我们先来找到通讯协议的入口点吧。通过Protocol接口查找通讯协议入口点，我们根据接口的export方法搜索发现入口了，在ServiceConfig的doExportUrlsFor1Protocol方法，如下图：
然后我们进入 protocol.export(invoker)方法发现有很多实现类，根据spi（不懂的请看之前写的容器篇）查看配置文件能找到如下
registry=com.alibaba.dubbo.registry.integration.RegistryProtocol
dubbo=com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocol //这个是默认的，我们在Protocol接口上可以看到spi的注解
filter=com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper
listener=com.alibaba.dubbo.rpc.protocol.ProtocolListenerWrapper
mock=com.alibaba.dubbo.rpc.support.MockProtocol
injvm=com.alibaba.dubbo.rpc.protocol.injvm.InjvmProtocol
rmi=com.alibaba.dubbo.rpc.protocol.rmi.RmiProtocol
hessian=com.alibaba.dubbo.rpc.protocol.hessian.HessianProtocol
com.alibaba.dubbo.rpc.protocol.http.HttpProtocol
com.alibaba.dubbo.rpc.protocol.webservice.WebServiceProtocol
thrift=com.alibaba.dubbo.rpc.protocol.thrift.ThriftProtocol
memcached=com.alibaba.dubbo.rpc.protocol.memcached.MemcachedProtocol
redis=com.alibaba.dubbo.rpc.protocol.redis.RedisProtocol
rest=com.alibaba.dubbo.rpc.protocol.rest.RestProtocol
进入DubboProtocol.export(Invoker<T> invoker)方法里面有个 openServer(url);
代码：
private void openServer(URL url) {
// find server.
String key = url.getAddress();
//client 也可以暴露一个只有server可以调用的服务。
boolean isServer = url.getParameter(Constants.IS_SERVER_KEY,true);
if (isServer) {
ExchangeServer server = serverMap.get(key);
if (server == null) {
serverMap.put(key, createServer(url)); //createServer是创建服务
} else {
//server支持reset,配合override功能使用
server.reset(url);
}
}
}
继续进入createServer，上源码
private ExchangeServer createServer(URL url) {
//默认开启server关闭时发送readonly事件
url = url.addParameterIfAbsent(Constants.CHANNEL_READONLYEVENT_SENT_KEY, Boolean.TRUE.toString());
//默认开启heartbeat
url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
String str = url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_SERVER);
if (str != null && str.length() > 0 && ! ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str))
throw new RpcException("Unsupported server type: " + str + ", url: " + url);
url = url.addParameter(Constants.CODEC_KEY, Version.isCompatibleVersion() ? COMPATIBLE_CODEC_NAME : DubboCodec.NAME);
ExchangeServer server;
try {
server = Exchangers.bind(url, requestHandler);
} catch (RemotingException e) {
throw new RpcException("Fail to start server(url: " + url + ") " + e.getMessage(), e);
}
str = url.getParameter(Constants.CLIENT_KEY);
if (str != null && str.length() > 0) {
Set<String> supportedTypes = ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions();
if (!supportedTypes.contains(str)) {
throw new RpcException("Unsupported client type: " + str);
}
}
return server;
}
dubbo从要暴漏的服务的URL中取得相关的配置（host，port等）进行服务端server的创建，同上面的server = Exchangers.bind(url, requestHandler) 正式创建服务。
所以基本的创建步骤是
export() --> openServer() --> createServer() --> server = Exchangers.bind(url, requestHandler);
我们进行来看 Exchangers.bind(url, requestHandler)
源码：
public static ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
if (url == null) {
throw new IllegalArgumentException("url == null");
}
if (handler == null) {
throw new IllegalArgumentException("handler == null");
}
url = url.addParameterIfAbsent(Constants.CODEC_KEY, "exchange");
return getExchanger(url).bind(url, handler);
}
然后通过getExchanger(url).bind(url, handler）的bing进入 HeaderExchanger类
public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}
在进入Transporters类的bing的
public static Server bind(URL url, ChannelHandler... handlers) throws RemotingException {
if (url == null) {
throw new IllegalArgumentException("url == null");
}
if (handlers == null || handlers.length == 0) {
throw new IllegalArgumentException("handlers == null");
}
ChannelHandler handler;
if (handlers.length == 1) {
handler = handlers[0];
} else {
handler = new ChannelHandlerDispatcher(handlers);
}
return getTransporter().bind(url, handler);
}
通过bing可以知道他讲调用:GrizzlyTransporter,MinaTransporter,NettyTransporter 通过spi默认是调用NettyTransporter
到这里我们基本明白dubbo的通讯默认是交给了netty来处理，
我们在看下doOPen方法
@Override
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.setPipelineFactory(new ChannelPipelineFactory() {
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());//解码
pipeline.addLast("encoder", adapter.getEncoder());//编码
pipeline.addLast("handler", nettyHandler);
return pipeline;
}
});
// bind
channel = bootstrap.bind(getBindAddress());
}
了解netty的同学，肯定早已习惯这个方法的写法，就是创建了netty的server嘛，到这里dubbo的服务创建完毕了,这个时候控制台见打印：
[DUBBO] Start NettyServer bind /0.0.0.0:20880, export /192.168.4.241:20880, dubbo version: 2.8.4, current host: 127.0.0.1