Dubbo SPI扩展

SPI

是dubbo中一个设计和巧妙的机制。

Dubbo对JDK SPI进行了扩展，对服务提供者配置文件中的内容进行了改造，由原来的提供者类的全限定名列表改成了KV形式的列表，这也导致了Dubbo中无法直接使用JDK ServiceLoader，所以，与之对应的，在Dubbo中有ExtensionLoader，ExtensionLoader是扩展点载入器，用于载入Dubbo中的各种可配置组件，比如：动态代理方式（ProxyFactory）、负载均衡策略（LoadBalance）、RCP协议（Protocol）、拦截器（Filter）、容器类型（Container）、集群方式（Cluster）和注册中心类型（RegistryFactory）等。

总之，Dubbo为了应对各种场景，它的所有内部组件都是通过这种SPI的方式来管理的，这也是为什么Dubbo需要将服务提供者配置文件设计成KV键值对形式，这个K就是我们在Dubbo配置文件或注解中用到的K，Dubbo直接通过服务接口（上面提到的ProxyFactory、LoadBalance、Protocol、Filter等）和配置的K从ExtensionLoader拿到服务提供的实现类。

同时，由于Dubbo使用了URL总线的设计，即很多参数通过URL对象来传递，在实际中，具体要用到哪个值，可以通过URL中的参数值来指定。

####SPI初始化

ServiceConfig

/**
 * The {@link Protocol} implementation with adaptive functionality,it will be different in different scenarios.
 * A particular {@link Protocol} implementation is determined by the protocol attribute in the {@link URL}.
 * For example:
 *
 * <li>when the url is registry://224.5.6.7:1234/org.apache.dubbo.registry.RegistryService?application=dubbo-sample,
 * then the protocol is <b>RegistryProtocol</b></li>
 *
 * <li>when the url is dubbo://224.5.6.7:1234/org.apache.dubbo.config.api.DemoService?application=dubbo-sample, then
 * the protocol is <b>DubboProtocol</b></li>
 * <p>
 * Actually，when the {@link ExtensionLoader} init the {@link Protocol} instants,it will automatically wraps two
 * layers, and eventually will get a <b>ProtocolFilterWrapper</b> or <b>ProtocolListenerWrapper</b>
 */
private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();

/**
 * A {@link ProxyFactory} implementation that will generate a exported service proxy,the JavassistProxyFactory is its
 * default implementation
 */
private static final ProxyFactory PROXY_FACTORY = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();

getExtensionLoader会根据传入的type值来获取不通的ExtensionLoader，ExtensionLoader是由ConcurrentMap保存，ConcurrentMap<Class, ExtensionLoader>，key是class

getAdaptiveExtension其中的createAdaptiveExtension()-》getAdaptiveExtensionClass-》getExtensionClasses-》loadExtensionClasses会扫描

1. META-INF/dubbo/
2. META-INF/dubbo/internal
3. META-INF/services/ 下面的配置文件

因为是通过url来保存参数，所以能通过url来重新生成新的proxyFactory

关键方法在createAdaptiveExtensionClass()

String code = new AdaptiveClassCodeGenerator(type, cachedDefaultName).generate();

这一段会生成新的class文件，以ProxyFactory为例： 生成后的代码

package org.apache.dubbo.rpc;
import org.apache.dubbo.common.extension.ExtensionLoader;
public class ProxyFactory$Adaptive implements org.apache.dubbo.rpc.ProxyFactory {

public java.lang.Object getProxy(org.apache.dubbo.rpc.Invoker arg0) throws org.apache.dubbo.rpc.RpcException {
	if (arg0 == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument == null");
	if (arg0.getUrl() == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument getUrl() == null");
	org.apache.dubbo.common.URL url = arg0.getUrl();
	String extName = url.getParameter("proxy", "javassist");
	if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.ProxyFactory) name from url (" + url.toString() + ") use keys([proxy])");
	org.apache.dubbo.rpc.ProxyFactory extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
	return extension.getProxy(arg0);
}

public java.lang.Object getProxy(org.apache.dubbo.rpc.Invoker arg0, boolean arg1) throws org.apache.dubbo.rpc.RpcException {
	if (arg0 == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument == null");
	if (arg0.getUrl() == null) throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument getUrl() == null");
	org.apache.dubbo.common.URL url = arg0.getUrl();
	String extName = url.getParameter("proxy", "javassist");
	if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.ProxyFactory) name from url (" + url.toString() + ") use keys([proxy])");
	org.apache.dubbo.rpc.ProxyFactory extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
	return extension.getProxy(arg0, arg1);
}

public org.apache.dubbo.rpc.Invoker getInvoker(java.lang.Object arg0, java.lang.Class arg1, org.apache.dubbo.common.URL arg2) throws org.apache.dubbo.rpc.RpcException {
	if (arg2 == null) throw new IllegalArgumentException("url == null");
	org.apache.dubbo.common.URL url = arg2;
	//这个javassist是在初始化ExtensionLoader时传的
	String extName = url.getParameter("proxy", "javassist");
	if(extName == null) throw new IllegalStateException("Failed to get extension (org.apache.dubbo.rpc.ProxyFactory) name from url (" + url.toString() + ") use keys([proxy])");
	org.apache.dubbo.rpc.ProxyFactory extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
	return extension.getInvoker(arg0, arg1, arg2);
}
}

这样返回给ServiceBean的

private static final ProxyFactory PROXY_FACTORY = 
	ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();

这样在doExportUrlsFor1Protocol导出方法中

Invoker<?> invoker = PROXY_FACTORY.getInvoker(ref, (Class) interfaceClass, 

能调到JavassistProxyFactory的getInvoker