Dubbo SPI

淡淡的烟草味﹌ 2022-05-01 12:54 165阅读 0赞

## 前言 ##

前面已经讲过[JDK SPI][], dubbo SPI主要基于JDK SPI机制，进行了增强，实现可扩展

为什么不直接用JDK的SPI呢，主要有哪些改变呢，先列举一下
    1. JDK的SPI使用的是for循环通过if判断过去指定的spi对象，而dubbo使用key-value形式获取
    2. JDK的SPI不支持默认值，dubbo增加了默认值的设计 比如dubbo的Protocol默认dubbo
    3. JDK标准的SPI会一次性实例化扩展点所有实现，如果有扩展实现初始化很耗时，但如果没用上也加载，会很浪费资源
    4. 如果扩展点加载失败，连扩展点的名称都拿不到了
    5. dubbo增加了对扩展点IoC和AOP的支持，一个扩展点可以直接setter注入其它扩展点

## 代码示例 ##

public class TestExtension {
        public static void main(String[] args) {
            ExtensionLoader<Protocol> loader = ExtensionLoader.getExtensionLoader(Protocol.class);
            final Protocol dubboProtocol = loader.getExtension("dubbo");
            final Protocol adaptiveExtension = loader.getAdaptiveExtension();
        }
    }
    复制代码

## 源码解析 ##

dubbo的SPI 主要是ExtensionLoader这个类可以类比为JDK-SPI中的ServiceLoader

首先来看一下ExtensionLoader的类属性：
    /** 存放SPI文件的三个目录,其中META-INF/services/也是jdk的SPI文件的存放目录 */
        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/";//这个是最终jar包中存放spi文件的位置
    
        private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*");
        /** key: SPI接口Class value: 该接口的ExtensionLoader */
        private static final ConcurrentMap, ExtensionLoader> EXTENSION_LOADERS = new ConcurrentHashMap, ExtensionLoader>();
        /** key: SPI接口Class value: SPI实现类的对象实例 */
        private static final ConcurrentMap, Object> EXTENSION_INSTANCES = new ConcurrentHashMap, Object>();
    
    再来看一下ExtensionLoader的实例属性：
    /** SPI接口Class */
        private final Class type;
        /** SPI实现类对象实例的创建工厂 */
        private final ExtensionFactory objectFactory;
        /** key: ExtensionClass的Class value: SPI实现类的key */
        private final ConcurrentMap, String> cachedNames = new ConcurrentHashMap, String>();
        /** 存放所有的extensionClass */
        private final Holder>> cachedClasses = new Holder>>();
    
        private final Map cachedActivates = new ConcurrentHashMap();
        /** 缓存创建好的extensionClass实例 */
        private final ConcurrentMap> cachedInstances = new ConcurrentHashMap>();
        /** 缓存创建好的适配类实例 */
        private final Holder cachedAdaptiveInstance = new Holder();
        /** 存储类上带有@Adaptive注解的Class */
        private volatile Class cachedAdaptiveClass = null;
        /** 默认的SPI文件中的key */
        private String cachedDefaultName;
        /** 存储在创建适配类实例这个过程中发生的错误 */
        private volatile Throwable createAdaptiveInstanceError;
        /** 存放具有一个type入参的构造器的实现类的Class对象 */
        private Set> cachedWrapperClasses;
        /** key :实现类的全类名  value: exception, 防止真正的异常被吞掉 */
        private Map exceptions = new ConcurrentHashMap();

### ExtensionLoader loader = ExtensionLoader.getExtensionLoader(Protocol.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!");
            }
    
            ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
            if (loader == null) {
                EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
                loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
            }
            return loader;
        }
    复制代码

通过type获取到对应的ExtensionLoader并放入全局缓存中EXTENSION\_LOADERS 创建ExtensionLoader

private ExtensionLoader(Class<?> type) {
            this.type = type;
            objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
        }
    复制代码

当前创建的ExtensionLoader对象的type是com.alibaba.dubbo.rpc.Protocol
    所以此时会执行：ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()
    1. 先创建ExtensionLoader.getExtensionLoader(ExtensionFactory.class)
    2. 因为type是com.alibaba.dubbo.common.extension.ExtensionFactory，
    所以objectFactory是null。之后，这个ExtensionLoader被放入EXTENSION_LOADERS缓存
    3. ExtensionLoader对象的getAdaptiveExtension()

