引言

上篇文章： Dubbo源码篇06—SPI神秘的面纱—原理篇—上我们追踪了getAdaptiveExtension获取自适应扩展点的整个流程，整个流程核心如下:

private T createAdaptiveExtension() {
        T instance = (T) getAdaptiveExtensionClass().newInstance();
        instance = postProcessBeforeInitialization(instance, null);
        injectExtension(instance);
        instance = postProcessAfterInitialization(instance, null);
        initExtension(instance);
        return instance;
}

private Class<?> getAdaptiveExtensionClass() {
    getExtensionClasses();
    if (cachedAdaptiveClass != null) {
        return cachedAdaptiveClass;
    }
    return cachedAdaptiveClass = createAdaptiveExtensionClass();
}

因为自适应扩展点在dubbo中的用意是用来实现运行时动态选择实现类的，所以不会对自适应扩展点赋予AOP能力，从上面的流程中我们也没有发现哪里存在Wrapper机制的处理。

所以本文我们顺着普通扩展类加载流程，来过一遍dubbo对AOP的处理过程:

        ApplicationModel applicationModel = ApplicationModel.defaultModel();
        ExtensionLoader<FrameWork> extensionLoader = applicationModel.getExtensionLoader(FrameWork.class);
        FrameWork frameWork = extensionLoader.getExtension("guice");

根据name获取扩展实例对象

根据传入的name作为serviceKey去加载对应的扩展实现：

    public T getExtension(String name) {
        //第二个参数表明是否对当前扩展类启动Wrapper装饰
        T extension = getExtension(name, true);
        if (extension == null) {
            throw new IllegalArgumentException("Not find extension: " + name);
        }
        return extension;
    }

public T getExtension(String name, boolean wrap) {
        ...
        //如果name为true,那么去获取默认扩展实现
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        String cacheKey = name;
        if (!wrap) {
            cacheKey += "_origin";
        }
        //查询缓存--没有新建一个Holder返回
        final Holder<Object> holder = getOrCreateHolder(cacheKey);
        Object instance = holder.get();
        //缓存有,直接返回,否则进入创建逻辑
        if (instance == null) {
            synchronized (holder) {
                instance = holder.get();
                if (instance == null) {
                    //创建扩展类的核心方法
                    instance = createExtension(name, wrap);
                    holder.set(instance);
                }
            }
        }
        return (T) instance;
    }

private T createExtension(String name, boolean wrap) {
        //getExtensionClasses方法上篇文章解析过了,这里跳过
        Class<?> clazz = getExtensionClasses().get(name);
        //如果没有key=name的扩展实现,则抛出异常
        if (clazz == null || unacceptableExceptions.contains(name)) {
            throw findException(name);
        }
        try {
           // 判断对应的扩展实现类型是否已经创建过了实例对象--确保单例性 
            T instance = (T) extensionInstances.get(clazz);
            //如果只是解析了SPI文件,构成了<name,class>缓存,下一步就是为当前扩展类型构建<class,signleInstance>缓存
            if (instance == null) {
                //利用instantiationStrategy实例化扩展实例对象---具体逻辑在InstantiationStrategy中
                //实例化逻辑比较简单: 要不就是默认构造,要么构造函数可以有参数,但是参数类型必须是ScopeModel子类
                extensionInstances.putIfAbsent(clazz, createExtensionInstance(clazz));
                instance = (T) extensionInstances.get(clazz);
                //前后置处理--依赖注入
                instance = postProcessBeforeInitialization(instance, name);
                injectExtension(instance);
                instance = postProcessAfterInitialization(instance, name);
            }
            //和自适应扩展点创建的不同逻辑: 判断是否需要对当前扩展实例进行装饰
            if (wrap) {
                List<Class<?>> wrapperClassesList = new ArrayList<>();
                //当前扩展类相关wrapper类型搜集工作在getExtensionClasses中完成
                if (cachedWrapperClasses != null) {
                    wrapperClassesList.addAll(cachedWrapperClasses);
                    wrapperClassesList.sort(WrapperComparator.COMPARATOR);
                    Collections.reverse(wrapperClassesList);
                }
                
                //wrapper class搜集是满足存在一个单参数的拷贝构造函数，并且参数类型为当前扩展类类型
                if (CollectionUtils.isNotEmpty(wrapperClassesList)) {
                    //不断循环,套娃创建一层层的装饰器对象
                    for (Class<?> wrapperClass : wrapperClassesList) {
                        //Wrapper注解用于实现按条件装饰
                        Wrapper wrapper = wrapperClass.getAnnotation(Wrapper.class);
                        //如果wrapper class类上不存在Wrapper注解,那么表示装饰不需要满足任何条件
                        //否则,需要判断条件是否满足,满足才会进行装饰
                        boolean match = (wrapper == null) ||
                            ((ArrayUtils.isEmpty(wrapper.matches()) || ArrayUtils.contains(wrapper.matches(), name)) &&
                                !ArrayUtils.contains(wrapper.mismatches(), name));
                        if (match) {
                            //满足则进入装饰流程
                            //1.实例化当前装饰类,采用的是单参的拷贝构造函数
                            //2.执行依赖注入流程
                            instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
                            //3.执行后置处理流程
                            instance = postProcessAfterInitialization(instance, name);
                        }
                    }
                }
            }

