从ServiceConfig类的protocol属性开始看起：
private static final Protocol protocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension(); 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); //初始化loader if (loader == null) { EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type)); loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type); } return loader; }loader是通过new ExtensionLoader<T>(type)初始化的，进入new ExtensionLoader<Protocol.class>(type)构造方法：
private ExtensionLoader(Class<?> type) { //将type赋值给全局变量type this.type = type; objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()); }因为此时type为Protocol.class，不等于ExtensionFactory.class，所以走的是ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()。
第二次进入getExtensionLoader办法，此时type为ExtensionFactory，初始化loader，进入new ExtensionLoader<ExtensionFactory.class>(type)，type等于ExtensionFactory.class，那么objectFactory 赋值为null，构造方法执行结束，返回loader为 ExtensionLoader<ExtensionFactory.class>，戒指执行loader.getAdaptiveExtension())，即ExtensionLoader<ExtensionFactory.class>.getAdaptiveExtension()办法：
@SuppressWarnings("unchecked") public T getAdaptiveExtension() { Object instance = cachedAdaptiveInstance.get(); if (instance == null) { if (createAdaptiveInstanceError == null) { synchronized (cachedAdaptiveInstance) { instance = cachedAdaptiveInstance.get(); if (instance == null) { try { //进入这里 instance = createAdaptiveExtension(); 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; } @SuppressWarnings("unchecked") private T createAdaptiveExtension() { try { //先看getAdaptiveExtensionClass return injectExtension((T) getAdaptiveExtensionClass().newInstance()); } catch (Exception e) { throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e); } } private Class<?> getAdaptiveExtensionClass() { //进入办法 getExtensionClasses(); if (cachedAdaptiveClass != null) { return cachedAdaptiveClass; } return cachedAdaptiveClass = createAdaptiveExtensionClass(); } private Map<String, Class<?>> getExtensionClasses() { Map<String, Class<?>> classes = cachedClasses.get(); if (classes == null) { synchronized (cachedClasses) { classes = cachedClasses.get(); if (classes == null) { //进入load办法 classes = loadExtensionClasses(); cachedClasses.set(classes); } } } return classes; } // synchronized in getExtensionClasses private Map<String, Class<?>> loadExtensionClasses() { //若type有spi注解且有value，则将cachedDefaultName赋值为该value final SPI defaultAnnotation = type.getAnnotation(SPI.class); if (defaultAnnotation != null) { String value = defaultAnnotation.value(); if ((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<?>>(); // 别离装载META-INF/resources目录下 // "META-INF/services/"， // "META-INF/dubbo/"， //"META-INF/dubbo/internal/"三个目录的合乎type的类 loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY); loadDirectory(extensionClasses, DUBBO_DIRECTORY); loadDirectory(extensionClasses, SERVICES_DIRECTORY); return extensionClasses; }type即为上文的ExtensionFactory类.