/**
     * 首先从cachedAdaptiveInstance缓存中获取AdaptiveExtension实例
     * 如果不为null, 直接返回;
     * 如果为null, 先创建AdaptiveExtension实例, 之后放入cachedAdaptiveInstance缓存中,最后返回
     */
    public T getAdaptiveExtension() {
            Object instance = cachedAdaptiveInstance.get();
            if (instance == null) {
                if (createAdaptiveInstanceError == null) {
                    synchronized (cachedAdaptiveInstance) {
                        instance = cachedAdaptiveInstance.get();
                        if (instance == null) {
                            try {
                                instance = createAdaptiveExtension();
                                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;
        }
    复制代码

创建AdaptiveExtension
    终返回对应的实例对象

/**
     * createAdaptiveExtension()
     * --getAdaptiveExtensionClass()
     *   //从dubbo-spi配置文件中获取AdaptiveExtensionClass
     *   --getExtensionClasses()
     *     --loadExtensionClasses()
     *       --loadFile(Map<String, Class<?>> extensionClasses, String dir)
     *   //创建动态代理类
     *   --createAdaptiveExtensionClass()
     *
     * --injectExtension(T instance)  //dubbo-ioc
     */
    private T createAdaptiveExtension() {
            try {
                return injectExtension((T) getAdaptiveExtensionClass().newInstance());
            } catch (Exception e) {
                throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
            }
        }
    复制代码

调用层级看注释。
    injectExtension(T instance)方法只对objectFactory有用，
    如果objectFactory==null，则直接返回T instance。
    所以这里返回的是getAdaptiveExtensionClass().newInstance()

/**
         * 获取ExtensionClasses和适配类
         * 如果实现类上带有@Adaptive注解，直接创建修饰类
         * 如果方法上带有@Adaptive注解，动态生成代理类
         */
        private Class<?> getAdaptiveExtensionClass() {
            getExtensionClasses();
            if (cachedAdaptiveClass != null) {
                return cachedAdaptiveClass;
            }
            return cachedAdaptiveClass = createAdaptiveExtensionClass();
        }
    复制代码

/**
         * 先从cachedClasses缓存中获取所有的ExtensionClass,如果有,直接返回;
         * 如果没有,通过loadExtensionClasses()从SPI文件中去读取,之后写入缓存
         */
        private Map<String, Class<?>> getExtensionClasses() {
            Map<String, Class<?>> classes = cachedClasses.get();
            if (classes == null) {
                synchronized (cachedClasses) {
                    classes = cachedClasses.get();
                    if (classes == null) {
                        classes = loadExtensionClasses();
                        cachedClasses.set(classes);
                    }
                }
            }
            return classes;
        }
    复制代码

/**
         * 1 从@SPI注解中将默认值解析出来,并缓存到cachedDefaultName中
         * 2 从SPI文件中获取extensionClass并存储到extensionClasses中,最后返回extensionClasses
         * 注意：此方法已经getExtensionClasses方法同步过。
         */
        private Map<String, Class<?>> loadExtensionClasses() {
            final SPI defaultAnnotation = type.getAnnotation(SPI.class);
            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];
                }
            }
    
            Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
            loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
            loadFile(extensionClasses, DUBBO_DIRECTORY);
            loadFile(extensionClasses, SERVICES_DIRECTORY);
            return extensionClasses;
        }
    复制代码

之后来看一下非常重要的一个方法loadFile(Map<String, Class<?>> extensionClasses, String dir)

/**
         * 1 加载dir目录下的指定type名称的文件(例如:dubbo-2.5.5.jar中的/META-INF/dubbo/internal/com.alibaba.dubbo.common.extension.ExtensionFactory)
         * 2 遍历该文件中的每一行
         * (1)获取实现类key和value, 例如 name=spi, line=com.alibaba.dubbo.common.extension.factory.SpiExtensionFactory
         * (2)根据line创建Class对象
         * (3)将具有@Adaptive注解的实现类的Class对象放在cachedAdaptiveClass缓存中, 注意该缓存只能存放一个具有@Adaptive注解的实现类的Class对象,如果有两个满足条件,则抛异常
         * 下面的都是对不含@Adaptive注解的实现类的Class对象:
         * (4)查看是否具有含有一个type入参的构造器, 如果有（就是wrapper类）, 将当前的Class对象放置到cachedWrapperClasses缓存中
         * (5)如果没有含有一个type入参的构造器, 获取无参构造器. 如果Class对象具有@Active注解, 将该对象以<实现类的key, active>存储起来
         * (6)最后,将<Class对象, 实现类的key>存入cachedNames缓存,并将这些Class存入extensionClasses中.
         * @param extensionClasses
         * @param dir
         */
        private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
            String fileName = dir + type.getName();
            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('=');
                                            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)) {
                                                    if (cachedAdaptiveClass == null) {
                                                        cachedAdaptiveClass = clazz;
                                                    } else if (!cachedAdaptiveClass.equals(clazz)) {
                                                        throw new IllegalStateException("More than 1 adaptive class found: "
                                                                + cachedAdaptiveClass.getClass().getName()
                                                                + ", " + clazz.getClass().getName());
                                                    }
                                                } else {
                                                    try {
                                                        clazz.getConstructor(type);
                                                        Set<Class<?>> wrappers = cachedWrapperClasses;
                                                        if (wrappers == null) {
                                                            cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
                                                            wrappers = cachedWrapperClasses;
                                                        }
                                                        wrappers.add(clazz);
                                                    } catch (NoSuchMethodException e) {
                                                        clazz.getConstructor();
                                                        if (name == null || name.length() == 0) {
                                                            name = findAnnotationName(clazz);
                                                            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);
                                                        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) {
                                                                    extensionClasses.put(n, clazz);
                                                                } else if (c != clazz) {
                                                                    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);
            }
        }
    复制代码