            // Warning: After an instance of Lifecycle is wrapped by cachedWrapperClasses, it may not still be Lifecycle instance, this application may not invoke the lifecycle.initialize hook.
            //调用初始化接口---注意上面警告信息,也就是说经过包装后,我们的包装对象未必继承lifecycle接口,因此初始化调用也就不会发生了
            initExtension(instance);
            return instance;
        } catch (Throwable t) {
            throw new IllegalStateException("Extension instance (name: " + name + ", class: " +
                type + ") couldn't be instantiated: " + t.getMessage(), t);
        }
    }

获取默认扩展实例对象

采用@SPI注解中的val值,作为serviceKey去加载对应的扩展实现:

    public T getDefaultExtension() {
        //加载SPI文件,构建相关缓存,如: <name,class>
        getExtensionClasses();
        //cachedDefaultName来自@SPI注解中的val值
        if (StringUtils.isBlank(cachedDefaultName) || "true".equals(cachedDefaultName)) {
            return null;
        }
        //这个流程上面讲过了
        return getExtension(cachedDefaultName);
    }

按条件批量获取扩展实例对象

到现在为止，我们还差extensionLoader.getActivateExtensions()流程没有讲解，下面我们来看看按条件批量获取扩展实例对象是怎样实现的:

    public List<T> getActivateExtension(URL url, String key, String group) {
        //根据传入的key从url中提取出value值
        String value = url.getParameter(key);
        //如果value不为空,则按照","分割，作为serviceKey
        return getActivateExtension(url, StringUtils.isEmpty(value) ? null : COMMA_SPLIT_PATTERN.split(value), group);
    }

    public List<T> getActivateExtension(URL url, String[] values, String group) {
        checkDestroyed();
        // solve the bug of using @SPI's wrapper method to report a null pointer exception.
        Map<Class<?>, T> activateExtensionsMap = new TreeMap<>(activateComparator);
        List<String> names = values == null ? new ArrayList<>(0) : asList(values);
        Set<String> namesSet = new HashSet<>(names);
        if (!namesSet.contains(REMOVE_VALUE_PREFIX + DEFAULT_KEY)) {
            //集合条件缓存构建: <serviceKey,groups>  和 <serviceKey,keyParis>
            if (cachedActivateGroups.size() == 0) {
                synchronized (cachedActivateGroups) {
                    // cache all extensions
                    if (cachedActivateGroups.size() == 0) {
                        //加载当前扩展类对应的SPI资源文件,并建立好相关缓存映射,此处主要为cachedActivates映射
                        //cachedActivates缓存了<serviceKey,@Activate注解> (serviceKey就是我们在SPI文件: serviceKey=serivceImpl全类名)
                        //如果配置文件中没有指定serviceKey,那么为@Extension注解中指定的val值,如果没有注解,那么就为实现类的简单类名
                        getExtensionClasses();
                        //依次处理当前扩展类下所有标注了@Activate注解的实现类
                        for (Map.Entry<String, Object> entry : cachedActivates.entrySet()) {
                            //key为serviceKey
                            String name = entry.getKey();
                            //activate注解
                            Object activate = entry.getValue();

                            String[] activateGroup, activateValue;
                            //提取注解中的值
                            if (activate instanceof Activate) {
                                activateGroup = ((Activate) activate).group();
                                activateValue = ((Activate) activate).value();
                            } else if (activate instanceof com.alibaba.dubbo.common.extension.Activate) {
                                activateGroup = ((com.alibaba.dubbo.common.extension.Activate) activate).group();
                                activateValue = ((com.alibaba.dubbo.common.extension.Activate) activate).value();
                            } else {
                                continue;
                            }
                            //缓存<serviceKey,groups>映射,即激活当前实现类,需要满足哪些分组要求(满足其一即可)
                            cachedActivateGroups.put(name, new HashSet<>(Arrays.asList(activateGroup)));
                            //缓存<serviceKey,keyParis>映射
                            //activate注解中的value属性有两种写法: key1:val1 或者 key2
                            //前者表示URL中存在key1=val1的键值对才算满足条件
                            //后置表示URL中存在key2即满足条件
                            String[][] keyPairs = new String[activateValue.length][];
                            for (int i = 0; i < activateValue.length; i++) {
                                if (activateValue[i].contains(":")) {
                                    keyPairs[i] = new String[2];
                                    String[] arr = activateValue[i].split(":");
                                    keyPairs[i][0] = arr[0];
                                    keyPairs[i][1] = arr[1];
                                } else {
                                    keyPairs[i] = new String[1];
                                    keyPairs[i][0] = activateValue[i];
                                }
                            }
                            cachedActivateValues.put(name, keyPairs);
                        }
                    }
                }
            }

