性能、高扩展性等特性。Netty提供了统一的底层协议接口，使得开发者从底层的网络协议（比如TCP/IP、UDP）中解脱出来。就使用来说，开发者只要参考 Netty提供的若干例子和它的指南文档，就可以放手开发基于Netty的服务端程序了。在Java社区，最知名的开源Java NIO框架要属Mina和Netty，而且两者渊源颇多，对两者的比较自然不少。实际上，Netty的作者原来就是Mina作者之一，所以可以想到，Netty和Mina在设计理念上会有很多共同点。我对Mina没什么研究，但其作者介绍，Netty的设计对开发者有更友好的扩展性，并且性能方面要优于Mina，而Netty完善的文档也很吸引人。所以，如果你在寻找Java NIO框架，Netty是个很不错的选择。本文的内容就是围绕一个demo介绍使用Netty的点点滴滴。2、服务端程序2.1、ChannelHandler服务端程序通常的处理过程是：解码请求数据、业务逻辑处理、编码响应。从框架角度来说，可以提供3个接口来控制并调度该处理过程；从更通用的角度来说，并不特化处理其中的每一步，而把每一步当做过滤器链中的一环，这也是Netty的做法。Netty对请求处理过程实现了过滤器链模式（ChannelPipeline），每个过滤器实现了ChannelHandler接口。Netty中有两种请求事件流类型也做了细分：1）downstream event：其对应的ChannelHandler子接口是ChannelDownstreamHandler。downstream event是说从头到尾执行ChannelPipeline中的ChannelDownstreamHandler，这一过程相当于向外发送数据的过程。 downstream event有：”write”、”bind”、”unbind”、 “connect”、 “disconnect”、”close”。2）upstream event：其对应的ChannelHandler子接口是ChannelUpstreamHandler。upstream event处理的事件方向和downstream event相反，这一过程相当于接收处理外来请求的过程。upstream event有：”messageReceived”、 “exceptionCaught”、”channelOpen”、”channelClosed”、 “channelBound”、”channelUnbound”、 “channelConnected”、”writeComplete”、”channelDisconnected”、”channelInterestChanged”。Netty中有个注释@interface ChannelPipelineCoverage，它表示被注释的ChannelHandler是否能添加到多个ChannelPipeline中，其可选的值是”all”和”one”。”all”表示ChannelHandler是无状态的，可被多个ChannelPipeline共享，而”one”表示ChannelHandler只作用于单个ChannelPipeline中。但ChannelPipelineCoverage只是个注释而已，并没有实际的检查作用。对于ChannelHandler是”all”还是”one”，还是根据逻辑需要而定。比如，像解码请求handler，因为可能解码的数据不完整，需要等待下一次读事件来了之后再继续解析，所以解码请求handler就需要是”one”的（否则多个Channel共享数据就乱了）。而像业务逻辑处理hanlder通常是”all”的。下面以一个简单的例子说明如何编写“解码请求数据、业务逻辑处理、编码响应”这一过程中涉及的ChannelHandler。该例子实现的协议格式很简单，请求和响应流中头4个字节表示后面跟的内容长度，根据该长度可得到内容体。首先看下解码器的实现：public class MessageDecoder extends FrameDecoder { @Override protected Object decode( ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception { if (buffer.readableBytes() < 4) { return null;//(1) } int dataLength = buffer.getInt(buffer.readerIndex()); if (buffer.readableBytes() < dataLength + 4) { return null;//(2) } buffer.skipBytes(4);//(3) byte[] decoded = new byte[dataLength]; buffer.readBytes(decoded); String msg = new String(decoded);//(4) return msg; }}MessageDecoder继承自FrameDecoder，FrameDecoder是Netty codec包中的辅助类，它是个ChannelUpstreamHandler，decode方法是FrameDecoder子类需要实现的。在上面的代码中，有：(1)检查ChannelBuffer中的字节数，如果ChannelBuffer可读的字节数少于4,则返回null等待下次读事件。(2)继续检查ChannelBuffer中的字节数，如果ChannelBuffer可读的字节数少于dataLength + 4，则返回null等待下次读事件。(3)越过dataLength的字节。(4)构造解码的字符串返回。@ChannelPipelineCoverage("all")public class MessageServerHandler extends SimpleChannelUpstreamHandler { private static final Logger logger = Logger.getLogger( MessageServerHandler.class.getName()); @Override public void messageReceived( ChannelHandlerContext ctx, MessageEvent e) { if (!