比方META-INF/dubbo/internal/com.alibaba.dubbo.common.extension.ExtensionFactory的内容如下：
adaptive=com.alibaba.dubbo.common.extension.factory.AdaptiveExtensionFactory
spi=com.alibaba.dubbo.common.extension.factory.SpiExtensionFactory
private void loadDirectory(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()) { java.net.URL resourceURL = urls.nextElement(); //进入这里 loadResource(extensionClasses, classLoader, resourceURL); } } } catch (Throwable t) { logger.error("Exception when load extension class(interface: " + type + ", description file: " + fileName + ").", t); } } private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, java.net.URL resourceURL) { try { BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), "utf-8")); try { String line; while ((line = reader.readLine()) != null) { final int ci = line.indexOf('#'); if (ci >= 0) line = line.substring(0, ci); line = line.trim(); if (line.length() > 0) { try { String name = null; int i = line.indexOf('='); if (i > 0) { name = line.substring(0, i).trim(); line = line.substring(i + 1).trim(); } if (line.length() > 0) { //进入这里 loadClass(extensionClasses, resourceURL, Class.forName(line, true, classLoader), name); } } catch (Throwable t) { IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + resourceURL + ", cause: " + t.getMessage(), t); exceptions.put(line, e); } } } } finally { reader.close(); } } catch (Throwable t) { logger.error("Exception when load extension class(interface: " + type + ", class file: " + resourceURL + ") in " + resourceURL, t); } } private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException { if (!type.isAssignableFrom(clazz)) { throw new IllegalStateException("Error when load extension class(interface: " + type + ", class line: " + clazz.getName() + "), class " + clazz.getName() + "is not subtype of interface."); } //判断类上有没有Adaptive注解 if (clazz.isAnnotationPresent(Adaptive.class)) { // 有的话则将cachedAdaptiveClass赋值为该类 //比方 AdaptiveExtensionFactory if (cachedAdaptiveClass == null) { cachedAdaptiveClass = clazz; } else if (!cachedAdaptiveClass.equals(clazz)) { throw new IllegalStateException("More than 1 adaptive class found: " + cachedAdaptiveClass.getClass().getName() + ", " + clazz.getClass().getName()); } } //判断是不是包装类 else if (isWrapperClass(clazz)) { Set<Class<?>> wrappers = cachedWrapperClasses; if (wrappers == null) { cachedWrapperClasses = new ConcurrentHashSet<Class<?>>(); wrappers = cachedWrapperClasses; } wrappers.add(clazz); } else { clazz.getConstructor(); //获取类简名，比方SpringExtensionFactory就是spring，AdaptiveExtensionFactory就是adaptive if (name == null || name.length() == 0) { name = findAnnotationName(clazz); if (name.length() == 0) { throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL); } } String[] names = NAME_SEPARATOR.split(name); if (names != null && names.length > 0) { //若类上有注解就放入map，AdaptiveExtensionFactory有注解，SpringExtensionFactory没有 //那么 Activate activate = clazz.getAnnotation(Activate.class); if (activate != null) { cachedActivates.put(names[0], activate); } for (String n : names) { if (!cachedNames.containsKey(clazz)) { cachedNames.put(clazz, n); } Class<?> c = extensionClasses.get(n); if (c == null) { extensionClasses.put(n, clazz); } else if (c != clazz) { throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName()); } } } } }/**判断办法就是有没有type为参数的构造方法**/ private boolean isWrapperClass(Class<?> clazz) { try { clazz.getConstructor(type); return true; } catch (NoSuchMethodException e) { return false; } }下面的代码简略说就是扫描META-INF/services，META-INF/dubbo，META-INF/dubbo/internal下的文件，找到合乎type实现类放到map里。
再回到getAdaptiveExtensionClass办法，cachedAdaptiveClass对象在方才代码中被赋值为了
AdaptiveExtensionFactory，如果扫描过程中cachedAdaptiveClass没有被赋值，即没有实现类上有@Adaptive注解，那么就会执行createAdaptiveExtensionClass办法：
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); }@SPI("javassist")public interface Compiler { /** * Compile java source code. * * @param code Java source code * @param classLoader classloader * @return Compiled class */ Class<?> compile(String code, ClassLoader classLoader);}通过同样的代码，找到了Compiler的实现类AdaptiveCompiler：