上述的方法分别从三个目录查找SPI文件并进行加载。在这里只有在META-INF/dubbo/internal/com.alibaba.dubbo.common.extension.ExtensionFactory中有值，内容如下：
    1 adaptive=com.alibaba.dubbo.common.extension.factory.AdaptiveExtensionFactory
    2 spi=com.alibaba.dubbo.common.extension.factory.SpiExtensionFactory
    3 spring=com.alibaba.dubbo.config.spring.extension.SpringExtensionFactory

执行之后：

getAdaptiveExtensionClass()返回的com.alibaba.dubbo.common.extension.factory.AdaptiveExtensionFactory类
    之后来看一下其newInstance()代码，调用这个方法，默认会执行AdaptiveExtensionFactory的无参构造器。这里给出AdaptiveExtensionFactory的完整代码

@Adaptive
    public class AdaptiveExtensionFactory implements ExtensionFactory {
    
        private final List<ExtensionFactory> factories;
    
        public AdaptiveExtensionFactory() {
            ExtensionLoader<ExtensionFactory> loader = ExtensionLoader.getExtensionLoader(ExtensionFactory.class);
            List<ExtensionFactory> list = new ArrayList<ExtensionFactory>();
            for (String name : loader.getSupportedExtensions()) {
                list.add(loader.getExtension(name));
            }
            factories = Collections.unmodifiableList(list);
        }
    
        @Override
        public <T> T getExtension(Class<T> type, String name) {
            for (ExtensionFactory factory : factories) {
                T extension = factory.getExtension(type, name);
                if (extension != null) {
                    return extension;
                }
            }
            return null;
        }
    
    }
    复制代码

从上可以看出，这个装饰类只是实例化好了各个ExtensionFactory（这里是SpiExtensionFactory和SpringExtensionFactory），后续通过工厂获取实现类实例都是由具体工厂来完成。

小结：
    ExtensionLoader loader = ExtensionLoader.getExtensionLoader(Class type)最终得到的实例变量是：
    1. Class type = interface T
    2. ExtensionFactory objectFactory = AdaptiveExtensionFactory（适配类）
            factories = [SpringExtensionFactory实例, SpiExtensionFactory实例]

### Protocol dubboProtocol = loader.getExtension("dubbo") ###

/**
         * 从cachedInstances缓存中获取name对应的实例,如果没有,通过createExtension(name)创建,之后放入缓存
         * getExtension(String name)
         * --createExtension(String name)
         * ----injectExtension(T instance)
         */
        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);
            }
            Object instance = holder.get();
            if (instance == null) {
                synchronized (holder) {
                    instance = holder.get();
                    if (instance == null) {
                        instance = createExtension(name);
                        holder.set(instance);
                    }
                }
            }
            return (T) instance;
        }
    复制代码

private T createExtension(String name) {
            /** 从cachedClasses缓存中获取所有的实现类map,之后通过name获取到对应的实现类的Class对象 */
            Class<?> clazz = getExtensionClasses().get(name);
            if (clazz == null) {
                throw findException(name);
            }
            try {
                /** 从EXTENSION_INSTANCES缓存中获取对应的实现类的Class对象,如果没有,直接创建,之后放入缓存 */
                T instance = (T) EXTENSION_INSTANCES.get(clazz);
                if (instance == null) {
                    EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
                    instance = (T) EXTENSION_INSTANCES.get(clazz);
                }
                injectExtension(instance);//ioc
                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);
            }
        }
    复制代码

转载于:https://juejin.im/post/5c91fd756fb9a070d90f5cca

[JDK SPI]: https://juejin.im/post/5c90d14ce51d45206a18befa