Dubbo SPI-蒲公英云

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();
    }
}
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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;
    }
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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());
    }
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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;
    }
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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);
        }
    }
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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();
    }
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/**
     * 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;
    }
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/**
     * 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;
    }
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之后来看一下非常重要的一个方法loadFile(Map> 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);
        }
    }
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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;
    }
}
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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;
    }
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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);
        }
    }
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