            // traverse all cached extensions
            //遍历<serviceKey,groups>映射
            cachedActivateGroups.forEach((name, activateGroup) -> {
                // 如果函数调用中传入的group匹配条件为空,或者group存在于groups集合,则满足分组匹配这个条件
                if (isMatchGroup(group, activateGroup)
                 //nameSet是从传入函数中的key,从url中获取value后,按照","分割,得到的集合
                 //这里去掉serviceKey=name的处理,因为该逻辑会在下面被处理   
                    && !namesSet.contains(name)
                    && !namesSet.contains(REMOVE_VALUE_PREFIX + name)
                    //从<serviceKey,keyParis>集合中获取激活当前扩展实现类,需要满足哪些用户自定义条件
                    //这里判断逻辑就是: 如果用户指定的是形如@Active(value="key1:value1, key2:value2")
                    //那么会从url先中取出key1的值,与value1进行比较,相等直接返回true，否则取出key2值继续判断,也就是说这里是任意条件满足就返回true
                    //如果用户注解中只指定了@Active(value="key1"),那么只要url中存在key1，就满足条件
                    && isActive(cachedActivateValues.get(name), url)) {
                    
                    //如果分组条件和用户自定义条件都满足,则加入activateExtensionsMap集合<class,Instance>
                    activateExtensionsMap.put(getExtensionClass(name), getExtension(name));
                }
            });
        }

        if (namesSet.contains(DEFAULT_KEY)) {
           ...
        } else {
            // add extensions, will be sorted by its order
            for (int i = 0; i < names.size(); i++) {
                String name = names.get(i);
                if (!name.startsWith(REMOVE_VALUE_PREFIX)
                    && !namesSet.contains(REMOVE_VALUE_PREFIX + name)) {
                    if (!DEFAULT_KEY.equals(name)) {
                        //<serviceKey,class>集合中包含serviceKey=name
                        if (containsExtension(name)) {
                            //则将serviceKey=name的扩展实现类也加入结果集合
                            activateExtensionsMap.put(getExtensionClass(name), getExtension(name));
                        }
                    }
                }
            }
            //返回最终得到的集合扩展实现类集合
            return new ArrayList<>(activateExtensionsMap.values());
        }
    }

上面这一大段看下来可能会比较懵逼，但是没关系，下图详细解释了按照激活条件筛选的整个流程:
在这里插入图片描述

如果某个serviceKey对应的keyParis为空，也就是说用户没有自定义匹配条件，那么该条件分支默认返回true。
如果函数传入的group为空，那么不考虑分组匹配条件，该条件分支默认返回true

注意: 放入结果集合前，扩展类的获取调用的是getExtension方法，意味着按条件批量获取扩展实例对象场景下，实现类是享有AOP(Wrapper机制)支持的:

   activateExtensionsMap.put(getExtensionClass(name), getExtension(name));

实例演示

扩展接口,及其实现类

@SPI("spring")
public interface FrameWork {
    @Adaptive
    String getName(URL url);
    String getInfo();
}

@Activate(value = {"name:dhy","age:18"},group = "test")
public class Guice implements FrameWork{
    @Override
    public String getName(URL url) {
        return "guice";
    }

    @Override
    public String getInfo() {
        return "google 开源的轻量级IOC框架";
    }
}

@Activate(value = {"name","sex"},group = "test")
public class Spring implements FrameWork{
    @Override
    public String getName(URL url) {
        return "spring";
    }

    @Override
    public String getInfo() {
        return "流行的Spring框架";
    }
}


@Activate(group = "prod")
public class SpringBoot implements FrameWork{
    @Override
    public String getName(URL url) {
        return "springBoot";
    }

    @Override
    public String getInfo() {
        return "自动化的SpringBoot框架";
    }
}

SPI文件内容

spring=com.adaptive.Spring
springBoot=com.adaptive.SpringBoot
guice=com.adaptive.Guice

测试类

class ActivateTest {
    @Test
    void activateTest() {
        ApplicationModel applicationModel = ApplicationModel.defaultModel();
        ExtensionLoader<FrameWork> extensionLoader = applicationModel.getExtensionLoader(FrameWork.class);
        List<FrameWork> frameWorkList = extensionLoader.getActivateExtension(URL.valueOf("dubbo://127.0.0.1:80/?age=18"), "", "test");
        frameWorkList.forEach(frameWork -> {
            System.out.println(frameWork.getInfo());
        });
    }
}