1. 源码分析---SOFARPC可扩展的机制SPI

@Extensible(singleton = false)
public abstract class ConsumerBootstrap<T> {
    ....
}

@Extension("sofa")
public class DefaultConsumerBootstrap<T> extends ConsumerBootstrap<T> {
    ...
}

sofa=com.alipay.sofa.rpc.bootstrap.DefaultConsumerBootstrap

ConsumerBootstrap sofa =  ExtensionLoaderFactory.getExtensionLoader(ConsumerBootstrap.class).getExtension("sofa");

    /**
     * All extension loader {Class : ExtensionLoader}
     * 这个map里面装的是所有ExtensionLoader
     */
    private static final ConcurrentMap<Class, ExtensionLoader> LOADER_MAP = new ConcurrentHashMap<Class, ExtensionLoader>();


    public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> clazz, ExtensionLoaderListener<T> listener) {
        ExtensionLoader<T> loader = LOADER_MAP.get(clazz);
        if (loader == null) {
            //get不到则加上锁
            synchronized (ExtensionLoaderFactory.class) {
                //防止其他线程操作再get一次
                loader = LOADER_MAP.get(clazz);
                if (loader == null) {
                    loader = new ExtensionLoader<T>(clazz, listener);
                    LOADER_MAP.put(clazz, loader);
                }
            }
        }
        return loader;
    }

    protected ExtensionLoader(Class<T> interfaceClass, boolean autoLoad, ExtensionLoaderListener<T> listener) {
        //如果正在执行关闭，则将属性置空后直接返回
        if (RpcRunningState.isShuttingDown()) {
            this.interfaceClass = null;
            this.interfaceName = null;
            this.listener = null;
            this.factory = null;
            this.extensible = null;
            this.all = null;
            return;
        }
        // 接口为空，既不是接口，也不是抽象类
        if (interfaceClass == null ||
                !(interfaceClass.isInterface() || Modifier.isAbstract(interfaceClass.getModifiers()))) {
            throw new IllegalArgumentException("Extensible class must be interface or abstract class!");
        }
        //当前加载的接口类名
        this.interfaceClass = interfaceClass;
        //接口名字
        this.interfaceName = ClassTypeUtils.getTypeStr(interfaceClass);
        this.listener = listener;
        //接口上必须要有Extensible注解
        Extensible extensible = interfaceClass.getAnnotation(Extensible.class);
        if (extensible == null) {
            throw new IllegalArgumentException(
                    "Error when load extensible interface " + interfaceName + ", must add annotation @Extensible.");
        } else {
            this.extensible = extensible;
        }
        // 如果是单例，那么factory不为空
        this.factory = extensible.singleton() ? new ConcurrentHashMap<String, T>() : null;
        //这个属性里面是这个接口的所有实现类
        this.all = new ConcurrentHashMap<String, ExtensionClass<T>>();
        if (autoLoad) {
            //获取到扩展点加载的路径
            List<String> paths = RpcConfigs.getListValue(RpcOptions.EXTENSION_LOAD_PATH);
            for (String path : paths) {
                //根据路径加载文件
                loadFromFile(path);
            }
        }
    }

    protected synchronized void loadFromFile(String path) {
        if (LOGGER.isDebugEnabled()) {
            LOGGER.debug("Loading extension of extensible {} from path: {}", interfaceName, path);
        }
        // 默认如果不指定文件名字，就是接口名
        String file = StringUtils.isBlank(extensible.file()) ? interfaceName : extensible.file().trim();
        String fullFileName = path + file;
        try {
            ClassLoader classLoader = ClassLoaderUtils.getClassLoader(getClass());
            loadFromClassLoader(classLoader, fullFileName);
        } catch (Throwable t) {
            if (LOGGER.isErrorEnabled()) {
                LOGGER.error("Failed to load extension of extensible " + interfaceName + " from path:" + fullFileName,
                    t);
            }
        }
    }
    
    
    protected void loadFromClassLoader(ClassLoader classLoader, String fullFileName) throws Throwable {
        Enumeration<URL> urls = classLoader != null ? classLoader.getResources(fullFileName)
            : ClassLoader.getSystemResources(fullFileName);
        // 可能存在多个文件。
        if (urls != null) {
            while (urls.hasMoreElements()) {
                // 读取一个文件
                URL url = urls.nextElement();
                if (LOGGER.isDebugEnabled()) {
                    LOGGER.debug("Loading extension of extensible {} from classloader: {} and file: {}",
                        interfaceName, classLoader, url);
                }
                BufferedReader reader = null;
                try {
                    reader = new BufferedReader(new InputStreamReader(url.openStream(), "UTF-8"));
                    String line;
                    while ((line = reader.readLine()) != null) {
                        readLine(url, line);
                    }
                } catch (Throwable t) {
                    if (LOGGER.isWarnEnabled()) {
                        LOGGER.warn("Failed to load extension of extensible " + interfaceName
                            + " from classloader: " + classLoader + " and file:" + url, t);
                    }
                } finally {
                    if (reader != null) {
                        reader.close();
                    }
                }
            }
        }
    }

