关于java:SpringBoot进阶之道Enable模块驱动

明天来说说 @Enable 模块驱动。Spring Framework 是从 3.1 版本开始反对“@Enable 模块驱动”的。所谓“模块”是指具备雷同畛域的性能组件汇合，组合造成一个独立的单元。

图表是 Spring Framework、Spring Boot 和spring Cloud的 @Enable 注解模块:

• 引入 @Enable 模块驱动的意义在于能 简化拆卸步骤，实现了“按需拆卸”，同时屏蔽组件汇合拆卸的细节。

1、分析 @Enable“模块驱动”

这就要从 Spring Framework3.0 新引入的 @Import 说起。@Import 用于导入一个或多个 ConfigurationClass，将其注册为 Spring Bean。这里须要留神的是:

• Spring Framework3.0 中存在肯定的限度，仅反对 @Configuration 标注的类
• 在 Spring Framework3.1 中，@Import 则扩充了指摘范畴，还能够用于申明至多一个 @Bean 办法的类，以及 ImportSelector 或 ImportBeanDefinitionRegistrar 的实现类

预计有小伙伴曾经发现了，Spring Framework 提供了 两类 对于 @Enable 模块驱动的实现形式:

• 被 @Configuration 类和 @Bean 办法申明的类归类为“注解驱动”。
• 以 ImportSelector 或 ImportBeanDefinitionRegistrar 的实现类归类为“接口编程”。

1）首先，咱们举个栗子，拿 @EnableAsync 注解来说：

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import({AsyncConfigurationSelector.class})
public @interface EnableAsync {...}

咱们发现下面 @EnableAsync 的源码标注了@Import({AsyncConfigurationSelector.class})。@Import 的作用我曾经提及过了，接下来点开 AsyncConfigurationSelector 类，看看是啥子货色:

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {...}

好，咱们发现 AsyncConfigurationSelector 继承了 AdviceModeImportSelector 类，那么咱们在点开 AdviceModeImportSelector 类，看看有什么机密：

public abstract class AdviceModeImportSelector<A extends Annotation> implements ImportSelector {...}

是不是很眼生，没错，AdviceModeImportSelector 实现了 ImportSelector 类，那么咱们是不是能够推断，@EnableAysnc 是以 “接口编程” 的形式实现 @Enbale 模块的。

2）那么，接下来咱们再举个栗子，看一下 @EnableWebMvc 注解有什么神秘之处：

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Import({DelegatingWebMvcConfiguration.class})
public @interface EnableWebMvc {}

以上源码置信小伙伴们曾经猜到了，@EnableWebMvc标注了@Import({DelegatingWebMvcConfiguration.class})。好，咱们关上 DelegatingWebMvcConfiguration 看看源码是什么：

@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {...}

有没有看出什么特别之处呢？DelegatingWebMvcConfiguration 类是一个 @Configuration 的类。所以咱们一样能够推断，@EnableWebMvc 是以 “注解驱动” 实现 @Enbale 模块的。

置信看到这里，不少小伙伴曾经能够自定义 @Enable 模块驱动了。

2、@Enable 模块驱动原理

后面提到过 @Enable 模块驱动应用 @Import 实现，并且 @Import 的职责在于装载导入类（Importing Class）, 将其定义为 Spring Bean。导入类次要为 @Configuration Class、ImportSelector 实现 以及ImportBeanDefinitionRegistrar 实现。

1）装载 @Configuration Class

• @Configuration 是 Spring Framework3.0 开始引入的，但该版本还未引入 @ComponentScan 注解驱动，因而配套的导入注解是 @Import。
• 只管 Spring Framework3.0 提供了注解驱动上下文实现 AnnotationConfigApplicationContext，但与 @Import 配合比拟繁琐，仅反对 Spring 组件类的一一导入，如这样 @Import({A.class,B.class,…})，因而过后还是无奈齐全代替 XML 元素<context:component-scan/>。
• 即便 Spring 利用上下文与 <context:component-scan/> 联合应用，@Import 的解决仍旧无奈执行。因而咱们以前在开发 spring 我的项目时，同时配置 XML 元素 <context:component-scan/> 与<context:annotation-config/>。

