dubbo SPI机制

源码分析：

/**
     * 获取扩展类
     */
    @SuppressWarnings("unchecked")
    public T getExtension(String name) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        // 获取默认的拓展实现类
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        // Holder用于持有目标对象
        final Holder<Object> holder = getOrCreateHolder(name);
        Object instance = holder.get();
        // 双重检查
        if (instance == null) {
            synchronized (holder) {
                instance = holder.get();
                if (instance == null) {
                    // 创建拓展实例
                    instance = createExtension(name);
                    // 设置实例到 holder 中
                    holder.set(instance);
                }
            }
        }
        return (T) instance;
    }

// 创建拓展对象
    @SuppressWarnings("unchecked")
    private T createExtension(String name) {
        Class<?> clazz = getExtensionClasses().get(name);
        if (clazz == null) {
            throw findException(name);
        }
        try {
            T instance = (T) EXTENSION_INSTANCES.get(clazz);
            if (instance == null) {
                // 通过反射创建实例
                EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
                instance = (T) EXTENSION_INSTANCES.get(clazz);
            }
            // 向实例中注入依赖
            injectExtension(instance);
            Set<Class<?>> wrapperClasses = cachedWrapperClasses;
            if (CollectionUtils.isNotEmpty(wrapperClasses)) {
                // 循环创建 Wrapper 实例
                for (Class<?> wrapperClass : wrapperClasses) {
                    // 将当前 instance 作为参数传给 Wrapper 的构造方法，并通过反射创建 Wrapper 实例。
                    // 然后向 Wrapper 实例中注入依赖，最后将 Wrapper 实例再次赋值给 instance 变量
                    instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
                }
            }
            return instance;
        } catch (Throwable t) {
            throw new IllegalStateException("Extension instance (name: " + name + ", class: " + type
                    + ") couldn't be instantiated: " + t.getMessage(), t);
        }
    }

// 从配置文件中加载所有的拓展类，可得到“配置项名称”到“配置类”的映射关系表
    private Map<String, Class<?>> getExtensionClasses() {
        // 从缓存中获取已加载的拓展类
        Map<String, Class<?>> classes = cachedClasses.get();
        // 双重检查
        if (classes == null) {
            synchronized (cachedClasses) {
                classes = cachedClasses.get();
                if (classes == null) {
                    // 加载拓展类
                    classes = loadExtensionClasses();
                    cachedClasses.set(classes);
                }
            }
        }
        return classes;
    }

    // synchronized in getExtensionClasses
    private Map<String, Class<?>> loadExtensionClasses() {
        cacheDefaultExtensionName();

        Map<String, Class<?>> extensionClasses = new HashMap<>();
        // 加载指定文件夹下的配置文件
        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName());
        loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName());
        loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName());
        loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
        return extensionClasses;
    }

private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir, String type) {
        // fileName = 文件夹路径 + type 全限定名
        String fileName = dir + type;
        try {
            Enumeration<java.net.URL> urls;
            ClassLoader classLoader = findClassLoader();
            // 根据文件名加载所有的同名文件
            if (classLoader != null) {
                urls = classLoader.getResources(fileName);
            } else {
                urls = ClassLoader.getSystemResources(fileName);
            }
            if (urls != null) {
                while (urls.hasMoreElements()) {
                    java.net.URL resourceURL = urls.nextElement();
                    // 加载资源
                    loadResource(extensionClasses, classLoader, resourceURL);
                }
            }
        } catch (Throwable t) {
            logger.error("Exception occurred when loading extension class (interface: " + type + ", description file: "
                    + fileName + ").", t);
        }
    }