DubboSPI

org.apache.dubbo.common.extension.ExtensionLoader#getExtensionClasses

• Dubbo扩展点加载机制的核心方法，负责从配置文件中读取并解析所有的扩展点实现类。
• 理解Dubbo SPI机制的核心入口

private Map<String, Class<?>> getExtensionClasses() {// 从缓存中读取已加载的扩展类Map<String, Class<?>> classes = cachedClasses.get();if (classes == null) {loadExtensionClassesLock.lock();try {classes = cachedClasses.get();if (classes == null) {try {// 双重检测锁，确保只加载一次classes = loadExtensionClasses();} catch (InterruptedException e) {logger.error(COMMON_ERROR_LOAD_EXTENSION,"","","Exception occurred when loading extension class (interface: " + type + ")",e);throw new IllegalStateException("Exception occurred when loading extension class (interface: " + type + ")", e);}cachedClasses.set(classes);}} finally {loadExtensionClassesLock.unlock();}}return classes;
}

org.apache.dubbo.common.extension.ExtensionLoader#loadExtensionClasses

• 加载扩展类

private Map<String, Class<?>> loadExtensionClasses() throws InterruptedException {// 检查扩展加载器是否已被销毁，防止在应用关闭过程中继续加载扩展点checkDestroyed();// 缓存默认扩展点名称（通过 @SPI 注解指定）cacheDefaultExtensionName();Map<String, Class<?>> extensionClasses = new HashMap<>();// 遍历所有加载策略// LoadingStrategy：用于定义扩展点加载策略// 		getDirectory()：返回扩展实现的配置文件所在目录（如 META-INF/dubbo/）// 		getPriority()：定义加载策略的优先级，数值越小优先级越高// 默认实现类// 		DubboInternalLoadingStrategy：加载Dubbo内部组件，路径为 META-INF/dubbo/internal/// 		DubboLoadingStrategy：加载用户自定义扩展，路径为 META-INF/dubbo/// 		ServicesLoadingStrategy：兼容 JDK SPI 标准，路径为 META-INF/services/for (LoadingStrategy strategy : strategies) {// 从当前策略指定的路径加载扩展类loadDirectory(extensionClasses, strategy, type.getName());// 兼容旧版本的 ExtensionFactory 实现if (this.type == ExtensionInjector.class) {loadDirectory(extensionClasses, strategy, ExtensionFactory.class.getName());}}return extensionClasses;
}

org.apache.dubbo.common.extension.ExtensionLoader#loadDirectory

• 从特定目录加载扩展类的配置文件，并将其映射关系存储到 extensionClasses 中
• 为了兼容从 com.alibaba 迁移到 org.apache 的类路径变更，它会尝试加载旧路径下的同名类

private void loadDirectory(Map<String, Class<?>> extensionClasses, LoadingStrategy strategy, String type)throws InterruptedException {// 实际执行加载逻辑的内部方法，它会根据 strategy 提供的目录（如 META-INF/dubbo/）读取配置文件，并将扩展名与实现类的映射存入 extensionClassesloadDirectoryInternal(extensionClasses, strategy, type);// 用于判断是否启用 Dubbo 2.x 兼容性模式if (Dubbo2CompactUtils.isEnabled()) {try {// 将类路径中的 org.apache 替换为 com.alibaba（例如 org.apache.dubbo.xxx → com.alibaba.dubbo.xxx）String oldType = type.replace("org.apache", "com.alibaba");if (oldType.equals(type)) {return;}// 通过 ClassUtils.forName(oldType) 检查旧类是否存在，若不存在则跳过加载ClassUtils.forName(oldType);// 若旧类存在，调用 loadDirectoryInternal 加载旧路径下的配置文件，避免因类路径变更导致的兼容性问题。loadDirectoryInternal(extensionClasses, strategy, oldType);} catch (ClassNotFoundException classNotFoundException) {}}
}

org.apache.dubbo.common.extension.ExtensionLoader#loadDirectoryInternal

• 参数

  • extensionClasses: 用于存储加载的扩展类的映射，键为扩展名，值为对应的 Class 对象

  • loadingStrategy: 加载策略，包含目录、是否覆盖、包含 / 排除的包等配置

  • type: 扩展点类型的名称

• 用处

  • 多种类加载器的处理
  • 特殊 SPI 加载策略的定制
  • 范围模型类加载器的支持
  • 资源加载的错误处理

private void loadDirectoryInternal(Map<String, Class<?>> extensionClasses, LoadingStrategy loadingStrategy, String type)throws InterruptedException {// 构建文件名并初始化类加载器列表String fileName = loadingStrategy.directory() + type;try {List<ClassLoader> classLoadersToLoad = new LinkedList<>();// 优先处理扩展类加载器// 如果加载策略优先使用扩展类加载器，则将扩展加载器本身的类加载器加入列表if (loadingStrategy.preferExtensionClassLoader()) {ClassLoader extensionLoaderClassLoader = ExtensionLoader.class.getClassLoader();if (ClassLoader.getSystemClassLoader() != extensionLoaderClassLoader) {classLoadersToLoad.add(extensionLoaderClassLoader);}}// 处理特殊 SPI 加载策略// 如果存在针对该类型的特殊加载策略，则检查当前加载策略名称是否匹配。若匹配则清空类加载器列表，仅使用扩展类加载器。if (specialSPILoadingStrategyMap.containsKey(type)) {String internalDirectoryType = specialSPILoadingStrategyMap.get(type);// skip to load spi when name don't matchif (!LoadingStrategy.ALL.equals(internalDirectoryType)&& !internalDirectoryType.equals(loadingStrategy.getName())) {return;}classLoadersToLoad.clear();classLoadersToLoad.add(ExtensionLoader.class.getClassLoader());} else {// 处理范围模型类加载器// 如果没有特殊加载策略，则获取范围模型的类加载器。若类加载器集合为空，则直接从系统类加载器加载资源；否则将这些类加载器添加到待处理列表。Set<ClassLoader> classLoaders = scopeModel.getClassLoaders();if (CollectionUtils.isEmpty(classLoaders)) {Enumeration<java.net.URL> resources = ClassLoader.getSystemResources(fileName);if (resources != null) {while (resources.hasMoreElements()) {loadResource(extensionClasses,null,resources.nextElement(),loadingStrategy.overridden(),loadingStrategy.includedPackages(),loadingStrategy.excludedPackages(),loadingStrategy.onlyExtensionClassLoaderPackages());}}} else {classLoadersToLoad.addAll(classLoaders);}}// 加载并处理资源// 使用类加载器资源加载器加载所有匹配的资源文件，然后遍历每个类加载器及其对应的资源 URL，调用 loadFromClass 方法处理这些资源。Map<ClassLoader, Set<java.net.URL>> resources =ClassLoaderResourceLoader.loadResources(fileName, classLoadersToLoad);resources.forEach(((classLoader, urls) -> {loadFromClass(extensionClasses,loadingStrategy.overridden(),urls,classLoader,loadingStrategy.includedPackages(),loadingStrategy.excludedPackages(),loadingStrategy.onlyExtensionClassLoaderPackages());}));} catch (InterruptedException e) {throw e;} catch (Throwable t) {logger.error(COMMON_ERROR_LOAD_EXTENSION,"","","Exception occurred when loading extension class (interface: " + type + ", description file: "+ fileName + ").",t);}
}