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1、Dubbo 的 SPI 例子
| @SPI | |
| public interface Robot {void sayHello(); | |
| } | |
| public class OptimusPrime implements Robot{ | |
| @Override | |
| public void sayHello() {System.out.println("Hello, I am Optimus Prime."); | |
| } | |
| } | |
| public class Bumblebee implements Robot{ | |
| @Override | |
| public void sayHello() {System.out.println("Hello, I am Bumblebee."); | |
| } | |
| } |
| public class DubboSPITest { | |
| @Test | |
| public void sayHelloDubbo() throws Exception {ExtensionLoader<Robot> extensionLoader = ExtensionLoader.getExtensionLoader(Robot.class); | |
| Robot optimusPrime = extensionLoader.getExtension("optimusPrime"); | |
| optimusPrime.sayHello(); | |
| Robot bumblebee = extensionLoader.getExtension("bumblebee"); | |
| bumblebee.sayHello();} | |
| } |
| 输出: | |
| Hello, I am Optimus Prime. | |
| Hello, I am Bumblebee. |
2、Dubbo 的 SPI 源码分析
| ExtensionLoader<Robot> extensionLoader = ExtensionLoader.getExtensionLoader(Robot.class); | |
| 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!"); | |
| } | |
| // 从缓存中获取与拓展类对应的 ExtensionLoader | |
| ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type); | |
| if (loader == null) {// 若缓存未命中, 则创建一个新的实例, 先简单看下 new ExtensionLoader<T>(type) | |
| EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type)); | |
| loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type); | |
| } | |
| return loader; | |
| } |
| private ExtensionLoader(Class<?> type) { | |
| this.type = type; | |
| // 这里的 type 是 Robot.class, 所以会执行后面的代码, 加载并创建 SpiExtensionFactory 和 SpringExtensionFactory, | |
| // 后面再分析 | |
| objectFactory = (type == ExtensionFactory.class ? null : | |
| ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()); | |
| } |
Robot optimusPrime = extensionLoader.getExtension("optimusPrime");
| public T getExtension(String name) {if ("true".equals(name)) {return getDefaultExtension(); | |
| } | |
| // Holder 也是用于持有对象的, 用作缓存 | |
| Holder<Object> holder = cachedInstances.get(name); | |
| if (holder == null) {cachedInstances.putIfAbsent(name, new Holder<Object>()); | |
| holder = cachedInstances.get(name); | |
| } | |
| Object instance = holder.get(); | |
| if (instance == null) {synchronized (holder) {instance = holder.get(); | |
| if (instance == null) {// 创建拓展实例, 下面分析 createExtension(name) | |
| instance = createExtension(name); | |
| holder.set(instance); | |
| } | |
| } | |
| } | |
| return (T) instance; | |
| } |
| private T createExtension(String name) { | |
| // 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 | |
| // 加载完后根据 name 获取, 得到形如 com.alibaba.dubbo.demo.provider.spi.OptimusPrime | |
| // 待会重点分析 getExtensionClasses(), 删去了一些非关键代码 | |
| Class<?> clazz = getExtensionClasses().get(name); | |
| try { | |
| // 也是尝试先从缓存获取, 获取不到通过反射创建一个并放到缓存中 | |
| T instance = (T) EXTENSION_INSTANCES.get(clazz); | |
| if (instance == null) {EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance()); | |
| instance = (T) EXTENSION_INSTANCES.get(clazz); | |
| } | |
| // 依赖注入和 cachedWrapperClasses, 后面分析 | |
| injectExtension(instance); | |
| Set<Class<?>> wrapperClasses = cachedWrapperClasses; | |
| if (wrapperClasses != null && !wrapperClasses.isEmpty()) {for (Class<?> wrapperClass : wrapperClasses) {instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance)); | |
| } | |
| } | |
| return instance; | |
| } | |
| } |
| private Map<String, Class<?>> getExtensionClasses() { | |
| // 从缓存中获取映射关系表 | |
| Map<String, Class<?>> classes = cachedClasses.get(); | |
| // 双重检查 | |
| if (classes == null) {synchronized (cachedClasses) {classes = cachedClasses.get(); | |
