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); } }