DSL 系列（2） - 插件的论述与实现

前言

本文主要探讨基于 DSL（domain specific language） 之上的插件设计，他们是领域的附属，为领域提供额外的服务，但领域不依赖于他们。

1. 论述

领域应当尽可能地去专注他的核心业务规则，应当尽可能地与其他辅助性的代码解耦，一些通用的功能可以耦合进框架或者设计为中间件；但还存在有一些是与核心功能无关的，且又与业务逻辑密不可分，譬如特定的监控、特定的埋点、为领域定制的稳定性保障等，把他们定义为插件再合适不过，其依赖关系如前言所述。

2. 设计方案

暂不讨论特定的插件要实现哪些特定的能力，后续系列中将逐步展开构建一个完整的 DSL 具体需要哪些插件及其实现方案，这里我想展开思考的是怎样设计一个比较通用的 DSL 插件方案。

论述中对插件的定义与 AOP 的思想相当吻合，也当首选使用 AOP 来实现，但这其中还存在一个问题，我希望插件只专注其自身职责的表达，至于哪些节点需要接入哪些插件应当在 DSL 中配置（即我期望插件与 DSL 之间只存在配置关系），而配置应当支持动态更新，因此这就导致了 AOP 的代理对象事先是不确定的，需要去动态生成。

最后落到实现上，插件这块我需要去攻克两个核心技术点：

1、怎样去更新 AOP 的代理及切点表达式？

2、怎样去更新 IOC 容器？

3. 实现方案

3.1 配置入口

若不考虑动态更新，那么入口要实现的基本功能有两个：1、按需引入，这很简单，用一个 Conditional 即可；2、加载一个表达式类型的通知器，示例如下：

@Configuration
@ConditionalOnBean(DSL.class)
public class PluginConfig {
    @Bean
    public AspectJExpressionPointcutAdvisor pluginAdvisor() {
        AspectJExpressionPointcutAdvisor advisor = new AspectJExpressionPointcutAdvisor();
        advisor.setExpression(DSL.getExpression());
        advisor.setAdvice(new PluginAdvice());
        return advisor;
    }
}

public class PluginAdvice implements MethodInterceptor {
    @Override
    public Object invoke(MethodInvocation invocation) throws Throwable {
        System.out.println("do plugin_work start...");
        Object resObj = invocation.proceed();
        System.out.println("do plugin_work end...");
        return resObj;
    }
}

测试：

@RunWith(SpringRunner.class)
@SpringBootTest
public class DefaultTest {

    @ExtensionNode
    private Engine engine;

    @Test
    public void test() {
        DslUtils.setDslA();
        engine.launch();
    }
}

3.2 监听 DLS 变更

怎么监听配置的更新是所有的配置中心都需要去深入设计的（后续系列中探讨），此处暂用伪代码代替：

@Configuration
public class PluginListenerImpl implements DslListener {

    @Override
    public void refresh(DslContext dslContext) {
        // do something...
    }
}

3.3 更新切点表达式

3.1 中我们已经注入了一个表达式通知器的 Bean：AspectJExpressionPointcutAdvisor，因此仅仅更新表达式的字符串非常简单，但查看查看源码会发现起匹配作用的是他的内部对象 AspectJExpressionPointcut，而他在首次执行匹配时会构建一个 PointcutExpression 并保存起来：

private PointcutExpression obtainPointcutExpression() {
	if (getExpression() == null) {
		throw new IllegalStateException("Must set property 'expression' before attempting to match");
	}
	if (this.pointcutExpression == null) {
		this.pointcutClassLoader = determinePointcutClassLoader();
		this.pointcutExpression = buildPointcutExpression(this.pointcutClassLoader);
	}
	return this.pointcutExpression;
}

因此我们还需要通过反射将这个私有字段置空，让 ClassFilter 重新执行构建，示例如下：

@Configuration
public class PluginListenerImpl implements DslListener {

    @Autowired
    private AspectJExpressionPointcutAdvisor aspectJExpressionPointcutAdvisor;

    @Override
    public void refresh(DslContext dslContext) throws NoSuchFieldException, IllegalAccessException {
        refreshExpression(dslContext);
        // next...
    }

    private void refreshExpression(DslContext dslContext) throws NoSuchFieldException, IllegalAccessException {
        aspectJExpressionPointcutAdvisor.setExpression(dslContext.getExpression());
        AspectJExpressionPointcut pointcut = (AspectJExpressionPointcut) aspectJExpressionPointcutAdvisor.getPointcut().getClassFilter();