| if (classes == null) { | |
| // 若缓存中没有, 去加载映射关系 | |
| classes = loadExtensionClasses(); | |
| cachedClasses.set(classes); | |
| } | |
| } | |
| } | |
| return classes; | |
| } |
| private Map<String, Class<?>> loadExtensionClasses() { | |
| // 获取 SPI 注解, 这里的 type 是在调用 getExtensionLoader 方法时传入的 | |
| 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) {// 抛异常} | |
| // 获取 @SPI 注解中的值, 并缓存起来, 可以关注下, 后面会用到 | |
| if (names.length == 1) cachedDefaultName = names[0]; | |
| } | |
| } | |
| Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>(); | |
| // 加载指定文件夹下的配置文件,META-INF/services/、META-INF/dubbo/、META-INF/dubbo/internal/ | |
| loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY); | |
| // 我是放在这个目录下的 | |
| loadDirectory(extensionClasses, DUBBO_DIRECTORY); | |
| loadDirectory(extensionClasses, SERVICES_DIRECTORY); | |
| return extensionClasses; | |
| } |
| private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir) { | |
| // fileName 是 META-INF/dubbo/com.alibaba.dubbo.demo.provider.spi.Robot | |
| String fileName = dir + type.getName(); | |
| try { | |
| Enumeration<java.net.URL> urls; | |
| ClassLoader classLoader = findClassLoader(); | |
| // 根据文件名加载所有同名文件 | |
| if (classLoader != null) {urls = classLoader.getResources(fileName); | |
| } else {urls = ClassLoader.getSystemResources(fileName); | |
| } | |
| if (urls != null) {while (urls.hasMoreElements()) { | |
| // 一个 resourceURL 就是一个文件 | |
| java.net.URL resourceURL = urls.nextElement(); | |
| loadResource(extensionClasses, classLoader, resourceURL); | |
| } | |
| } | |
| } | |
| } |
| 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; | |
| // 按行读取配置内容 | |
| while ((line = reader.readLine()) != null) {final int ci = line.indexOf('#'); | |
| // 定位 #字符,# 之后的为注释, 跳过 | |
| if (ci >= 0) line = line.substring(0, ci); | |
| line = line.trim(); | |
| if (line.length() > 0) { | |
| try { | |
| String name = null; | |
| // 按等号切割 | |
| int i = line.indexOf('='); | |
| if (i > 0) {name = line.substring(0, i).trim(); | |
| line = line.substring(i + 1).trim();} | |
| if (line.length() > 0) { | |
| // 真正的去加载类 | |
| loadClass(extensionClasses, resourceURL, | |
| Class.forName(line, true, classLoader), name); | |
| } | |
| } | |
| } | |
| } | |
| } | |
| } | |
| } |
| private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, | |
| Class<?> clazz, String name) throws NoSuchMethodException { | |
| // clazz 必须是 type 类型的, 否则抛异常 | |
| if (!type.isAssignableFrom(clazz)) { } | |
| // 判断 clazz 是否为标注了 @Adaptive 注解, 后面分析 | |
| if (clazz.isAnnotationPresent(Adaptive.class)) {if (cachedAdaptiveClass == null) {cachedAdaptiveClass = clazz;} else if (!cachedAdaptiveClass.equals(clazz)) {// 抛异常, 不能有多个标注有 @Adaptive 注解的类} | |
| } | |
| // 判断是否是 Wrapper 类型, 后面分析 | |
| else if (isWrapperClass(clazz)) { | |
| Set<Class<?>> wrappers = cachedWrapperClasses; | |
| if (wrappers == null) {cachedWrapperClasses = new ConcurrentHashSet<Class<?>>(); | |
| wrappers = cachedWrapperClasses; | |
| } | |
| wrappers.add(clazz); | |
| } | |
| // 程序进入此分支, 表明 clazz 是一个普通的拓展类,Robot 就是一个普通的拓展类 | |
| else { | |
| // 检测 clazz 是否有默认的构造方法, 如果没有, 则抛出异常 | |
| clazz.getConstructor(); | |
| if (name == null || name.length() == 0) {name = findAnnotationName(clazz); | |
| if (name.length() == 0) {// 抛异常} | |
| } | |
| String[] names = NAME_SEPARATOR.split(name); | |
| if (names != null && names.length > 0) {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) { | |
| // 存储名称到 class 的映射关系, 这样就解析好了一行 | |
| extensionClasses.put(n, clazz); | |
| } else if (c != clazz) {// 抛异常} | |
| } | |
| } | |
| } | |
| } |
正文完
