JDK 的 SPI 有什么缺陷？dubbo 做了什么改进？

private static final String SERVICES_DIRECTORY = "META-INF/services/";
private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";    private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";

registry=com.alibaba.dubbo.registry.integration.RegistryProtocolfilter=com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapperlistener=com.alibaba.dubbo.rpc.protocol.ProtocolListenerWrappermock=com.alibaba.dubbo.rpc.support.MockProtocolinjvm=com.alibaba.dubbo.rpc.protocol.injvm.InjvmProtocoldubbo=com.alibaba.dubbo.rpc.protocol.dubbo.DubboProtocolrmi=com.alibaba.dubbo.rpc.protocol.rmi.RmiProtocolhessian=com.alibaba.dubbo.rpc.protocol.hessian.HessianProtocolcom.alibaba.dubbo.rpc.protocol.http.HttpProtocolcom.alibaba.dubbo.rpc.protocol.webservice.WebServiceProtocolthrift=com.alibaba.dubbo.rpc.protocol.thrift.ThriftProtocolmemcached=memcom.alibaba.dubbo.rpc.protocol.memcached.MemcachedProtocolredis=com.alibaba.dubbo.rpc.protocol.redis.RedisProtocol

public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {    // 静态方法，意味着可以直接通过类调用    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);    // 从map中获取该类型的ExtensionLoader数据    if (loader == null) {        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));        // 如果没有则，创建一个新的ExtensionLoader对象，并且以该类型存储        loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);        // 再从map中获取        // 这里这样做的原因就是为了防止并发的问题，而且map本是也是个ConcurrentHashMap    }    return loader;}

private ExtensionLoader(Class<?> type) {    this.type = type;    objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());}// 传入的type是Protocol.class，所以需要获取ExtensionFactory.class最合适的实现类

public T getAdaptiveExtension() {    Object instance = cachedAdaptiveInstance.get();    if (instance == null) {        if(createAdaptiveInstanceError == null) {            synchronized (cachedAdaptiveInstance) {                instance = cachedAdaptiveInstance.get();                if (instance == null) {                    try {                        instance = createAdaptiveExtension();                        // 创建对象，也是需要关注的函数                        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;}

private T createAdaptiveExtension() {    try {        return injectExtension((T) getAdaptiveExtensionClass().newInstance());        // (T) getAdaptiveExtensionClass().newInstance() 创建一个具体的实例对象        // getAdaptiveExtensionClass() 生成相关的class        // injectExtension 往该对象中注入数据    } catch (Exception e) {        throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);    }}

private Class<?> getAdaptiveExtensionClass() {    getExtensionClasses();    // 通过加载SPI配置，获取到需要的所有的实现类存储到map中    // 通过可能会去修改cachedAdaptiveClass数据，具体原因在spi配置文件解析中分析    if (cachedAdaptiveClass != null) {        return cachedAdaptiveClass;    }    return cachedAdaptiveClass = createAdaptiveExtensionClass();}

private Class<?> createAdaptiveExtensionClass() {    String code = createAdaptiveExtensionClassCode();    // 动态生成需要的代码内容字符串    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);    // 编译，生成相应的类}

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 {          // 暴露远程服务，传入的参数是invoke对象        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() );          // 如果没有具体协议，则使用dubbo协议          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 {          // 引用远程对象，生成相对应的远程invoke对象        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 T getExtension(String name) {    if (name == null || name.length() == 0)        throw new IllegalArgumentException("Extension name == null");    if ("true".equals(name)) {        return getDefaultExtension();    }    Holder<Object> holder = cachedInstances.get(name);    if (holder == null) {        cachedInstances.putIfAbsent(name, new Holder<Object>());        holder = cachedInstances.get(name);    }    // 无论是否真有数据，在cachedInstances存储的是一个具体的Holder对象    Object instance = holder.get();    if (instance == null) {        synchronized (holder) {            instance = holder.get();            if (instance == null) {                instance = createExtension(name);                // 创建对象                holder.set(instance);            }        }    }    return (T) instance;}

