Dubbo分析之Transport层

系列文章

Dubbo分析Serialize层
Dubbo分析之Transport层
Dubbo分析之Exchange 层
Dubbo分析之Protocol层

前言

上一篇文章Dubbo分析之Serialize层，介绍了最底层的序列化/反序列化层，本文继续分析Serialize层的上一层transport网络传输层，此层使用了现有的一些通讯开源框架(ex:netty,mina,grizzly)来做底层通讯，上文也做了简单介绍，本文将做更深入的了解；

Transporter类分析

dubbo为通讯框架提供了统一的bind和connet接口，方便进行管理和扩展，封装在接口类：Transporter中：

@SPI("netty") public interface Transporter { @Adaptive({Constants.SERVER_KEY, Constants.TRANSPORTER_KEY}) Server bind(URL url, ChannelHandler handler) throws RemotingException; @Adaptive({Constants.CLIENT_KEY, Constants.TRANSPORTER_KEY}) Client connect(URL url, ChannelHandler handler) throws RemotingException; }

提供了bind和connect接口，分别对应这服务器端和客户端，具体有哪些实现类，如下图所示：

以默认使用的netty框架为例，代码如下：

public class NettyTransporter implements Transporter { public static final String NAME = "netty"; @Override public Server bind(URL url, ChannelHandler listener) throws RemotingException { return new NettyServer(url, listener); } @Override public Client connect(URL url, ChannelHandler listener) throws RemotingException { return new NettyClient(url, listener); } }

具体的服务器端封装在NettyServer中，客户端封装在NettyClient；url参数是包含了xml配置的信息(包括：对外的接口，使用的协议，使用的序列化方式，使用的通讯框架等)，listener是一个Handler，在解码之后将数据交给它做后续的业务处理；对应以上的几种通讯开源框架，分别提供了对应的Transporter包括：NettyTransporter，NettyTransporter(netty4)，MinaTransporter以及GrizzlyTransporter，具体使用哪种类型的Transporter，在Transporters类中提供了getTransporter方法：

public static Transporter getTransporter() { return ExtensionLoader.getExtensionLoader(Transporter.class).getAdaptiveExtension(); }

这里并没有像在获取具体serialization类一样，通过在url指定transporter参数，然后加载具体的transporter类，而是生成了一个动态的transporter，由此动态transporter去加载具体的类；
因为Server端和Client可以分别设置成不同的通讯框架，一次获取唯一的Transporter不能满足此需求；具体的生成动态代码的方法在ExtensionLoader的createAdaptiveExtensionClassCode方法中，此处不在列出源码，在此展示一下默认生成的动态代码扩展类：

package com.alibaba.dubbo.remoting; import com.alibaba.dubbo.common.extension.ExtensionLoader; public class Transporter$Adaptive implements com.alibaba.dubbo.remoting.Transporter { public com.alibaba.dubbo.remoting.Server bind( com.alibaba.dubbo.common.URL arg0, com.alibaba.dubbo.remoting.ChannelHandler arg1) throws com.alibaba.dubbo.remoting.RemotingException { if (arg0 == null) { throw new IllegalArgumentException("url == null"); } com.alibaba.dubbo.common.URL url = arg0; String extName = url.getParameter("server", url.getParameter("transporter", "netty")); if (extName == null) { throw new IllegalStateException( "Fail to get extension(com.alibaba.dubbo.remoting.Transporter) name from url(" + url.toString() + ") use keys([server, transporter])"); } com.alibaba.dubbo.remoting.Transporter extension = (com.alibaba.dubbo.remoting.Transporter) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.remoting.Transporter.class) .getExtension(extName); return extension.bind(arg0, arg1); } public com.alibaba.dubbo.remoting.Client connect( com.alibaba.dubbo.common.URL arg0, com.alibaba.dubbo.remoting.ChannelHandler arg1) throws com.alibaba.dubbo.remoting.RemotingException { if (arg0 == null) { throw new IllegalArgumentException("url == null"); } com.alibaba.dubbo.common.URL url = arg0; String extName = url.getParameter("client", url.getParameter("transporter", "netty")); if (extName == null) { throw new IllegalStateException( "Fail to get extension(com.alibaba.dubbo.remoting.Transporter) name from url(" + url.toString() + ") use keys([client, transporter])"); } com.alibaba.dubbo.remoting.Transporter extension = (com.alibaba.dubbo.remoting.Transporter) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.remoting.Transporter.class) .getExtension(extName); return extension.connect(arg0, arg1); } }

