可以看到其中有一个重要方法createServer(url)。
private ExchangeServer createServer(URL url) { // 当服务关闭时，默认启动发送只读事件 url = url.addParameterIfAbsent(Constants.CHANNEL_READONLYEVENT_SENT_KEY, Boolean.TRUE.toString()); // 默认启动心跳 url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT)); //str默认为netty 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); //添加编解码器 url = url.addParameter(Constants.CODEC_KEY, DubboCodec.NAME); ExchangeServer server; try { //启动服务，并传入请求处理器 server = Exchangers.bind(url, requestHandler); } catch (RemotingException e) { throw new RpcException("Fail to start server(url: " + url + ") " + e.getMessage(), e); } //判断客户端使用的是网络传输层框架是否支持服务端的网络传输层。 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; } Exchangers进入Exchangers.bind方法一探究竟。
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"); } //如果编码码器没有，则添加参数exchange url = url.addParameterIfAbsent(Constants.CODEC_KEY, "exchange"); return getExchanger(url).bind(url, handler); }getExchanger(url)默认得到的是HeaderExchanger,可通过exchanger参数配置。
到HeaderExchanger中看看bind方法
看下Transporters的bind方法。
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getTransporter方法获取一个自适应的Transporter，类名为Transporter$Adaptive，我们来看一下其源码：
package org.apache.dubbo.remoting; import org.apache.dubbo.common.URL; import org.apache.dubbo.common.extension.ExtensionLoader; public class Transporter$Adaptive implements Transporter { public Transporter$Adaptive() { } public Client connect(URL var1, ChannelHandler var2) throws RemotingException { if (var1 == null) { throw new IllegalArgumentException("url == null"); } else { String var4 = var1.getParameter("client", var1.getParameter("transporter", "netty")); if (var4 == null) { throw new IllegalStateException("Fail to get extension(org.apache.dubbo.remoting.Transporter) name from url(" + var1.toString() + ") use keys([client, transporter])"); } else { Transporter var5 = (Transporter)ExtensionLoader.getExtensionLoader(Transporter.class).getExtension(var4); return var5.connect(var1, var2); } } } public Server bind(URL var1, ChannelHandler var2) throws RemotingException { if (var1 == null) { throw new IllegalArgumentException("url == null"); } else { String var4 = var1.getParameter("server", var1.getParameter("transporter", "netty")); if (var4 == null) { throw new IllegalStateException("Fail to get extension(org.apache.dubbo.remoting.Transporter) name from url(" + var1.toString() + ") use keys([server, transporter])"); } else { Transporter var5 = (Transporter)ExtensionLoader.getExtensionLoader(Transporter.class).getExtension(var4); return var5.bind(var1, var2); } } } }可以看到Transporter$Adaptive通过判断URL中是否有transporter参数，如果没有，就默认为netty。
示例中服务的URL为
dubbo://192.168.124.1:20880/org.apache.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=192.168.124.1&bind.port=20880&dubbo=2.0.2&generic=false&interface=org.apache.dubbo.demo.DemoService&methods=sayHello&pid=8004&qos.port=22222&side=provider×tamp=1536114090787