上面咱们剖析 <context:annotation-config/> 所对应的 BeanDefinitionParser 的实现类AnnotationConfigBeanDefinitionParser:

public class AnnotationConfigBeanDefinitionParser implements BeanDefinitionParser {
    ...
    @Nullable
    public BeanDefinition parse(Element element, ParserContext parserContext) {Object source = parserContext.extractSource(element);
        Set<BeanDefinitionHolder> processorDefinitions = AnnotationConfigUtils.registerAnnotationConfigProcessors(parserContext.getRegistry(), source);
        ...
        return null;
    }
}

咱们看源码得悉，parse(Element,ParserContext)办法调用 AnnotationConfigUtils#registerAnnotationConfigProcessors(BeanDefinitionRegistry,Object) 办法实现 BeanDefinition(用于存储 Bean 的定义信息 ) 的实例解析。

该办法从 Spring Framework3.0 开始，新增了 @Configuration Class 的解决实现ConfigurationClassPostProcessor:

public abstract class AnnotationConfigUtils {
    ...
    public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(BeanDefinitionRegistry registry, @Nullable Object source) {
        ...
        Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet(8);
        RootBeanDefinition def;
        if (!registry.containsBeanDefinition("org.springframework.context.annotation.internalConfigurationAnnotationProcessor")) {def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, "org.springframework.context.annotation.internalConfigurationAnnotationProcessor"));
        }
        ...
        return beanDefs;
    }
}

从源码我能够看出，ConfigurationClassPostProcessor 被封装成 Spring Bean 定义（BeanDefinition），接着注册为 Spring Bean，且 Bean 的名称为“org.springframework.context.annotation.internalConfigurationAnnotationProcessor”。

那么咱们脑洞下，所以在 Spring Framework 中:

• <context:annotation-config/>的底层实现类 AnnotationConfigBeanDefinitionParser 调用了 AnnotationConfigUtils#registerAnnotationConfigProcessors(BeanDefinitionRegistry, Object) 办法注册 ConfigurationClassPostProcessor Bean。
• <context:component-scan/>的底层实现类 ComponentScanBeanDefinitionParser 也调用了 AnnotationConfigUtils#registerAnnotationConfigProcessors(BeanDefinitionRegistry, Object) 办法注册 ConfigurationClassPostProcessor Bean。

其实 ConfigurationClassPostProcessor 不仅在 XML 配置驱动下才可拆卸，Spring Framework3.0 注解驱动上下文实现 AnnotationConfigApplicationContext 也能够拆卸：

public class AnnotationConfigApplicationContext extends GenericApplicationContext implements AnnotationConfigRegistry {
    private final AnnotatedBeanDefinitionReader reader;
    ...
    public AnnotationConfigApplicationContext() {this.reader = new AnnotatedBeanDefinitionReader(this);
        ...
    }
    ...
}

咱们持续察看成员变量 AnnotatedBeanDefinitionReader 的构造函数：

public class AnnotatedBeanDefinitionReader {
    ...
    public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {this(registry, getOrCreateEnvironment(registry));
    }

    public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
        ...
        this.registry = registry;
        ...
        AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
    }
}

咱们剖析以上源码，能够发现 AnnotationConfigApplicationContext 的AnnotatedBeanDefinitionReader类型成员 reader 在结构时，也显式的调用了 AnnotationConfigUtils#registerAnnotationConfigProcessors(BeanDefinitionRegistry, Object) 办法。

在 Spring 利用上下文启动中（AbstractApplicationContext#refresh()办法被调用时），Spring 容器（BeanFactory）将 ConfigurationClassPostProcessor 初始化为 Spring Bean。它作为 BeanFactoryPostProcessor 实现，随后其 postProcessBeanFactory(ConfigurableListableBeanFactory)办法被调用。

咱们先来看看 ConfigurationClassPostProcessor 源码：

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor, ... {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {...}
}

ConfigurationClassPostProcessor 实现了 BeanDefinitionRegistryPostProcessor，而后者继承了 BeanFactoryPostProcessor

public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {...}

public interface BeanFactoryPostProcessor {void postProcessBeanFactory(ConfigurableListableBeanFactory var1) throws BeansException;
}

能够看出 ConfigurationClassPostProcessor 实现了 BeanFactoryPostProcessor 并重写了 postProcessBeanFactory 办法。postProcessBeanFactory 办法被调用后，随之解决 @Configuration 类和 @Bean 办法：