可以发现Server端可以通过transporter和server两个参数来设置扩展类，而且server参数设置的值是可以覆盖transporter参数的值，同理Client也类似；最后不管是bind()还是connet()都是通过ExtensionLoader的getExtension方法来获取具体的transporter类；同serialize层，相关的transporter也同样定义在META-INF/dubbo/internal/com.alibaba.dubbo.remoting.Transporter文件中：

netty=com.alibaba.dubbo.remoting.transport.netty.NettyTransporter netty4=com.alibaba.dubbo.remoting.transport.netty4.NettyTransporter mina=com.alibaba.dubbo.remoting.transport.mina.MinaTransporter grizzly=com.alibaba.dubbo.remoting.transport.grizzly.GrizzlyTransporter

Server端和Client分析

1.Server端

在实例化具体的Server类时，会首先调用父类的构造器，进行参数初始化，同时调用bind()方法，启动服务器；父类AbstractServer构造器如下：

public AbstractServer(URL url, ChannelHandler handler) throws RemotingException { super(url, handler); localAddress = getUrl().toInetSocketAddress(); String bindIp = getUrl().getParameter(Constants.BIND_IP_KEY, getUrl().getHost()); int bindPort = getUrl().getParameter(Constants.BIND_PORT_KEY, getUrl().getPort()); if (url.getParameter(Constants.ANYHOST_KEY, false) || NetUtils.isInvalidLocalHost(bindIp)) { bindIp = NetUtils.ANYHOST; } bindAddress = new InetSocketAddress(bindIp, bindPort); this.accepts = url.getParameter(Constants.ACCEPTS_KEY, Constants.DEFAULT_ACCEPTS); this.idleTimeout = url.getParameter(Constants.IDLE_TIMEOUT_KEY, Constants.DEFAULT_IDLE_TIMEOUT); try { doOpen(); if (logger.isInfoEnabled()) { logger.info("Start " + getClass().getSimpleName() + " bind " + getBindAddress() + ", export " + getLocalAddress()); } } catch (Throwable t) { throw new RemotingException(url.toInetSocketAddress(), null, "Failed to bind " + getClass().getSimpleName() + " on " + getLocalAddress() + ", cause: " + t.getMessage(), t); } //fixme replace this with better method DataStore dataStore = ExtensionLoader.getExtensionLoader(DataStore.class).getDefaultExtension(); executor = (ExecutorService) dataStore.get(Constants.EXECUTOR_SERVICE_COMPONENT_KEY, Integer.toString(url.getPort())); }

主要从url获取启动参数包括：ip，port，accepts(可接受的连接数，0表示不受限制数量，默认为0)，idleTimeout等；然后调用doOpen方法通过具体的通讯框架绑定端口启动服务；已默认使用的Netty为例，查看doOpen()方法如下：

protected void doOpen() throws Throwable { NettyHelper.setNettyLoggerFactory(); ExecutorService boss = Executors.newCachedThreadPool(new NamedThreadFactory("NettyServerBoss", true)); ExecutorService worker = Executors.newCachedThreadPool(new NamedThreadFactory("NettyServerWorker", true)); ChannelFactory channelFactory = new NioServerSocketChannelFactory(boss, worker, getUrl().getPositiveParameter(Constants.IO_THREADS_KEY, Constants.DEFAULT_IO_THREADS)); bootstrap = new ServerBootstrap(channelFactory); final NettyHandler nettyHandler = new NettyHandler(getUrl(), this); channels = nettyHandler.getChannels(); // https://issues.jboss.org/browse/NETTY-365 // https://issues.jboss.org/browse/NETTY-379 // final Timer timer = new HashedWheelTimer(new NamedThreadFactory("NettyIdleTimer", true)); bootstrap.setOption("child.tcpNoDelay", true); bootstrap.setPipelineFactory(new ChannelPipelineFactory() { @Override public ChannelPipeline getPipeline() { NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec(), getUrl(), NettyServer.this); ChannelPipeline pipeline = Channels.pipeline(); /*int idleTimeout = getIdleTimeout(); if (idleTimeout > 10000) { pipeline.addLast("timer", new IdleStateHandler(timer, idleTimeout / 1000, 0, 0)); }*/ pipeline.addLast("decoder", adapter.getDecoder()); pipeline.addLast("encoder", adapter.getEncoder()); pipeline.addLast("handler", nettyHandler); return pipeline; } }); // bind channel = bootstrap.bind(getBindAddress()); }

以上是常规的启动netty程序，需要指定编解码器，nettyHandler；编解码已经在上文中介绍过了，此处不在详细介绍，重点介绍nettyHandler；server端在数据经过解码之后就交给NettyHandler来处理，NettyHandler继承于Netty的SimpleChannelHandler类，重写了channelConnected，channelDisconnected，messageReceived，writeRequested以及exceptionCaught方法，基本上就是常规的几种操作：建立连接，断开连接，接收消息，发送消息，异常处理；看一下部分源码：