private T createExtension(String name) {    Class<?> clazz = getExtensionClasses().get(name);    // 获取具体的实现类的类    if (clazz == null) {        throw findException(name);    }    try {        T instance = (T) EXTENSION_INSTANCES.get(clazz);        if (instance == null) {            EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());            // clazz.newInstance 才是真正创建对象的操作            instance = (T) EXTENSION_INSTANCES.get(clazz);        }        injectExtension(instance);        // 往实例中反射注入参数        Set<Class<?>> wrapperClasses = cachedWrapperClasses;        if (wrapperClasses != null && wrapperClasses.size() > 0) {            for (Class<?> wrapperClass : wrapperClasses) {                instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));            }        }        return instance;    } catch (Throwable t) {        throw new IllegalStateException("Extension instance(name: " + name + ", class: " +                type + ")  could not be instantiated: " + t.getMessage(), t);    }}

private Map<String, Class<?>> loadExtensionClasses() {    final SPI defaultAnnotation = type.getAnnotation(SPI.class);    // 查看该类是否存在SPI注解信息    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];            // 设置默认的名称，如果注解的值经过切割，发现超过1个的数据，则同样会认为错误        }    }        Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();    loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);    // 加载文件    loadFile(extensionClasses, DUBBO_DIRECTORY);    loadFile(extensionClasses, SERVICES_DIRECTORY);    return extensionClasses;}    private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {    String fileName = dir + type.getName();    // type.getName就是类名称，和java的类似    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 url = urls.nextElement();                try {                    BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));                    try {                        String line = null;                        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('=');                                    // 对k=v这样的格式进行分割操作，分别获取                                    if (i > 0) {                                        name = line.substring(0, i).trim();                                        line = line.substring(i + 1).trim();                                    }                                    if (line.length() > 0) {                                        Class<?> clazz = Class.forName(line, true, classLoader);                                        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.");                                        }                                        if (clazz.isAnnotationPresent(Adaptive.class)) {                                            // 如果获取的类包含了Adaptive注解                                            if(cachedAdaptiveClass == null) {                                                cachedAdaptiveClass = clazz;                                            } else if (! cachedAdaptiveClass.equals(clazz)) {                                               // 已经存在了该数据，现在又出现了，则抛出异常                                                throw new IllegalStateException("More than 1 adaptive class found: "                                                        + cachedAdaptiveClass.getClass().getName()                                                        + ", " + clazz.getClass().getName());                                            }                                        } else {  // 不包含Adaptive注解信息                                            try {                                                clazz.getConstructor(type);                                                // 查看构造函数是否包含了type的类型参数                                                // 如果不存在，则抛出NoSuchMethodException异常                                                Set<Class<?>> wrappers = cachedWrapperClasses;                                                if (wrappers == null) {                                                    cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();                                                    wrappers = cachedWrapperClasses;                                                }                                                wrappers.add(clazz);                                                // 往wrappers中添加该类                                            } catch (NoSuchMethodException e) {                                                clazz.getConstructor();                                                if (name == null || name.length() == 0) {                                                   // 没用名字的那种，也就是不存在k=v这种样式                                                   // 例如上面的HttpProtocol                                                    name = findAnnotationName(clazz);                                                    // 查看该类是否存在注解                                                    if (name == null || name.length() == 0) {                                                        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);                                                        }                                                    }                                                }                                                String[] names = NAME_SEPARATOR.split(name);                                                // 可能存在多个，切割开                                                if (names != null && names.length > 0) {                                                    Activate activate = clazz.getAnnotation(Activate.class);                                                    if (activate != null) {                                                        // 如果类存在Activate的注解信息                                                        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);                                                            // 往容器中填充该键值对信息，k和v                                                        } else if (c != clazz) {                                                             // 存在多个同名扩展类，则抛出异常信息                                                            throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());                                                        }                                                    }                                                }                                            }                                        }                                    }                                } catch (Throwable t) {                                    IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + url + ", cause: " + t.getMessage(), t);                                    exceptions.put(line, e);                                }                            }                        } // end of while read lines                    } finally {                        reader.close();                    }                } catch (Throwable t) {                    logger.error("Exception when load extension class(interface: " +                                        type + ", class file: " + url + ") in " + url, t);                }            } // end of while urls        }    } catch (Throwable t) {        logger.error("Exception when load extension class(interface: " +                type + ", description file: " + fileName + ").", t);    }}