(e.getMessage() instanceof String)) { return;//(1) } String msg = (String) e.getMessage(); System.err.println("got msg:"+msg); e.getChannel().write(msg);//(2) } @Override public void exceptionCaught( ChannelHandlerContext ctx, ExceptionEvent e) { logger.log( Level.WARNING, "Unexpected exception from downstream.", e.getCause()); e.getChannel().close(); }}MessageServerHandler是服务端业务处理handler，其继承自SimpleChannelUpstreamHandler，并主要实现messageReceived事件。关于该类，有如下注解：(1)该upstream事件流中，首先经过MessageDecoder，其会将decode返回的解码后的数据构造成 MessageEvent.getMessage()，所以在handler上下文关系中，MessageEvent.getMessage()并不一定都返回ChannelBuffer类型的数据。(2)MessageServerHandler只是简单的将得到的msg再写回给客户端。e.getChannel().write(msg);操作将触发DownstreamMessageEvent事件，也就是调用下面的MessageEncoder将编码的数据返回给客户端。@ChannelPipelineCoverage("all")public class MessageEncoder extends OneToOneEncoder { @Override protected Object encode( ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception { if (!(msg instanceof String)) { return msg;//(1) } String res = (String)msg; byte[] data = res.getBytes(); int dataLength = data.length; ChannelBuffer buf = ChannelBuffers.dynamicBuffer();//(2) buf.writeInt(dataLength); buf.writeBytes(data); return buf;//(3) }}MessageEncoder是个ChannelDownstreamHandler。对该类的注解如下：(1)如果编码的msg不是合法类型，就直接返回该msg，之后OneToOneEncoder会调用 ctx.sendDownstream(evt);来调用下一个ChannelDownstreamHandler。对于该例子来说，这种情况是不应该出现的。(2)开发者创建ChannelBuffer的用武之地就是这儿了，通常使用dynamicBuffer即可，表示得到的ChannelBuffer可动态增加大小。(3)返回编码后的ChannelBuffer之后，OneToOneEncoder会调用Channels.write将数据写回客户端。2.2、MessageServerPipelineFactory创建了3个ChannelHandler，需要将他们注册到ChannelPipeline，而ChannelPipeline又是和Channel对应的（是全局单例还是每个Channel对应一个ChannelPipeline实例依赖于实现）。可以实现ChannelPipeline的工厂接口 ChannelPipelineFactory实现该目的。MessageServerPipelineFactory的代码如下：public class MessageServerPipelineFactory implements ChannelPipelineFactory { public ChannelPipeline getPipeline() throws Exception { ChannelPipeline pipeline = pipeline(); pipeline.addLast("decoder", new MessageDecoder()); pipeline.addLast("encoder", new MessageEncoder()); pipeline.addLast("handler", new MessageServerHandler()); return pipeline; }}2.3、MessageServer服务端程序就剩下启动代码了，使用Netty的ServerBootstrap三下五除二完成之。public class MessageServer { public static void main(String[] args) throws Exception { // Configure the server. ServerBootstrap bootstrap = new ServerBootstrap( new NioServerSocketChannelFactory( Executors.newCachedThreadPool(), Executors.newCachedThreadPool())); // Set up the default event pipeline. bootstrap.setPipelineFactory(new MessageServerPipelineFactory()); // Bind and start to accept incoming connections. bootstrap.bind(new InetSocketAddress(8080)); }}稍加补充的是，该Server程序并不完整，它没有处理关闭时的资源释放，尽管暴力的来看并不一定需要做这样的善后工作。3、客户端程序客户端程序和服务端程序处理模型上是很相似的，这里还是付上代码并作简要说明。3.1、 ChannelHandler客户端是先发送数据到服务端（downstream事件流），然后是处理从服务端接收的数据（upstream事件流）。这里有个问题是，怎么把需要发送的数据送到downstream事件流里呢？这就用到了ChannelUpstreamHandler的channelConnected事件了。