@Override public void channelConnected(ChannelHandlerContext ctx, ChannelStateEvent e) throws Exception { NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler); try { if (channel != null) { channels.put(NetUtils.toAddressString((InetSocketAddress) ctx.getChannel().getRemoteAddress()), channel); } handler.connected(channel); } finally { NettyChannel.removeChannelIfDisconnected(ctx.getChannel()); } } @Override public void channelDisconnected(ChannelHandlerContext ctx, ChannelStateEvent e) throws Exception { NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler); try { channels.remove(NetUtils.toAddressString((InetSocketAddress) ctx.getChannel().getRemoteAddress())); handler.disconnected(channel); } finally { NettyChannel.removeChannelIfDisconnected(ctx.getChannel()); } } @Override public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception { NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler); try { handler.received(channel, e.getMessage()); } finally { NettyChannel.removeChannelIfDisconnected(ctx.getChannel()); } } @Override public void writeRequested(ChannelHandlerContext ctx, MessageEvent e) throws Exception { super.writeRequested(ctx, e); NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler); try { handler.sent(channel, e.getMessage()); } finally { NettyChannel.removeChannelIfDisconnected(ctx.getChannel()); } } @Override public void exceptionCaught(ChannelHandlerContext ctx, ExceptionEvent e) throws Exception { NettyChannel channel = NettyChannel.getOrAddChannel(ctx.getChannel(), url, handler); try { handler.caught(channel, e.getCause()); } finally { NettyChannel.removeChannelIfDisconnected(ctx.getChannel()); } }

将netty原生的channel包装成dubbo的NettyChannel，同时将NettyChannel保存在NettyChannel的内部静态变量channelMap中；这里的方法都统一调用了getOrAddChannel方法，先添加进去，最后在finally中判定channel是否已经关闭，如果关闭从channelMap中移除；中间部分调用了handler对应的方法，此处的handler就是在实例化时传入的NettyServer，NettyServer本身也是一个ChannelHandler，可以看一下channelHandler接口类：

public interface ChannelHandler { void connected(Channel channel) throws RemotingException; void disconnected(Channel channel) throws RemotingException; void sent(Channel channel, Object message) throws RemotingException; void received(Channel channel, Object message) throws RemotingException; void caught(Channel channel, Throwable exception) throws RemotingException; }

具体的server类中也可以做一些处理，比如connected时判段是否超过accepts，如果超过拒绝连接；处理完之后交给实例化Server时传入的ChannelHandler处理，此类具体是在HeaderExchanger中被初始化的：

public class HeaderExchanger implements Exchanger { public static final String NAME = "header"; @Override public ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException { return new HeaderExchangeClient(Transporters.connect(url, new DecodeHandler(new HeaderExchangeHandler(handler))), true); } @Override public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException { return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler)))); } }

可以发现这里具体的ChannelHandler是DecodeHandler，注这里的Decode和Netty本身的decode不一样，Netty本身的decode在执行NettyHandler之前就执行解码了；后续的操作在Exchange层进行处理，本文暂时先不做介绍；

2.Client端

同样查看父类AbstractClient，构造方法如下：

public AbstractClient(URL url, ChannelHandler handler) throws RemotingException { super(url, handler); send_reconnect = url.getParameter(Constants.SEND_RECONNECT_KEY, false); shutdown_timeout = url.getParameter(Constants.SHUTDOWN_TIMEOUT_KEY, Constants.DEFAULT_SHUTDOWN_TIMEOUT); // The default reconnection interval is 2s, 1800 means warning interval is 1 hour. reconnect_warning_period = url.getParameter("reconnect.waring.period", 1800); try { doOpen(); } catch (Throwable t) { close(); throw new RemotingException(url.toInetSocketAddress(), null, "Failed to start " + getClass().getSimpleName() + " " + NetUtils.getLocalAddress() + " connect to the server " + getRemoteAddress() + ", cause: " + t.getMessage(), t); } try { // connect. connect(); if (logger.isInfoEnabled()) { logger.info("Start " + getClass().getSimpleName() + " " + NetUtils.getLocalAddress() + " connect to the server " + getRemoteAddress()); } } catch (RemotingException t) { if (url.getParameter(Constants.CHECK_KEY, true)) { close(); throw t; } else { logger.warn("Failed to start " + getClass().getSimpleName() + " " + NetUtils.getLocalAddress() + " connect to the server " + getRemoteAddress() + " (check == false, ignore and retry later!), cause: " + t.getMessage(), t); } } catch (Throwable t) { close(); throw new RemotingException(url.toInetSocketAddress(), null, "Failed to start " + getClass().getSimpleName() + " " + NetUtils.getLocalAddress() + " connect to the server " + getRemoteAddress() + ", cause: " + t.getMessage(), t); } executor = (ExecutorService) ExtensionLoader.getExtensionLoader(DataStore.class) .getDefaultExtension().get(Constants.CONSUMER_SIDE, Integer.toString(url.getPort())); ExtensionLoader.getExtensionLoader(DataStore.class) .getDefaultExtension().remove(Constants.CONSUMER_SIDE, Integer.toString(url.getPort())); }