    protected void readLine(URL url, String line) {
        //读取文件里面的一行记录，并将这行记录用=号分割
        String[] aliasAndClassName = parseAliasAndClassName(line);
        if (aliasAndClassName == null || aliasAndClassName.length != 2) {
            return;
        }
        //别名
        String alias = aliasAndClassName[0];
        //包名
        String className = aliasAndClassName[1];
        // 读取配置的实现类
        Class tmp;
        try {
            tmp = ClassUtils.forName(className, false);
        } catch (Throwable e) {
            if (LOGGER.isWarnEnabled()) {
                LOGGER.warn("Extension {} of extensible {} is disabled, cause by: {}",
                    className, interfaceName, ExceptionUtils.toShortString(e, 2));
            }
            if (LOGGER.isDebugEnabled()) {
                LOGGER.debug("Extension " + className + " of extensible " + interfaceName + " is disabled.", e);
            }
            return;
        }
        if (!interfaceClass.isAssignableFrom(tmp)) {
            throw new IllegalArgumentException("Error when load extension of extensible " + interfaceName +
                " from file:" + url + ", " + className + " is not subtype of interface.");
        }
        Class<? extends T> implClass = (Class<? extends T>) tmp;

        // 检查是否有可扩展标识
        Extension extension = implClass.getAnnotation(Extension.class);
        if (extension == null) {
            throw new IllegalArgumentException("Error when load extension of extensible " + interfaceName +
                " from file:" + url + ", " + className + " must add annotation @Extension.");
        } else {
            String aliasInCode = extension.value();
            if (StringUtils.isBlank(aliasInCode)) {
                // 扩展实现类未配置@Extension 标签
                throw new IllegalArgumentException("Error when load extension of extensible " + interfaceClass +
                    " from file:" + url + ", " + className + "'s alias of @Extension is blank");
            }
            if (alias == null) {
                // spi文件里没配置，用代码里的
                alias = aliasInCode;
            } else {
                // spi文件里配置的和代码里的不一致
                if (!aliasInCode.equals(alias)) {
                    throw new IllegalArgumentException("Error when load extension of extensible " + interfaceName +
                        " from file:" + url + ", aliases of " + className + " are " +
                        "not equal between " + aliasInCode + "(code) and " + alias + "(file).");
                }
            }
            // 接口需要编号，实现类没设置
            if (extensible.coded() && extension.code() < 0) {
                throw new IllegalArgumentException("Error when load extension of extensible " + interfaceName +
                    " from file:" + url + ", code of @Extension must >=0 at " + className + ".");
            }
        }
        // 不可以是default和*
        if (StringUtils.DEFAULT.equals(alias) || StringUtils.ALL.equals(alias)) {
            throw new IllegalArgumentException("Error when load extension of extensible " + interfaceName +
                " from file:" + url + ", alias of @Extension must not \"default\" and \"*\" at " + className + ".");
        }
        // 检查是否有存在同名的
        ExtensionClass old = all.get(alias);
        ExtensionClass<T> extensionClass = null;
        if (old != null) {
            // 如果当前扩展可以覆盖其它同名扩展
            if (extension.override()) {
                // 如果优先级还没有旧的高，则忽略
                if (extension.order() < old.getOrder()) {
                    if (LOGGER.isDebugEnabled()) {
                        LOGGER.debug("Extension of extensible {} with alias {} override from {} to {} failure, " +
                            "cause by: order of old extension is higher",
                            interfaceName, alias, old.getClazz(), implClass);
                    }
                } else {
                    if (LOGGER.isInfoEnabled()) {
                        LOGGER.info("Extension of extensible {} with alias {}: {} has been override to {}",
                            interfaceName, alias, old.getClazz(), implClass);
                    }
                    // 如果当前扩展可以覆盖其它同名扩展
                    extensionClass = buildClass(extension, implClass, alias);
                }
            }
            // 如果旧扩展是可覆盖的
            else {
                if (old.isOverride() && old.getOrder() >= extension.order()) {
                    // 如果已加载覆盖扩展，再加载到原始扩展
                    if (LOGGER.isInfoEnabled()) {
                        LOGGER.info("Extension of extensible {} with alias {}: {} has been loaded, ignore origin {}",
                            interfaceName, alias, old.getClazz(), implClass);
                    }
                } else {
                    // 如果不能被覆盖，抛出已存在异常
                    throw new IllegalStateException(
                        "Error when load extension of extensible " + interfaceClass + " from file:" + url +
                            ", Duplicate class with same alias: " + alias + ", " + old.getClazz() + " and " + implClass);
                }
            }
        } else {
            extensionClass = buildClass(extension, implClass, alias);
        }
        if (extensionClass != null) {
            // 检查是否有互斥的扩展点
            for (Map.Entry<String, ExtensionClass<T>> entry : all.entrySet()) {
                ExtensionClass existed = entry.getValue();
                if (extensionClass.getOrder() >= existed.getOrder()) {
                    // 新的优先级 >= 老的优先级，检查新的扩展是否排除老的扩展
                    String[] rejection = extensionClass.getRejection();
                    if (CommonUtils.isNotEmpty(rejection)) {
                        for (String rej : rejection) {
                            existed = all.get(rej);
                            if (existed == null || extensionClass.getOrder() < existed.getOrder()) {
                                continue;
                            }
                            ExtensionClass removed = all.remove(rej);
                            if (removed != null) {
                                if (LOGGER.isInfoEnabled()) {
                                    LOGGER.info(
                                        "Extension of extensible {} with alias {}: {} has been reject by new {}",
                                        interfaceName, removed.getAlias(), removed.getClazz(), implClass);
                                }
                            }
                        }
                    }
                } else {
                    String[] rejection = existed.getRejection();
                    if (CommonUtils.isNotEmpty(rejection)) {
                        for (String rej : rejection) {
                            if (rej.equals(extensionClass.getAlias())) {
                                // 被其它扩展排掉
                                if (LOGGER.isInfoEnabled()) {
                                    LOGGER.info(
                                        "Extension of extensible {} with alias {}: {} has been reject by old {}",
                                        interfaceName, alias, implClass, existed.getClazz());
                                    return;
                                }
                            }
                        }
                    }
                }
            }