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
        PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
    ...
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {int factoryId = System.identityHashCode(beanFactory);
        if (this.factoriesPostProcessed.contains(factoryId)) {
            throw new IllegalStateException("postProcessBeanFactory already called on this post-processor against" + beanFactory);
        }
        this.factoriesPostProcessed.add(factoryId);
        if (!this.registriesPostProcessed.contains(factoryId)) {
            // BeanDefinitionRegistryPostProcessor hook apparently not supported...
            // Simply call processConfigurationClasses lazily at this point then.
            processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
        }

        enhanceConfigurationClasses(beanFactory);
        beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
    }

    /**
     * Build and validate a configuration model based on the registry of
     * {@link Configuration} classes.
     */
    public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
        String[] candidateNames = registry.getBeanDefinitionNames();

        for (String beanName : candidateNames) {BeanDefinition beanDef = registry.getBeanDefinition(beanName);
            if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
                    ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {if (logger.isDebugEnabled()) {logger.debug("Bean definition has already been processed as a configuration class:" + beanDef);
                }
            }
            else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
            }
        }

        // Return immediately if no @Configuration classes were found
        if (configCandidates.isEmpty()) {return;}

        // Sort by previously determined @Order value, if applicable
        configCandidates.sort((bd1, bd2) -> {int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
            int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
            return Integer.compare(i1, i2);
        });

        // Detect any custom bean name generation strategy supplied through the enclosing application context
        SingletonBeanRegistry sbr = null;
        if (registry instanceof SingletonBeanRegistry) {sbr = (SingletonBeanRegistry) registry;
            if (!this.localBeanNameGeneratorSet) {BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
                if (generator != null) {
                    this.componentScanBeanNameGenerator = generator;
                    this.importBeanNameGenerator = generator;
                }
            }
        }

        if (this.environment == null) {this.environment = new StandardEnvironment();
        }

        // Parse each @Configuration class
        ConfigurationClassParser parser = new ConfigurationClassParser(
                this.metadataReaderFactory, this.problemReporter, this.environment,
                this.resourceLoader, this.componentScanBeanNameGenerator, registry);

        Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
        Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
        do {parser.parse(candidates);
            parser.validate();

            Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
            configClasses.removeAll(alreadyParsed);

            // Read the model and create bean definitions based on its content
            if (this.reader == null) {
                this.reader = new ConfigurationClassBeanDefinitionReader(
                        registry, this.sourceExtractor, this.resourceLoader, this.environment,
                        this.importBeanNameGenerator, parser.getImportRegistry());
            }
            this.reader.loadBeanDefinitions(configClasses);
            alreadyParsed.addAll(configClasses);

            candidates.clear();
            if (registry.getBeanDefinitionCount() > candidateNames.length) {String[] newCandidateNames = registry.getBeanDefinitionNames();
                Set<String> oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
                Set<String> alreadyParsedClasses = new HashSet<>();
                for (ConfigurationClass configurationClass : alreadyParsed) {alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
                }
                for (String candidateName : newCandidateNames) {if (!oldCandidateNames.contains(candidateName)) {BeanDefinition bd = registry.getBeanDefinition(candidateName);
                        if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
                                !alreadyParsedClasses.contains(bd.getBeanClassName())) {candidates.add(new BeanDefinitionHolder(bd, candidateName));
                        }
                    }
                }
                candidateNames = newCandidateNames;
            }
        }
        while (!candidates.isEmpty());

        // Register the ImportRegistry as a bean in order to support ImportAware @Configuration classes
        if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
        }

        if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
            // Clear cache in externally provided MetadataReaderFactory; this is a no-op
            // for a shared cache since it'll be cleared by the ApplicationContext.
            ((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();}
    }
}

2）装载 ImportSelector 和 ImportBeanDefinitionRegistrar 实现

因为 ImportSelector 和 ImportBeanDefinitionRegistrar 从 Spring Frameeork3.1 才开始引入，所以 3.0 版本中不会呈现两者的实现。因为 BeanDefinitionRegistryPostProcessor 从 Spring Framework3.0.1 开始引入，ConfigurationClassPostprocessor 的实现也随之发生变化，其实现接口从 BeanFactoryPostProcessor 替换为 BeanDefinitionRegistryPostProcessor，且 BeanDefinitionRegistryPostProcessor 扩大了 BeanFactoryPostProcessor 接口，所以 ConfigurationClassPostProcessor 存在两个阶段：

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
        PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {...}
    
