Dubbo分析之Transport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...