        Field f = AspectJExpressionPointcut.class
                .getDeclaredField("pointcutExpression");

        f.setAccessible(true);
        f.set(pointcut, null);
    }
}

3.3 更新动态代理

通过翻阅源码可得出 Spring AOP 主要通过：AbstractAdvisingBeanPostProcessor 、AbstractAutoProxyCreator 这两个 processor 来实现动态代理，其对应的实例为：MethodValidationPostProcessor、AnnotationAwareAspectJAutoProxyCreator，前者用于创建代理对象，后者用于标记切面（即织入代理）。由此，若我们需要去更新动态代理，我想到的最简单的方法就是对指定的节点重新执行以下这两个 processor（原理简单，就是一点点扣源码，麻烦...），其中还有一个小问题，和 3.2 中的一致，代理结果被缓存了，清空再执行即可，示例如下：

@Autowired
private DefaultListableBeanFactory defaultListableBeanFactory;

private void refreshTypes(DslContext dslContext) throws NoSuchFieldException, IllegalAccessException {
    List<Class<?>> refreshTypes = dslContext.getRefreshTypes();
    for (Class<?> refreshType : refreshTypes) {
        String[] beanNames = defaultListableBeanFactory.getBeanNamesForType(refreshType);
        for (String beanName : beanNames) {
            Object bean = defaultListableBeanFactory.getBean(beanName);
            for (BeanPostProcessor processor : defaultListableBeanFactory.getBeanPostProcessors()) {
                bean = getProxyBean(bean, beanName, processor);
            }

        }
    }
}

private Object getProxyBean(Object bean, String beanName, BeanPostProcessor processor) throws NoSuchFieldException, IllegalAccessException {
    if (processor instanceof MethodValidationPostProcessor
            || processor instanceof AnnotationAwareAspectJAutoProxyCreator) {
        removeAdvisedBeanCache(processor, bean, beanName);
        Object current = processor.postProcessAfterInitialization(bean, beanName);
        return current == null ? bean : current;
    }
    return bean;
}

private void removeAdvisedBeanCache(BeanPostProcessor processor, Object bean, String beanName) throws NoSuchFieldException, IllegalAccessException {
    if (processor instanceof AnnotationAwareAspectJAutoProxyCreator) {
        AnnotationAwareAspectJAutoProxyCreator annotationAwareAspectJAutoProxyCreator = (AnnotationAwareAspectJAutoProxyCreator) processor;
        Field f = AnnotationAwareAspectJAutoProxyCreator.class
                .getSuperclass()
                .getSuperclass()
                .getSuperclass()
                .getDeclaredField("advisedBeans");

        f.setAccessible(true);
        Map<Object, Boolean> advisedBeans = (Map<Object, Boolean>) f.get(annotationAwareAspectJAutoProxyCreator);
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        advisedBeans.remove(cacheKey);
    }
}

private Object getCacheKey(Class<?> beanClass, @Nullable String beanName) {
    if (StringUtils.hasLength(beanName)) {
        return (FactoryBean.class.isAssignableFrom(beanClass) ?
                BeanFactory.FACTORY_BEAN_PREFIX + beanName : beanName);
    } else {
        return beanClass;
    }
}

到此可以测试以下新生成的代理类：

public class PluginTest {

    @Autowired
    private BEngine bEngine;

    @Autowired
    private DslListener dslListener;

    @Test
    public void test() throws NoSuchFieldException, IllegalAccessException {
        System.out.println("--------proxy before-----------");
        System.out.println("BEngine.class:" + bEngine.getClass());
        bEngine.launch();

        DslContext dslContext = new DslContext();
        // 初始值为 execution( void com.youclk.et.car.a.AEngine.launch() )，BEngine 并未被代理
        dslContext.setExpression("execution( void com.youclk.et.car.b.BEngine.launch() )");
        dslContext.setRefreshTypes(Collections.singletonList(BEngine.class));
        dslListener.refresh(dslContext);
    }

}

结果如下：

通过这种方式更新可以不用担心多次刷新代理对象产生的副作用，因为最终变化的只是代理类所匹配切面通知而已。

3.4 更新 Spring Context

开码之前我一直认为这一步是难点，刷了一遍源码后发觉这一步异常简单（看源码还是很重要...）。DefaultListableBeanFactory 其实有提供 remove 和 register 方法用于更新 Bean，但是这两步的操作我认为太重了，而且在 remove 和 register 之间用到了这个 Bean 怎么办，因此存在极大风险。且看我们上一步做了什么，从 BeanDefinition 这个维度看我们只更新了 classType，其他的都没变，因此我考虑只要更新下 BeanDefinition，并清除对应的缓存即可，示例如下：

