Dubbo之SPI机制详解

spi机制的思想提供一种更加灵活的，可插拔式的机制。本文分别对比了java和dubbo的spi的实现的区别，重点讨论dubbo的实现原理。

java spi机制

SPI，Service Provider Interface，主要是被框架的开发人员使用，比如java.sql.Driver接口，其他不同厂商可以针对同一接口做出不同的实现，mysql和postgresql都有不同的实现提供给用户，而Java的SPI机制可以为某个接口寻找服务实现。

当服务的提供者提供了一种接口的实现之后，需要在classpath下的META-INF/services/目录里创建一个以服务接口命名的文件，这个文件里的内容就是这个接口的具体的实现类。当其他的程序需要这个服务的时候，就可以通过查找这个jar包（一般都是以jar包做依赖）的META-INF/services/中的配置文件，配置文件中有接口的具体实现类名，可以根据这个类名进行加载实例化，就可以使用该服务了。JDK中查找服务的实现的工具类是：java.util.ServiceLoader

因为本文的主体内容是dubbo，所以这边就不对ServiceLoader的源码进行深入的解析。这边写了一个例子。

package com.shang.spi;

/**
 * @author shang
 * @date 2019/1/5
 */
public interface DubboService {
    void sayHello();
}

DubboService的实现类AppleService

package com.shang.spi;

/**
 * @author shang
 * @date 2019/1/5
 */
public class AppleService implements DubboService {
    @Override
    public void sayHello() {
        System.out.println("apple");
    }
}


package com.shang.spi;

import java.util.Iterator;
import java.util.ServiceLoader;

/**
 * @author shang
 * @date 2019/1/5
 */
public class ServiceMain {

    public static void main(String[] args) {
        ServiceLoader<DubboService> spiLoader = ServiceLoader.load(DubboService.class);
        Iterator<DubboService> iteratorSpi = spiLoader.iterator();
        while (iteratorSpi.hasNext()) {
            DubboService dubboService = iteratorSpi.next();
            dubboService.sayHello();
        }
    }
}

dubbo spi机制

dubbo的spi实现原理和java spi相似，只不过增强了一些功能和优化。java spi的是把所有的spi都加载到内存，但对于dubbo来说可能只需要加载用户指定的实现方式，而不需要全部加载进来，全部加载也会有性能问题，所以dubbo实现的是在有用到的时候去加载这些扩展组件。

spi机制有几个重要的注解：

1、@SPI注解，被此注解标记的接口，就表示是一个可扩展的接口，并标注默认值。
2、@Adaptive注解，有两种注解方式：一种是注解在类上，一种是注解在方法上。
3、@Activate注解，此注解需要注解在类上或者方法上，并注明被激活的条件，以及所有的被激活实现类中的排序信息

本文重点分析1和2的解析过程，3其实是在loadFile的时候可能被激活。

什么时候加载扩展类？

public class ReferenceConfig<T> extends AbstractReferenceConfig {

    private static final long serialVersionUID = -5864351140409987595L;

    private static final Protocol refprotocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();

    private static final Cluster cluster = ExtensionLoader.getExtensionLoader(Cluster.class).getAdaptiveExtension();

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

...
}

我们来看下消费者生成refer的代码。从ReferenceConfig这个类中我们可以看到需要初始化的扩展类有Protocol、Cluster和ProxyFactory。那么其实就是需要的时候再去加载。所有的实现都在ExtensionLoader类中。这个类也不负责，一千行左右的代码。
下面我们就跟着Protocol的扩展来看下源码的实现方式。

Protocol refprotocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();

在看实现的代码之前我们先看加Protocol这个扩展类，为了尽量的简洁删除了一些包路径和注释。
可以看到Protocol的spi的默认值是dubbo，这个在初始化类的时候很有用，假如你的Protocol实现类有很多，在dubbo中就有：

@SPI("dubbo")
public interface Protocol {
    
    int getDefaultPort();

