com.alibaba.dubbo.demo.provider.DemoProvider#main

    com.alibaba.dubbo.container.Main.main(args);

com.alibaba.dubbo.container.Main#main

//初始化com.alibaba.dubbo.container.Main#loader静态变量
private static final ExtensionLoader<Container> loader = ExtensionLoader.getExtensionLoader(Container.class);

com.alibaba.dubbo.common.extension.ExtensionLoader#getExtensionLoader

//获取ExtensionLoader并缓存到com.alibaba.dubbo.common.extension.ExtensionLoader#EXTENSION_LOADERS
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);

//ExtensionLoader包含两个属性
private ExtensionLoader(Class<?> type) {
        this.type = type;
        objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
//关于objectFactory的细节：它就是为dubbo的IOC提供所有对象。

到这里获取到了
Container.class类型的ExtensionLoader（包含一个不为空的objectFactory）
ExtensionFactory.class类型的ExtensionLoader
并缓存到了类变量EXTENSION_LOADERS中

@Adaptive注解

注解在类上：代表人工实现编码，即实现一个装饰类（装饰模式），如：ExtensionFactory
注解在方法上：代表自动生成和编译一个动态的adaptive类，如：Protocol$Adaptive

@Adaptive注解在方法上(以Protocol为例)

com.alibaba.dubbo.config.spring.ServiceBean
com.alibaba.dubbo.config.ServiceConfig

//初始化类变量
private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();

com.alibaba.dubbo.common.extension.ExtensionLoader#getAdaptiveExtension

//创建并缓存到成员变量cachedAdaptiveInstance中
instance = createAdaptiveExtension();
cachedAdaptiveInstance.set(instance);

com.alibaba.dubbo.common.extension.ExtensionLoader#getAdaptiveExtensionClass
com.alibaba.dubbo.common.extension.ExtensionLoader#getExtensionClasses

classes = loadExtensionClasses();
cachedClasses.set(classes);

com.alibaba.dubbo.common.extension.ExtensionLoader#loadExtensionClasses

Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
//这个loadFile方法很复杂
//DUBBO_INTERNAL_DIRECTORY = META-INF\dubbo\internal
//DUBBO_DIRECTORY = META-INF/dubbo/
//SERVICES_DIRECTORY = META-INF/services/
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
loadFile(extensionClasses, DUBBO_DIRECTORY);
loadFile(extensionClasses, SERVICES_DIRECTORY);

com.alibaba.dubbo.common.extension.ExtensionLoader#loadFile

//fileName = META-INF\dubbo\internal\com.alibaba.dubbo.rpc.Protocol这个文件下有很多相关的实现类
String fileName = dir + type.getName();

image.png

关于loadFile的细节：
目的：通过把配置文件META-INF\dubbo\internal\com.alibaba.dubbo.rpc.Protocol的内容，储存在缓存变量里面
cachedAdaptiveClass:如果这个class含有adaptive注解就赋值，例如ExtensionFatory，而例如Protocol在这个环节是没有的。
cachedWrapperClasses：只有当该class没有adaptive注解，并且该class目标函数包含目标接口（type）类型，例如protocol里的spi就只有ProtocolFilterWrapper和ProtocolListenerWrapper能命中。

image.png

image.png

最后的extensionClasses值为：

image.png

因为是已Protocol这个类为例，这个类没有@Adaptive注解标注，所以成员变量cachedAdaptiveClass在这里肯定是为null的，即通过下面方法createAdaptiveExtensionClass去动态生成一个动态类Protocol$Adaptive。

 private Class<?> getAdaptiveExtensionClass() {
        getExtensionClasses();
        if (cachedAdaptiveClass != null) {
            return cachedAdaptiveClass;
        }
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }

动态生成的代码：

package com.alibaba.dubbo.rpc;

import com.alibaba.dubbo.common.extension.ExtensionLoader;

public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {
    public void destroy() {
        throw new UnsupportedOperationException(
                "method public abstract void com.alibaba.dubbo.rpc.Protocol.destroy() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
    }

    public int getDefaultPort() {
        throw new UnsupportedOperationException(
                "method public abstract int com.alibaba.dubbo.rpc.Protocol.getDefaultPort() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
    }

    public com.alibaba.dubbo.rpc.Exporter export(
            com.alibaba.dubbo.rpc.Invoker arg0)
            throws com.alibaba.dubbo.rpc.RpcException {
        if (arg0 == null)
            throw new IllegalArgumentException(
                    "com.alibaba.dubbo.rpc.Invoker argument == null");
        if (arg0.getUrl() == null)
            throw new IllegalArgumentException(
                    "com.alibaba.dubbo.rpc.Invoker argument getUrl() == null");
        com.alibaba.dubbo.common.URL url = arg0.getUrl();
        String extName = (url.getProtocol() == null ? "dubbo" : url
                .getProtocol());
        if (extName == null)
            throw new IllegalStateException(
                    "Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url("
                            + url.toString() + ") use keys([protocol])");
        com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader
                .getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class)
                .getExtension(extName);
        return extension.export(arg0);
    }

    public com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0,
            com.alibaba.dubbo.common.URL arg1)
            throws com.alibaba.dubbo.rpc.RpcException {
        if (arg1 == null)
            throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg1;
        String extName = (url.getProtocol() == null ? "dubbo" : url
                .getProtocol());
        if (extName == null)
            throw new IllegalStateException(
                    "Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url("
                            + url.toString() + ") use keys([protocol])");
        com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader
                .getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class)
                .getExtension(extName);
        return extension.refer(arg0, arg1);
    }
}

最后回到com.alibaba.dubbo.common.extension.ExtensionLoader#injectExtension ，这个方法的作用，IOC控制反转，实现动态注入。

image.png

最后：
com.alibaba.dubbo.common.extension.ExtensionLoader#getExtensionLoader 通过这个方法得到了对应实现类的具体对象Protocol$Adaptive,而生成此对象的ExtensionLoader的各个成员变量的值对应如下：

image.png

image.png