七Dubbo各模块的层次核心接口及组件类--2Exchange--Remoting模块
七Dubbo各模块的层次核心接口及组件类--2Exchange--Remoting模块
7.2 Exchange(remoting模块)
信息交换层：封装请求/响应模式，同步转异步，以request、response为中心，Exchanger 、ExchangeChannel ExchangeClient ExchangeServer为拓展接口。
| 模块 | Dubbo层 | 各层核心接口 |
|---|---|---|
| Service | ServiceBean ReferanceBean | |
| dubbo-config | Config | ServiceConfig ReferanceConfig |
| dubbo-RPC | Proxy | ServiceProxy ProxyFactory |
| dubbo-registry | Registry | RegistryFactory Registry RegistryService |
| dubbo-cluster | Cluster | Cluster Directory Router LoadBalance |
| dubbo-monitor | Monitor | |
| dubbo-RPC | Protocol | Filter Invoke Prococol Exporter |
| dubbo-remoting | Exchange | Exchanger ExchangeChannel ExchangeClient ExchangeServer |
| ： | Transport | Transport Channel Client Server Codec |
| dubbo-common | Serialize |
7.2.1 Exchangers
区别Exchanger与Exchangers
1 同样提供了bind和connect方法，但是Exchanger是Dubbo体系层级结构中的，具有继承结构关系的类，定义了SPI服务接口（包括默认adaptive参数的设置），以及定义了服务接口中的具体方法；但是Exchangers仅仅是Exchanger的门面类，代替后者做了具体的方法的实现，以及获取SPI对象（如getExtensionLoader）。
2 Exchangers的引入，作为一个代理层的作用，把原本需要调用Exchanger的bind和connect方法的，代理到其上，通过spi的getExtensionLoader，来调用对应的实现
//Exchanger定义了SPI的接口，以及方法上的自适应参数设置，以用来在URL中针对相应的参数，做对应服务实现类的选择
@SPI(HeaderExchanger.NAME)
public interface Exchanger {
/**
* bind.
*
* @param url
* @param handler
* @return message server
*/
@Adaptive({Constants.EXCHANGER_KEY})
ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException;
/**
* connect.
*
* @param url
* @param handler
* @return message channel
*/
@Adaptive({Constants.EXCHANGER_KEY})
ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException;
}
Exchangers为Exchanger层的facade门面，也就是remoting模块的集成的API，通过该类及其提供的接口方法bind、connect，创建客户端ExchangeClient，自动调用remoting层内Exchange和transport的通信底层构建，进行通信。
提供static的方法——bind和connect。
bind绑定，获取服务端ExchangeServer；connect，创建客户端ExchangeClient。
public class Exchangers {
static {
// check duplicate jar package
Version.checkDuplicate(Exchangers.class);
}
private Exchangers() {
}
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);
}
public static ExchangeClient connect(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).connect(url, handler);
}
//这里是通过dubbo的spi机制，动态的获取Exchanger的实现类，HeaderExchanger，调用connect、bind执行具体方法。
public static Exchanger getExchanger(URL url) {
String type = url.getParameter(Constants.EXCHANGER_KEY, Constants.DEFAULT_EXCHANGER);
return getExchanger(type);
}
public static Exchanger getExchanger(String type) {
return ExtensionLoader.getExtensionLoader(Exchanger.class).getExtension(type);
}
}
Exchanger的默认实现（spi服务）
public class HeaderExchanger implements Exchanger {
public static final String NAME = "header";
//connect建立客户端
// 这里包含了多个调用，分别如下：
// 1. 创建 HeaderExchangeHandler 对象
// 2. 创建 DecodeHandler 对象
// 3. 通过 Transporters 构建 Client 实例
// 4. 创建 HeaderExchangeClient 对象
public ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeClient(Transporters.connect(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}
//bind建立服务端
public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}
}
7.2.2 ExchangeChannel
7.2.3 ExchangeClient
ExchangeClient实际上并不具备通信能力，它需要基于更底层的客户端实例进行通信。比如NettyClient、MinaClient等，默认情况下，Dubbo使用NettyClient进行通信。