客户端需要提供重连机制，所以初始化的几个参数都和重连有关，send_reconnect表示在发送消息时发现连接已经断开是否发起重连，reconnect_warning_period表示多久报一次重连警告，shutdown_timeout表示连接服务器一直连接不上的超时时间；接下来就是调用doOpen()方法，同样已Netty为例：

protected void doOpen() throws Throwable { NettyHelper.setNettyLoggerFactory(); bootstrap = new ClientBootstrap(channelFactory); // config // @see org.jboss.netty.channel.socket.SocketChannelConfig bootstrap.setOption("keepAlive", true); bootstrap.setOption("tcpNoDelay", true); bootstrap.setOption("connectTimeoutMillis", getTimeout()); final NettyHandler nettyHandler = new NettyHandler(getUrl(), this); bootstrap.setPipelineFactory(new ChannelPipelineFactory() { @Override public ChannelPipeline getPipeline() { NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec(), getUrl(), NettyClient.this); ChannelPipeline pipeline = Channels.pipeline(); pipeline.addLast("decoder", adapter.getDecoder()); pipeline.addLast("encoder", adapter.getEncoder()); pipeline.addLast("handler", nettyHandler); return pipeline; } }); }

Netty客户端的常规代码，设置了和Server端相同的NettyHandler，decoder和encoder；下面重点看看connect方法：

protected void connect() throws RemotingException { connectLock.lock(); try { if (isConnected()) { return; } initConnectStatusCheckCommand(); doConnect(); if (!isConnected()) { throw new RemotingException(this, "Failed connect to server " + getRemoteAddress() + " from " + getClass().getSimpleName() + " " + NetUtils.getLocalHost() + " using dubbo version " + Version.getVersion() + ", cause: Connect wait timeout: " + getTimeout() + "ms."); } else { if (logger.isInfoEnabled()) { logger.info("Successed connect to server " + getRemoteAddress() + " from " + getClass().getSimpleName() + " " + NetUtils.getLocalHost() + " using dubbo version " + Version.getVersion() + ", channel is " + this.getChannel()); } } reconnect_count.set(0); reconnect_error_log_flag.set(false); } catch (RemotingException e) { throw e; } catch (Throwable e) { throw new RemotingException(this, "Failed connect to server " + getRemoteAddress() + " from " + getClass().getSimpleName() + " " + NetUtils.getLocalHost() + " using dubbo version " + Version.getVersion() + ", cause: " + e.getMessage(), e); } finally { connectLock.unlock(); } }

首先判定是否已经连接，如果连接直接return；接下来初始化连接状态检查器，定期检查channel是否连接，连接断开会进行重连操作，具体代码如下：

private synchronized void initConnectStatusCheckCommand() { //reconnect=false to close reconnect int reconnect = getReconnectParam(getUrl()); if (reconnect > 0 && (reconnectExecutorFuture == null || reconnectExecutorFuture.isCancelled())) { Runnable connectStatusCheckCommand = new Runnable() { @Override public void run() { try { if (!isConnected()) { connect(); } else { lastConnectedTime = System.currentTimeMillis(); } } catch (Throwable t) { String errorMsg = "client reconnect to " + getUrl().getAddress() + " find error . url: " + getUrl(); // wait registry sync provider list if (System.currentTimeMillis() - lastConnectedTime > shutdown_timeout) { if (!reconnect_error_log_flag.get()) { reconnect_error_log_flag.set(true); logger.error(errorMsg, t); return; } } if (reconnect_count.getAndIncrement() % reconnect_warning_period == 0) { logger.warn(errorMsg, t); } } } }; reconnectExecutorFuture = reconnectExecutorService.scheduleWithFixedDelay(connectStatusCheckCommand, reconnect, reconnect, TimeUnit.MILLISECONDS); } }

创建了一个Runnable，用来检测是否连接，如果连接断开，调用connect方法；定时调度交给ScheduledThreadPoolExecutor来执行；初始化之后就调用具体Client的doConnect操作，也是通讯框架的一些常规代码，此处不列出了；后续关于NettyChannel的介绍和Server端类似，不过多进行介绍；

总结

本文重点分析了dubbo架构中的transport层，具体围绕Transporter, Client, Server，ChannelHandler几个类展开，关于后续的处理将在exchange信息交换层；

示例代码地址

https://github.com/ksfzhaohui...
https://gitee.com/OutOfMemory...