    
    @Adaptive
    <T> Exporter<T> export(Invoker<T> invoker) throws RpcException;

   
    @Adaptive
    <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException;

   
    void destroy();

    
    default void destroyServer() {
        //空实现
    }
}

那么spi的Adaptive就是一种自适应的注解，具体是怎么实现的我们继续看代码。

public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if (type == null)
            throw new IllegalArgumentException("Extension type == null");
        if(!type.isInterface()) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
        }
        if(!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type(" + type + 
                    ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        }
        
        ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        if (loader == null) {
            EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
            loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
        }
        return loader;
    }

getExtensionLoader方法最主要的实现就是把所有的扩展放到EXTENSION_LOADERS这个容器中，避免二次加载。
private static final ConcurrentMap<Class, ExtensionLoader> EXTENSION_LOADERS = new ConcurrentHashMap<Class, ExtensionLoader>();

如何实现自适应扩展机制的？

下面其实就是为了获取自适应的扩展机制，Adaptive这个注解的自适应。调用的路径大概是这样的：

getAdaptiveExtension()-->
                createAdaptiveExtension()-->
                                getAdaptiveExtensionClass()-->
                                                getExtensionClasses()-->
                                                                loadExtensionClasses()

public T getAdaptiveExtension() {
        Object instance = cachedAdaptiveInstance.get();
        if (instance == null) {
            if(createAdaptiveInstanceError == null) {
                synchronized (cachedAdaptiveInstance) {
                    instance = cachedAdaptiveInstance.get();
                    if (instance == null) {
                        try {
                            //创建自适应扩展
                            instance = createAdaptiveExtension();
                            //设置缓存
                            cachedAdaptiveInstance.set(instance);
                        } catch (Throwable t) {
                            createAdaptiveInstanceError = t;
                            throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
                        }
                    }
                }
            }
            else {
                throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
            }
        }

        return (T) instance;
    }

createAdaptiveExtension通过名字就能可以知道这是创建自适应的扩展对象

private T createAdaptiveExtension() {
        try {
            //获取自适应扩展类，通过反射实例化
            return injectExtension((T) getAdaptiveExtensionClass().newInstance());
        } catch (Exception e) {
            throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
        }
    }

路径：getAdaptiveExtensionClass->getExtensionClasses->loadExtensionClasses->loadFile
cachedAdaptiveClass会在loadFile进行获取

private Class<?> getAdaptiveExtensionClass() {
        getExtensionClasses();
        //如果缓存中已经找到自适应类的话直接返回，意思也就是这个spi有Adaptive的注解类
        //比如：当前获取的自适应实现类是AdaptiveExtensionFactory或者是AdaptiveCompiler，就直接返回，这两个类是特殊用处的，不用代码生成，而是现成的代码
        if (cachedAdaptiveClass != null) {
            return cachedAdaptiveClass;
        }
        //否则需要代理类生成相关代理
        return cachedAdaptiveClass = createAdaptiveExtensionClass();
    }

getExtensionClasses->loadFile直接从文件加载

private Map<String, Class<?>> getExtensionClasses() {
        Map<String, Class<?>> classes = cachedClasses.get();
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                    classes = loadExtensionClasses();
                    cachedClasses.set(classes);
                }
            }
        }
        return classes;
	}

    // 此方法已经getExtensionClasses方法同步过。
    private Map<String, Class<?>> loadExtensionClasses() {
        final SPI defaultAnnotation = type.getAnnotation(SPI.class);
        if(defaultAnnotation != null) {
            String value = defaultAnnotation.value();
            if(value != null && (value = value.trim()).length() > 0) {
                String[] names = NAME_SEPARATOR.split(value);
                if(names.length > 1) {
                    throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
                            + ": " + Arrays.toString(names));
                }
                if(names.length == 1) cachedDefaultName = names[0];
            }
        }
        
        Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
        //从文件加载
        loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
        loadFile(extensionClasses, DUBBO_DIRECTORY);
        loadFile(extensionClasses, SERVICES_DIRECTORY);
        return extensionClasses;
    }

主动加载相关type的spi子类

private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
    String fileName = dir + type.getName();
    try {
        Enumeration<java.net.URL> urls;
        ClassLoader classLoader = findClassLoader();
        if (classLoader != null) {
            urls = classLoader.getResources(fileName);
        } else {
            urls = ClassLoader.getSystemResources(fileName);
        }
        if (urls != null) {
            while (urls.hasMoreElements()) {
                //配置文件路径
                java.net.URL url = urls.nextElement();
                try {
                    BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
                    try {
                        String line = null;
                        //每次处理一行
                        while ((line = reader.readLine()) != null) {
                            //#号以后的为注释
                            final int ci = line.indexOf('#');
                            //注释去掉
                            if (ci >= 0) line = line.substring(0, ci);
                            line = line.trim();
                            if (line.length() > 0) {
                                try {
                                    String name = null;
                                    //=号之前的为扩展名字，后面的为扩展类实现的全限定名
                                    int i = line.indexOf('=');
                                    if (i > 0) {
                                        name = line.substring(0, i).trim();
                                        line = line.substring(i + 1).trim();
                                    }
                                    if (line.length() > 0) {
                                        //加载扩展类的实现
                                        Class<?> clazz = Class.forName(line, true, classLoader);
                                        //查看类型是否匹配
                                        //type是Protocol接口
                                        //clazz就是Protocol的各个实现类
                                        if (! type.isAssignableFrom(clazz)) {
                                            throw new IllegalStateException();
                                        }
                                        //如果实现类是@Adaptive类型的，会赋值给cachedAdaptiveClass，这个用来存放被@Adaptive注解的实现类
                                        if (clazz.isAnnotationPresent(Adaptive.class)) {
                                            if(cachedAdaptiveClass == null) {
                                                cachedAdaptiveClass = clazz;
                                            } else if (! cachedAdaptiveClass.equals(clazz)) {
                                                throw new IllegalStateException();
                                            }
                                        } else {//不是@Adaptice类型的类，就是没有注解@Adaptive的实现类
                                            try {//判断是否是wrapper类型
                                            	//如果得到的实现类的构造方法中的参数是扩展点类型的，就是一个Wrapper类
                                                //比如ProtocolFilterWrapper，实现了Protocol类，
                                                //而它的构造方法是这样public ProtocolFilterWrapper(Protocol protocol)
                                                //就说明这个类是一个包装类
                                                clazz.getConstructor(type);
                                                //cachedWrapperClasses用来存放当前扩展点实现类中的包装类
                                                Set<Class<?>> wrappers = cachedWrapperClasses;
                                                if (wrappers == null) {
                                                    cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
                                                    wrappers = cachedWrapperClasses;
                                                }
                                                wrappers.add(clazz);
                                            } catch (NoSuchMethodException e) {
                                            //没有上面提到的构造器，则说明不是wrapper类型
                                            	//获取无参构造
                                                clazz.getConstructor();
                                                //没有名字，就是配置文件中没有xxx=xxxx.com.xxx这种
                                                if (name == null || name.length() == 0) {
                                                	//去找@Extension注解中配置的值
                                                    name = findAnnotationName(clazz);
                                                    //如果还没找到名字，从类名中获取
                                                    if (name == null || name.length() == 0) {
                                                    	//比如clazz是DubboProtocol，type是Protocol
                                                        //这里得到的name就是dubbo
                                                        if (clazz.getSimpleName().length() > type.getSimpleName().length()
                                                                && clazz.getSimpleName().endsWith(type.getSimpleName())) {
                                                            name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - type.getSimpleName().length()).toLowerCase();
                                                        } else {
                                                            throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);
                                                        }
                                                    }
                                                }
                                                //有可能配置了多个名字
                                                String[] names = NAME_SEPARATOR.split(name);
                                                if (names != null && names.length > 0) {
                                                    //是否是Active类型的类
                                                    Activate activate = clazz.getAnnotation(Activate.class);
                                                    if (activate != null) {
                                                    	//第一个名字作为键，放进cachedActivates这个map中缓存
                                                        cachedActivates.put(names[0], activate);
                                                    }
                                                    for (String n : names) {
                                                        if (! cachedNames.containsKey(clazz)) {
                                                            //放入Extension实现类与名称映射的缓存中去，每个class只对应第一个名称有效
                                                            cachedNames.put(clazz, n);
                                                        }
                                                        Class<?> c = extensionClasses.get(n);
                                                        if (c == null) {
                                                            //放入到extensionClasses缓存中去，多个name可能对应一份extensionClasses
                                                            extensionClasses.put(n, clazz);
                                                        } else if (c != clazz) {
                                                            throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
                                                        }
                                                    }
                                                }
                                            }
                                        }
                                    }
                                } catch (Throwable t) { }
                            }
                        } // end of while read lines
                    } finally {
                        reader.close();
                    }
                } catch (Throwable t) { }
            } // end of while urls
        }
    } catch (Throwable t) { }
}