private void refreshTypes(DslContext dslContext) throws NoSuchFieldException, IllegalAccessException {
    List<Class<?>> refreshTypes = dslContext.getRefreshTypes();
    for (Class<?> refreshType : refreshTypes) {
        String[] beanNames = defaultListableBeanFactory.getBeanNamesForType(refreshType);
        for (String beanName : beanNames) {
            Object bean = defaultListableBeanFactory.getBean(beanName);
            for (BeanPostProcessor processor : defaultListableBeanFactory.getBeanPostProcessors()) {
                bean = getProxyBean(bean, beanName, processor);
            }
            refreshBeanDefinition(beanName, bean.getClass());
        }
    }
}

private void refreshBeanDefinition(String beanName, Class<?> classType) throws NoSuchFieldException, IllegalAccessException {
    RootBeanDefinition rootBeanDefinition = (RootBeanDefinition) defaultListableBeanFactory.getMergedBeanDefinition(beanName);
    rootBeanDefinition.setBeanClass(classType);

    ScannedGenericBeanDefinition scannedGenericBeanDefinition = (ScannedGenericBeanDefinition) defaultListableBeanFactory.getBeanDefinition(beanName);
    scannedGenericBeanDefinition.setBeanClass(classType);

    removeBeanDefinitionCache(beanName);
}

private void removeBeanDefinitionCache(String beanName) throws NoSuchFieldException, IllegalAccessException {
    Field factoryBeanObjectCache_f = DefaultListableBeanFactory.class
            .getSuperclass()
            .getSuperclass()
            .getSuperclass()
            .getDeclaredField("factoryBeanObjectCache");
    factoryBeanObjectCache_f.setAccessible(true);
    Map<String, Object> factoryBeanObjectCache = (Map<String, Object>) factoryBeanObjectCache_f.get(defaultListableBeanFactory);
    factoryBeanObjectCache.remove(beanName);

    Field singletonObjects_f = DefaultListableBeanFactory.class
            .getSuperclass()
            .getSuperclass()
            .getSuperclass()
            .getSuperclass()
            .getDeclaredField("singletonObjects");
    singletonObjects_f.setAccessible(true);
    Map<String, Object> singletonObjects = (Map<String, Object>) singletonObjects_f.get(defaultListableBeanFactory);
    singletonObjects.remove(beanName);

    Field singletonFactories_f = DefaultListableBeanFactory.class
            .getSuperclass()
            .getSuperclass()
            .getSuperclass()
            .getSuperclass()
            .getDeclaredField("singletonFactories");
    singletonFactories_f.setAccessible(true);
    Map<String, Object> singletonFactories = (Map<String, Object>) singletonFactories_f.get(defaultListableBeanFactory);
    singletonFactories.remove(beanName);

    Field earlySingletonObjects_f = DefaultListableBeanFactory.class
            .getSuperclass()
            .getSuperclass()
            .getSuperclass()
            .getSuperclass()
            .getDeclaredField("earlySingletonObjects");
    earlySingletonObjects_f.setAccessible(true);
    Map<String, Object> earlySingletonObjects = (Map<String, Object>) earlySingletonObjects_f.get(defaultListableBeanFactory);
    earlySingletonObjects.remove(beanName);
}

测试下是否完成了我的预期：

@Autowired
private ApplicationContext applicationContext;

@Test
public void testRefreshBeanDefinition() throws NoSuchFieldException, IllegalAccessException {
    System.out.println("--------refresh before-----------");
    System.out.println("BEngine.class:" + applicationContext.getBean(bEngine.getClass()).getClass());
    refresh();
    System.out.println("--------refresh after-----------");
    System.out.println("BEngine.class:" + applicationContext.getBean(bEngine.getClass()).getClass());
}

private void refresh() throws NoSuchFieldException, IllegalAccessException {
    DslContext dslContext = new DslContext();
    //初始值为 execution( void com.youclk.et.car.a.AEngine.launch() )，BEngine 并未被代理
    dslContext.setExpression("execution( void com.youclk.et.car.b.BEngine.launch() )");
    dslContext.setRefreshTypes(Collections.singletonList(BEngine.class));
    dslListener.refresh(dslContext);
}

结果如下：

两次获取到的 classType 不同，说明更新成功。

3.5 更新 IOC 容器

这是最关键的一步，从操作数量上来看也是最重的一步，我们来回顾下，到此我们已经刷新了代理、刷新了切面通知、并将变更提交到了 Spring Context 中，我们还缺最后一步：更新目标对象所有的依赖注入。