@Override public Class<?> compile(String code, ClassLoader classLoader) { Compiler compiler; ExtensionLoader<Compiler> loader = ExtensionLoader.getExtensionLoader(Compiler.class); String name = DEFAULT_COMPILER; // copy reference //DEFAULT_COMPILER为空 if (name != null && name.length() > 0) { compiler = loader.getExtension(name); } else { //执行这里 compiler = loader.getDefaultExtension(); } return compiler.compile(code, classLoader); } /** * Return default extension, return <code>null</code> if it's not configured. */ public T getDefaultExtension() { getExtensionClasses(); if (null == cachedDefaultName || cachedDefaultName.length() == 0 || "true".equals(cachedDefaultName)) { return null; } // 加载cachedDefaultName的类 return getExtension(cachedDefaultName); }下面的代码会将接口里@SPI注解指定的值赋给cachedDefaultName，此时就会装载此类作为接口默认类。
通过spi获取ExtensionLoader实现，回到createAdaptiveExtension办法：
private T createAdaptiveExtension() { try { //getAdaptiveExtensionClass，通过spi实例化扩大类 //injectExtension管制反转 return injectExtension((T) getAdaptiveExtensionClass().newInstance()); } catch (Exception e) { throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e); } } private T injectExtension(T instance) { try { // objectFactory在构造方法进行过赋值,为null，所以间接返回instance if (objectFactory != null) { for (Method method : instance.getClass().getMethods()) { if (method.getName().startsWith("set") && method.getParameterTypes().length == 1 && Modifier.isPublic(method.getModifiers())) { /** * Check {@link DisableInject} to see if we need auto injection for this property */ // 过滤禁止主动注入的办法 if (method.getAnnotation(DisableInject.class) != null) { continue; } Class<?> pt = method.getParameterTypes()[0]; try { String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : ""; // 从objectFactory获取扩大类注入 Object object = objectFactory.getExtension(pt, property); if (object != null) { method.invoke(instance, object); } } catch (Exception e) { logger.error("fail to inject via method " + method.getName() + " of interface " + type.getName() + ": " + e.getMessage(), e); } } } } } catch (Exception e) { logger.error(e.getMessage(), e); } return instance; }回到getAdaptiveExtension办法，instance创立结束吗，赋值给cachedAdaptiveInstance，
ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()办法执行结束，一句话概括就是通过spi拿到ExtensionFactory接口的实现类（AdaptiveExtensionFactory），并赋值给objectFactory。
至此，new ExtensionLoader<Protocol.class>(Protocol.class)办法执行结束，
再回到getExtensionLoader<Protocol.class>办法，将new ExtensionLoader<Protocol.class>(Protocol.class)对象赋值给loader，即ExtensionLoader.getExtensionLoader(Protocol.class)办法获取到了该loader，而后继续执行loader.getAdaptiveExtension（）办法。
通过和下面一样的流程，始终到：
private Class<?> getAdaptiveExtensionClass() { //进入办法 getExtensionClasses(); //cachedAdaptiveClass为空，因为Protocol.class没有实现类有@Adaptive注解//所以接着执行createAdaptiveExtensionClass if (cachedAdaptiveClass != null) { return cachedAdaptiveClass; } return cachedAdaptiveClass = createAdaptiveExtensionClass(); }进入createAdaptiveExtensionClass（）：
private Class<?> createAdaptiveExtensionClass() { //拼接代理类代码的字符串 String code = createAdaptiveExtensionClassCode(); ClassLoader classLoader = findClassLoader(); com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension(); //生产代理类 return compiler.compile(code, classLoader); } /** * 生产代理类代码string * @return */ private String createAdaptiveExtensionClassCode() { StringBuilder codeBuilder = new StringBuilder(); Method[] methods = type.getMethods(); boolean hasAdaptiveAnnotation = false; for (Method m : methods) { if (m.isAnnotationPresent(Adaptive.class)) { hasAdaptiveAnnotation = true; break; } } //实现类必须至多一个上有@Adaptive注解，否则不予代理 // no need to generate adaptive class since there's no adaptive method found. if (!hasAdaptiveAnnotation) throw new IllegalStateException("No adaptive method on extension " + type.getName() + ", refuse to create the adaptive class!"); // 拼包名、类名 codeBuilder.append("package ").append(type.getPackage().getName()).append(";"); codeBuilder.append("\nimport ").append(ExtensionLoader.class.getName()).append(";"); codeBuilder.append("\npublic class ").append(type.getSimpleName()).append("$Adaptive").append(" implements ").append(type.getCanonicalName()).append(" {"); //遍历办法 