好了，还是要回到getAdaptiveExtensionClass这个方法，上面的loadFile是按规则加载了所有的type类型的spi类，那么如果生成自适应类呢？
我们继续看createAdaptiveExtensionClass这个方法的createAdaptiveExtensionClassCode

 private Class<?> createAdaptiveExtensionClass() {
        String code = createAdaptiveExtensionClassCode();
        ClassLoader classLoader = findClassLoader();
        com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
        return compiler.compile(code, classLoader);
    }

编写自适应代理类

createAdaptiveExtensionClassCode的代码有点长，具体就是生成代理类，我们先看下代码：

private String createAdaptiveExtensionClassCode() {
        StringBuilder codeBuidler = new StringBuilder();
        Method[] methods = type.getMethods();
        boolean hasAdaptiveAnnotation = false;
        for(Method m : methods) {
            if(m.isAnnotationPresent(Adaptive.class)) {
                hasAdaptiveAnnotation = true;
                break;
            }
        }
        // 完全没有Adaptive方法，则不需要生成Adaptive类
        if(! hasAdaptiveAnnotation)
            throw new IllegalStateException("No adaptive method on extension " + type.getName() + ", refuse to create the adaptive class!");
        
        codeBuidler.append("package " + type.getPackage().getName() + ";");
        codeBuidler.append("\nimport " + ExtensionLoader.class.getName() + ";");
        codeBuidler.append("\npublic class " + type.getSimpleName() + "$Adpative" + " implements " + type.getCanonicalName() + " {");
        
        for (Method method : methods) {
            Class<?> rt = method.getReturnType();
            Class<?>[] pts = method.getParameterTypes();
            Class<?>[] ets = method.getExceptionTypes();

            Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
            StringBuilder code = new StringBuilder(512);
            if (adaptiveAnnotation == null) {
                code.append("throw new UnsupportedOperationException(\"method ")
                        .append(method.toString()).append(" of interface ")
                        .append(type.getName()).append(" is not adaptive method!\");");
            } else {
                int urlTypeIndex = -1;
                for (int i = 0; i < pts.length; ++i) {
                    if (pts[i].equals(URL.class)) {
                        urlTypeIndex = i;
                        break;
                    }
                }
                // 有类型为URL的参数
                if (urlTypeIndex != -1) {
                    // Null Point check
                    String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"url == null\");",
                                    urlTypeIndex);
                    code.append(s);
                    
                    s = String.format("\n%s url = arg%d;", URL.class.getName(), urlTypeIndex); 
                    code.append(s);
                }
                // 参数没有URL类型
                else {
                    String attribMethod = null;
                    