            loadSuccess(alias, extensionClass);
        }
    }

ConsumerBootstrap sofa =  ExtensionLoaderFactory.getExtensionLoader(ConsumerBootstrap.class).getExtension("sofa");

    public ExtensionClass<T> getExtensionClass(String alias) {
        return all == null ? null : all.get(alias);
    }

    public T getExtension(String alias) {
        //从all属性中拿到加载的class
        ExtensionClass<T> extensionClass = getExtensionClass(alias);
        if (extensionClass == null) {
            throw new SofaRpcRuntimeException("Not found extension of " + interfaceName + " named: \"" + alias + "\"!");
        } else {
            //在加载class的时候，校验了是否是单例，如果是单例，那么factory不为null
            if (extensible.singleton() && factory != null) {
                T t = factory.get(alias);
                if (t == null) {
                    synchronized (this) {
                        t = factory.get(alias);
                        if (t == null) {
                            //实例化
                            t = extensionClass.getExtInstance();
                            //放入到factory，单例的class下次直接拿就好了，不需要重新创建
                            factory.put(alias, t);
                        }
                    }
                }
                return t;
            } else {
                //实例化
                return extensionClass.getExtInstance();
            }
        }
    }

    /**
     * 服务端实例对象（只在是单例的时候保留）
     * 用volatile修饰，保证了可见性
     */
    private volatile transient T       instance;
    
    /**
     * 得到服务端实例对象，如果是单例则返回单例对象，如果不是则返回新创建的实例对象
     *
     * @param argTypes 构造函数参数类型
     * @param args     构造函数参数值
     * @return 扩展点对象实例 ext instance
     */
    public T getExtInstance(Class[] argTypes, Object[] args) {
        if (clazz != null) {
            try {
                if (singleton) { // 如果是单例
                    if (instance == null) {
                        synchronized (this) {
                            if (instance == null) {
                                //通过反射创建实例
                                instance = ClassUtils.newInstanceWithArgs(clazz, argTypes, args);
                            }
                        }
                    }
                    return instance; // 保留单例
                } else {
                    //通过反射创建实例
                    return ClassUtils.newInstanceWithArgs(clazz, argTypes, args);
                }
            } catch (Exception e) {
                throw new SofaRpcRuntimeException("create " + clazz.getCanonicalName() + " instance error", e);
            }
        }
        throw new SofaRpcRuntimeException("Class of ExtensionClass is null");
    }