在看Dubbo源码的时候，总是出现下面的代码去加载一个类。
然后，看一下ProxyFactory这个类名上了加了一个注解。，
这里以ServiceConfig里的ProxyFactory为例，这里就是今天所要讲的Dubbo的SPI机制, 通过ExtensionLoader这个类去实现的。
这里最主要的的ETENTION_LOADERS是保存扩展点类和对应ExtensionLoader的的映射, EXTENSION_INSTANCES是缓存扩展类和反射实例化的映射，
getExtensionLoader函数，出入扩展类，如果没有，则放入IINSTENSION_LOADERS里面。映射的value是ExtensionLoader对象，可以看到新建的ExtensioLoader的扩展类
不是ExtensionFactory，则还是会调用ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()方法，来加载adaptive的扩展类。
然后，getAdaptiveExtension的，通过cacheAdaptiveExtension的get方法, 这里可以二次判断来防止的并发的操作。
因为开始都为空，createAdativeExtension的函数创建一个扩展点的对象。
然后调用getAdaptiveExtensionClass函数去加载扩展类，
首先是通过 getExtensionClasses函数，获取扩展点的类
是
因为开始都是空的，所以出事调用时候，会调用loadExtensionClasses函数，去加载所有的扩展类，
前面是获取@SPI注解的值，然后loadDirectoy方法去加载项目下 META-INF/dubbo/internal/，META-INF/dubbo/, META-INF/services/
这里就可以看出META-IN这下面的是都是扩展点的配置。
这里是利用classloade加载class的，还调用loaderResource就是I利用Class.forName去加载对应的类了。
加载完所有的扩展类后， 初始cacheAdaptiveClass为空，则创建createAdaptiveExtensionClass方法。
这里首先看一下，createAdapativeExtensionClassCode动态生成的代码，从下面可以看出它生成的类名=扩展类+$Adaptive，实现扩展类的接口，并且找到ClassLoader，并且加载ExtensionLoader的Complier，去编译生成的代码，并且加载到JVM，得到一个Class.
package org.apache.dubbo.rpc;
import org.apache.dubbo.common.extension.ExtensionLoader;
public class ProxyFactory$Adaptive implements org.apache.dubbo.rpc.ProxyFactory {
private static final org.apache.dubbo.common.logger.Logger logger = org.apache.dubbo.common.logger.LoggerFactory.getLogger(ExtensionLoader.class);
private java.util.concurrent.atomic.AtomicInteger count = new java.util.concurrent.atomic.AtomicInteger(0);
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;
String extName = url.getParameter("proxy", "javassist");
if(extName == null) throw new IllegalStateException("Fail to get extension(org.apache.dubbo.rpc.ProxyFactory) name from url(" + url.toString() + ") use keys([proxy])");
org.apache.dubbo.rpc.ProxyFactory extension = null;
try {
extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
}catch(Exception e){
if (count.incrementAndGet() == 1) {
logger.warn("Failed to find extension named " + extName + " for type org.apache.dubbo.rpc.ProxyFactory, will use default extension javassist instead.", e);
}
extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension("javassist");
}
return extension.getInvoker(arg0, arg1, arg2);
}
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("Fail to get extension(org.apache.dubbo.rpc.ProxyFactory) name from url(" + url.toString() + ") use keys([proxy])");
org.apache.dubbo.rpc.ProxyFactory extension = null;
try {
extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
}catch(Exception e){
if (count.incrementAndGet() == 1) {
logger.warn("Failed to find extension named " + extName + " for type org.apache.dubbo.rpc.ProxyFactory, will use default extension javassist instead.", e);
}
extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension("javassist");
}
return extension.getProxy(arg0, arg1);
}
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("Fail to get extension(org.apache.dubbo.rpc.ProxyFactory) name from url(" + url.toString() + ") use keys([proxy])");
org.apache.dubbo.rpc.ProxyFactory extension = null;
try {
extension = (org.apache.dubbo.rpc.ProxyFactory)ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension(extName);
}catch(Exception e){
if (count.incrementAndGet() == 1) {
logger.warn("Failed to find extension named " + extName + " for type org.apache.dubbo.rpc.ProxyFactory, will use default extension javassist instead.", e);
}
extension = (org.apache.dubbo.rpc.ProxyFactory).getExtensionLoader(org.apache.dubbo.rpc.ProxyFactory.class).getExtension("javassist");
}
return extension.getProxy(arg0);
}
}
总结：
本次对Dubbo 的SPI机制的一点粗略的理解。