实现的 MessageClientHandler代码如下：@ChannelPipelineCoverage("all")public class MessageClientHandler extends SimpleChannelUpstreamHandler { private static final Logger logger = Logger.getLogger( MessageClientHandler.class.getName()); @Override public void channelConnected( ChannelHandlerContext ctx, ChannelStateEvent e) { String message = "hello kafka0102"; e.getChannel().write(message); } @Override public void messageReceived( ChannelHandlerContext ctx, MessageEvent e) { // Send back the received message to the remote peer. System.err.println("messageReceived send message "+e.getMessage()); try { Thread.sleep(1000*3); } catch (Exception ex) { ex.printStackTrace(); } e.getChannel().write(e.getMessage()); } @Override public void exceptionCaught( ChannelHandlerContext ctx, ExceptionEvent e) { // Close the connection when an exception is raised. logger.log( Level.WARNING, "Unexpected exception from downstream.", e.getCause()); e.getChannel().close(); }}对于编码和解码Handler，复用MessageEncoder和MessageDecoder即可。3.2、 MessageClientPipelineFactory public class MessageClientPipelineFactory implements ChannelPipelineFactory { public ChannelPipeline getPipeline() throws Exception { ChannelPipeline pipeline = pipeline(); pipeline.addLast("decoder", new MessageDecoder()); pipeline.addLast("encoder", new MessageEncoder()); pipeline.addLast("handler", new MessageClientHandler()); return pipeline; }}3.3、MessageClientpublic class MessageClient { public static void main(String[] args) throws Exception { // Parse options. String host = "127.0.0.1"; int port = 8080; // Configure the client. ClientBootstrap bootstrap = new ClientBootstrap( new NioClientSocketChannelFactory( Executors.newCachedThreadPool(), Executors.newCachedThreadPool())); // Set up the event pipeline factory. bootstrap.setPipelineFactory(new MessageClientPipelineFactory()); // Start the connection attempt. ChannelFuture future = bootstrap.connect(new InetSocketAddress(host, port)); // Wait until the connection is closed or the connection attempt fails. future.getChannel().getCloseFuture().awaitUninterruptibly(); // Shut down thread pools to exit. bootstrap.releaseExternalResources(); }}在写客户端例子时，我想像的代码并不是这样的，对客户端的代码我也没做过多的研究，所以也可能没有找到更好的解决方案。在上面的例子中，bootstrap.connect方法中会触发实际的连接操作，接着触发 MessageClientHandler.channelConnected，使整个过程运转起来。但是，我想要的是一个连接池，并且如何写数据也不应该在channelConnected中，这样对于动态的数据，只能在构造函数中传递需要写的数据了。但到现在，我还不清楚如何将连接池和 ChannelPipeline有效的结合起来。或许，这样的需求可以跨过Netty来实现。4、总结关于Netty的初步使用，尚且总结到这里。关于这篇文章，写得断断续续，以至于到后来我都没兴趣把内容都整理出来。当然，这多少也是因为我是先整理 Netty原理方面的东西所致。我也只能卑微的期望，该文对Netty入门者会有些许帮助。=============================== 华丽的终止符 ================================ 1、简介Java1.4提供了NIO使开发者可以使用Java编写高性能的服务端程序，但使用原生的</p></div><br /><hr><link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap/5.2.1/css/bootstrap.min.css" 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