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); //没有@Adaptive注解的办法抛异样 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; //办法参数必须要蕴含URL.class for (int i = 0; i < pts.length; ++i) { if (pts[i].equals(URL.class)) { urlTypeIndex = i; break; } } // found parameter in URL type 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); } // did not find parameter in URL type else { String attribMethod = null; // find URL getter method 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 adaptive 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(); // value is not set, use the value generated from class name as the 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; //判断参数是否蕴含Invocation.class 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; //依据注解value拼接url调用代码 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(");"); } codeBuilder.append("\npublic ").append(rt.getCanonicalName()).append(" ").append(method.getName()).append("("); for (int i = 0; i < pts.length; i++) { if (i > 0) { codeBuilder.append(", "); } codeBuilder.append(pts[i].getCanonicalName()); codeBuilder.append(" "); codeBuilder.append("arg").append(i); } codeBuilder.append(")"); if (ets.length > 0) { codeBuilder.append(" throws "); for (int i = 0; i < ets.length; i++) { if (i > 0) { codeBuilder.append(", "); } codeBuilder.append(ets[i].getCanonicalName()); } } codeBuilder.append(" {"); codeBuilder.append(code.toString()); codeBuilder.append("\n}"); } codeBuilder.append("\n}"); if (logger.isDebugEnabled()) { logger.debug(codeBuilder.toString()); } return codeBuilder.toString(); }createAdaptiveExtensionClassCode（）办法就是拼接代理类代码的string，供后续javassist生产代理类。拿到complier的实现类AdaptiveCompiler后，开始生产代理类，compiler.compile(code, classLoader)：
@Override public Class<?> compile(String code, ClassLoader classLoader) { Compiler compiler; ExtensionLoader<Compiler> loader = ExtensionLoader.getExtensionLoader(Compiler.class); String name = DEFAULT_COMPILER; // copy reference //DEFAULT_COMPILER为空 if (name != null && name.length() > 0) { compiler = loader.getExtension(name); } else { //执行这里 compiler = loader.getDefaultExtension(); } return compiler.compile(code, classLoader); } /** * Return default extension, return <code>null</code> if it's not configured. */ public T getDefaultExtension() { getExtensionClasses(); if (null == cachedDefaultName || cachedDefaultName.length() == 0 || "true".equals(cachedDefaultName)) { return null; } // 加载cachedDefaultName的类，即compiler接口上spi注解的value，JavassistCompiler return getExtension(cachedDefaultName); }拿到JavassistCompiler后，进行compile：
@Override public Class<?> compile(String code, ClassLoader classLoader) { code = code.trim(); Matcher matcher = PACKAGE_PATTERN.matcher(code); String pkg; if (matcher.find()) { pkg = matcher.group(1); } else { pkg = ""; } matcher = CLASS_PATTERN.matcher(code); String cls; if (matcher.find()) { cls = matcher.group(1); } else { throw new IllegalArgumentException("No such class name in " + code); } String className = pkg != null && pkg.length() > 0 ? pkg + "." + cls : cls; try { //当类不存在时，办法会报错，就会执行catch里的doCompile办法，调用通过javassist进行生成类 return Class.forName(className, true, ClassHelper.getCallerClassLoader(getClass())); } catch (ClassNotFoundException e) { if (!code.endsWith("}")) { throw new IllegalStateException("The java code not endsWith \"}\", code: \n" + code + "\n"); } try { return doCompile(className, code); } catch (RuntimeException t) { throw t; } catch (Throwable t) { throw new IllegalStateException("Failed to compile class, cause: " + t.getMessage() + ", class: " + className + ", code: \n" + code + "\n, stack: " + ClassUtils.toString(t)); } } }最初会通过javassist生成代理类com.alibaba.dubbo.rpc.Protocol$Adaptive，残缺代码如下：
package com.alibaba.dubbo.rpc;import com.alibaba.dubbo.common.extension.ExtensionLoader;public class Protocol$Adaptive 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.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);}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);}}对于javassist生成类的形式在此不再阐明，有趣味同学能够自行搜寻。
至此，Protocol类通过dubbo的spi生成了扩大类Protocol$Adaptive。