因为我们需要将修改后的 Bean 重新注入所有依赖他的 Bean 中，这其中可能涉及到众多的修改操作，因此第一步我们要获取所有的依赖注入关系，他们维护在：AutowiredAnnotationBeanPostProcessor.injectionMetadataCache 中；由于一次提交可能涉及到多个目标对象的更新，他们之间又有存在依赖的可能性，因此第二步先把那一堆新的 bean 刷到 metadataCache，最后筛选出所有与更新相关的依赖，重新注入一遍，示例如下：

private AutowiredAnnotationBeanPostProcessor autowiredAnnotationBeanPostProcessor;

private void refreshTypes(DslContext dslContext) throws Exception {
    List<Class<?>> refreshTypes = dslContext.getRefreshTypes();
    HashMap<String, String> refreshBeans = new HashMap<>();
    for (Class<?> refreshType : refreshTypes) {
        String[] beanNames = defaultListableBeanFactory.getBeanNamesForType(refreshType);
        for (String beanName : beanNames) {
            Object bean = defaultListableBeanFactory.getBean(beanName);
            for (BeanPostProcessor processor : defaultListableBeanFactory.getBeanPostProcessors()) {
                if (processor instanceof AutowiredAnnotationBeanPostProcessor) {
                    autowiredAnnotationBeanPostProcessor = (AutowiredAnnotationBeanPostProcessor) processor;
                    continue;
                }
                bean = getProxyBean(bean, beanName, processor);
            }
            refreshBeanDefinition(beanName, bean.getClass());
            refreshBeans.put(beanName, getRealName(bean.getClass().getName()));
        }
    }
    refreshIoc(refreshBeans);
}

private void refreshIoc(HashMap<String, String> refreshBeans) throws Exception {
    for (String refreshBeanName : refreshBeans.keySet()) {
        resetInjectionMetadataCache(refreshBeanName);
    }
    Set<Object> beans = getReInjectionBeans(refreshBeans);
    for (Object bean : beans) {
        defaultListableBeanFactory.autowireBeanProperties(bean, 0, false);
    }
}

private void resetInjectionMetadataCache(String refreshBeanName) {
    autowiredAnnotationBeanPostProcessor.resetBeanDefinition(refreshBeanName);
    autowiredAnnotationBeanPostProcessor.determineCandidateConstructors(refreshBeanName.getClass(), refreshBeanName);

    RootBeanDefinition rootBeanDefinition = (RootBeanDefinition) defaultListableBeanFactory.getMergedBeanDefinition(refreshBeanName);
    Object bean = defaultListableBeanFactory.getBean(refreshBeanName);
    autowiredAnnotationBeanPostProcessor.postProcessMergedBeanDefinition(rootBeanDefinition, bean.getClass(), refreshBeanName);
}

private Set<Object> getReInjectionBeans(HashMap<String, String> refreshBeans) throws Exception {
    Field injectionMetadataCache_f = AutowiredAnnotationBeanPostProcessor.class.getDeclaredField("injectionMetadataCache");
    injectionMetadataCache_f.setAccessible(true);
    Map<String, InjectionMetadata> factoryBeanObjectCache = (Map<String, InjectionMetadata>) injectionMetadataCache_f.get(autowiredAnnotationBeanPostProcessor);

    Set<Object> injectedBeanNames = new HashSet<>();

    for (String beanName : factoryBeanObjectCache.keySet()) {
        Collection<InjectionMetadata.InjectedElement> injectedElements = getInjectedElements(factoryBeanObjectCache.get(beanName));
        if (injectedElements == null) {
            continue;
        }
        for (InjectionMetadata.InjectedElement injectedElement : injectedElements) {
            if (refreshBeans.values().contains(getRealName(getResourceType(injectedElement).getName()))) {
                injectedBeanNames.add(defaultListableBeanFactory.getBean(beanName));
            }
        }

    }

    return injectedBeanNames;
}

private Collection<InjectionMetadata.InjectedElement> getInjectedElements(InjectionMetadata injectionMetadata) throws Exception {
    Field injectedElements_f = InjectionMetadata.class.getDeclaredField("injectedElements");
    injectedElements_f.setAccessible(true);
    Collection<InjectionMetadata.InjectedElement> injectedElements = (Collection<InjectionMetadata.InjectedElement>) injectedElements_f.get(injectionMetadata);
    return injectedElements;

}

private Class<?> getResourceType(InjectionMetadata.InjectedElement injectedElement) throws Exception {
    Method getResourceType_m = InjectionMetadata.InjectedElement.class.getDeclaredMethod("getResourceType");
    getResourceType_m.setAccessible(true);
    return (Class<?>) getResourceType_m.invoke(injectedElement);
}

private String getRealName(String instanceName) {
    int index = instanceName.indexOf("$");
    if (index > 0) {
        instanceName = instanceName.substring(0, index);
    }
    return instanceName;
}

最后再来测试一波：

@Test
public void test() throws Exception {
    bEngine.launch();
    refresh();
    bEngine.launch();
}

正如预期效果：

结语

灵明无著，物来顺应，未来不迎，当下不杂，既过不恋~ 请关注公众号：