                    // 找到参数的URL属性
                    LBL_PTS:
                    for (int i = 0; i < pts.length; ++i) {
                        Method[] ms = pts[i].getMethods();
                        for (Method m : ms) {
                            String name = m.getName();
                            if ((name.startsWith("get") || name.length() > 3)
                                    && Modifier.isPublic(m.getModifiers())
                                    && !Modifier.isStatic(m.getModifiers())
                                    && m.getParameterTypes().length == 0
                                    && m.getReturnType() == URL.class) {
                                urlTypeIndex = i;
                                attribMethod = name;
                                break LBL_PTS;
                            }
                        }
                    }
                    if(attribMethod == null) {
                        throw new IllegalStateException("fail to create adative class for interface " + type.getName()
                        		+ ": not found url parameter or url attribute in parameters of method " + method.getName());
                    }
                    
                    // Null point check
                    String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"%s argument == null\");",
                                    urlTypeIndex, pts[urlTypeIndex].getName());
                    code.append(s);
                    s = String.format("\nif (arg%d.%s() == null) throw new IllegalArgumentException(\"%s argument %s() == null\");",
                                    urlTypeIndex, attribMethod, pts[urlTypeIndex].getName(), attribMethod);
                    code.append(s);

                    s = String.format("%s url = arg%d.%s();",URL.class.getName(), urlTypeIndex, attribMethod); 
                    code.append(s);
                }
                
                String[] value = adaptiveAnnotation.value();
                // 没有设置Key，则使用“扩展点接口名的点分隔 作为Key
                if(value.length == 0) {
                    char[] charArray = type.getSimpleName().toCharArray();
                    StringBuilder sb = new StringBuilder(128);
                    for (int i = 0; i < charArray.length; i++) {
                        if(Character.isUpperCase(charArray[i])) {
                            if(i != 0) {
                                sb.append(".");
                            }
                            sb.append(Character.toLowerCase(charArray[i]));
                        }
                        else {
                            sb.append(charArray[i]);
                        }
                    }
                    value = new String[] {sb.toString()};
                }
                
                boolean hasInvocation = false;
                for (int i = 0; i < pts.length; ++i) {
                    if (pts[i].getName().equals("com.alibaba.dubbo.rpc.Invocation")) {
                        // Null Point check
                        String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"invocation == null\");", i);
                        code.append(s);
                        s = String.format("\nString methodName = arg%d.getMethodName();", i); 
                        code.append(s);
                        hasInvocation = true;
                        break;
                    }
                }
                
                String defaultExtName = cachedDefaultName;
                String getNameCode = null;
                for (int i = value.length - 1; i >= 0; --i) {
                    if(i == value.length - 1) {
                        if(null != defaultExtName) {
                            if(!"protocol".equals(value[i]))
                                if (hasInvocation) 
                                    getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                                else
                                    getNameCode = String.format("url.getParameter(\"%s\", \"%s\")", value[i], defaultExtName);
                            else
                                getNameCode = String.format("( url.getProtocol() == null ? \"%s\" : url.getProtocol() )", defaultExtName);
                        }
                        else {
                            if(!"protocol".equals(value[i]))
                                if (hasInvocation) 
                                    getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                                else
                                    getNameCode = String.format("url.getParameter(\"%s\")", value[i]);
                            else
                                getNameCode = "url.getProtocol()";
                        }
                    }
                    else {
                        if(!"protocol".equals(value[i]))
                            if (hasInvocation) 
                                getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
                            else
                                getNameCode = String.format("url.getParameter(\"%s\", %s)", value[i], getNameCode);
                        else
                            getNameCode = String.format("url.getProtocol() == null ? (%s) : url.getProtocol()", getNameCode);
                    }
                }
                code.append("\nString extName = ").append(getNameCode).append(";");
                // check extName == null?
                String s = String.format("\nif(extName == null) " +
                		"throw new IllegalStateException(\"Fail to get extension(%s) name from url(\" + url.toString() + \") use keys(%s)\");",
                        type.getName(), Arrays.toString(value));
                code.append(s);
                
                s = String.format("\n%s extension = (%<s)%s.getExtensionLoader(%s.class).getExtension(extName);",
                        type.getName(), ExtensionLoader.class.getSimpleName(), type.getName());
                code.append(s);
                
                // return statement
                if (!rt.equals(void.class)) {
                    code.append("\nreturn ");
                }

                s = String.format("extension.%s(", method.getName());
                code.append(s);
                for (int i = 0; i < pts.length; i++) {
                    if (i != 0)
                        code.append(", ");
                    code.append("arg").append(i);
                }
                code.append(");");
            }
            
            codeBuidler.append("\npublic " + rt.getCanonicalName() + " " + method.getName() + "(");
            for (int i = 0; i < pts.length; i ++) {
                if (i > 0) {
                    codeBuidler.append(", ");
                }
                codeBuidler.append(pts[i].getCanonicalName());
                codeBuidler.append(" ");
                codeBuidler.append("arg" + i);
            }
            codeBuidler.append(")");
            if (ets.length > 0) {
                codeBuidler.append(" throws ");
                for (int i = 0; i < ets.length; i ++) {
                    if (i > 0) {
                        codeBuidler.append(", ");
                    }
                    codeBuidler.append(pts[i].getCanonicalName());
                }
            }
            codeBuidler.append(" {");
            codeBuidler.append(code.toString());
            codeBuidler.append("\n}");
        }
        codeBuidler.append("\n}");
        if (logger.isDebugEnabled()) {
            logger.debug(codeBuidler.toString());
        }
        return codeBuidler.toString();
    }

那么他生成的代理类是怎么样的呢？我们还是以com.alibaba.dubbo.rpc.Protocol 为例，他生成的代理类就是Protocol$Adpative，中间多了一个$。

import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {
  public com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0, com.alibaba.dubbo.common.URL arg1) throws java.lang.Class {
    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);
  }
  
  public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.Invoker {
    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 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!");
  }
}

其他的类型的代理代码生成是类似的，上面的代码是硬编码，需要把他编译并加载到内存中，具体还是要回到createAdaptiveExtensionClass这个方法，代码在上面，这里就不再贴了，需要通过Compiler这个类找到对应的自适应实现，这里得到的就是AdaptiveCompiler，最后调用compiler.compile(code, classLoader);来编译上面生成的类并返回，先进入AdaptiveCompiler的compile方法：

public Class<?> compile(String code, ClassLoader classLoader) {
    Compiler compiler;
    ExtensionLoader<Compiler> loader = ExtensionLoader.getExtensionLoader(Compiler.class);
    //默认的Compiler名字
    String name = DEFAULT_COMPILER; // copy reference
    //有指定了Compiler名字，就使用指定的名字来找到Compiler实现类
    if (name != null && name.length() > 0) {
        compiler = loader.getExtension(name);
    } else {//没有指定Compiler名字，就查找默认的Compiler的实现类
        compiler = loader.getDefaultExtension();
    }
   
    return compiler.compile(code, classLoader);
}

/*
 * Copyright 1999-2011 Alibaba Group.
 *  
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *  
 *      http://www.apache.org/licenses/LICENSE-2.0
 *  
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.dubbo.common.compiler;


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

/**
 * Compiler. (SPI, Singleton, ThreadSafe)
 * 
 * @author william.liangf
 */
@SPI("javassist")
public interface Compiler {

	/**
	 * Compile java source code.
	 * 
	 * @param code Java source code
	 * @param classLoader TODO
	 * @return Compiled class
	 */
	Class<?> compile(String code, ClassLoader classLoader);

}

至此整个自适应的代理方式都已经解析完了。但是我觉得实现的有点过于复杂了，是适应类的存在是很有必要的吗？我觉得也未必。
感觉在spi的实现中插入了Adaptive等的实现是把简单的spi机制搞得复杂化了，绕了一大圈去解决一个自适应代理类等方式是否可以简单化。