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {...}
}

除两阶段实现外，ConfigurationClassPostProcessor 在 Spring Framework3.1 中并没有太多变动，该版本的次要变动还是集中在 ConfigurationClassParser 的实现上，在其 doProcessConfigurationClass(ConfigurationClass,AnnotationMetadata)办法中，减少了 @PropertySource 和 @ComponentScan 注解解决，并且更新了 processImport(ConfigurationClass,String[],boolean)办法的实现：

class ConfigurationClassParser {protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
            throws IOException {if (configClass.getMetadata().isAnnotated(Component.class.getName())) {// Recursively process any member (nested) classes first
            processMemberClasses(configClass, sourceClass);
        }

        // Process any @PropertySource annotations
        for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(sourceClass.getMetadata(), PropertySources.class,
                org.springframework.context.annotation.PropertySource.class)) {if (this.environment instanceof ConfigurableEnvironment) {processPropertySource(propertySource);
            } else {logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
                        "]. Reason: Environment must implement ConfigurableEnvironment");
            }
        }

        // Process any @ComponentScan annotations
        Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
        if (!componentScans.isEmpty() &&
                !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {for (AnnotationAttributes componentScan : componentScans) {
                // The config class is annotated with @ComponentScan -> perform the scan immediately
                Set<BeanDefinitionHolder> scannedBeanDefinitions =
                        this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
                // Check the set of scanned definitions for any further config classes and parse recursively if needed
                for (BeanDefinitionHolder holder : scannedBeanDefinitions) {BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
                    if (bdCand == null) {bdCand = holder.getBeanDefinition();
                    }
                    if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {parse(bdCand.getBeanClassName(), holder.getBeanName());
                    }
                }
            }
        }

        // Process any @Import annotations
        processImports(configClass, sourceClass, getImports(sourceClass), true);

        // Process any @ImportResource annotations
        AnnotationAttributes importResource =
                AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
        if (importResource != null) {String[] resources = importResource.getStringArray("locations");
            Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
            for (String resource : resources) {String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
                configClass.addImportedResource(resolvedResource, readerClass);
            }
        }

        // Process individual @Bean methods
        Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
        for (MethodMetadata methodMetadata : beanMethods) {configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
        }

        // Process default methods on interfaces
        processInterfaces(configClass, sourceClass);

        // Process superclass, if any
        if (sourceClass.getMetadata().hasSuperClass()) {String superclass = sourceClass.getMetadata().getSuperClassName();
            if (superclass != null && !superclass.startsWith("java") &&
                    !this.knownSuperclasses.containsKey(superclass)) {this.knownSuperclasses.put(superclass, configClass);
                // Superclass found, return its annotation metadata and recurse
                return sourceClass.getSuperClass();}
        }

        // No superclass -> processing is complete
        return null;
    }
}

class ConfigurationClassParser {
    ...
    private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass, Collection<SourceClass> importCandidates, boolean checkForCircularImports) {if (importCandidates.isEmpty()) {return;}

        if (checkForCircularImports && isChainedImportOnStack(configClass)) {this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
        } else {this.importStack.push(configClass);
            try {for (SourceClass candidate : importCandidates) {if (candidate.isAssignable(ImportSelector.class)) {
                        // Candidate class is an ImportSelector -> delegate to it to determine imports
                        Class<?> candidateClass = candidate.loadClass();
                        ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
                        ParserStrategyUtils.invokeAwareMethods(selector, this.environment, this.resourceLoader, this.registry);
                        if (selector instanceof DeferredImportSelector) {this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
                        } else {String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                            Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
                            processImports(configClass, currentSourceClass, importSourceClasses, false);
                        }
                    } else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
                        // Candidate class is an ImportBeanDefinitionRegistrar ->
                        // delegate to it to register additional bean definitions
                        Class<?> candidateClass = candidate.loadClass();
                        ImportBeanDefinitionRegistrar registrar =
                                BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
                        ParserStrategyUtils.invokeAwareMethods(registrar, this.environment, this.resourceLoader, this.registry);
                        configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
                    } else {
                        // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
                        // process it as an @Configuration class
                        this.importStack.registerImport(currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
                        processConfigurationClass(candidate.asConfigClass(configClass));
                    }
                }
            } catch (BeanDefinitionStoreException ex) {throw ex;} catch (Throwable ex) {
                throw new BeanDefinitionStoreException(
                        "Failed to process import candidates for configuration class [" +
                                configClass.getMetadata().getClassName() + "]", ex);
            } finally {this.importStack.pop();
            }
        }
    }
}    

通过 AssignabletypeFilter 判断候选 Class 元注解 @Import 是否赋值 ImportSelector 或 ImportBeanDefinitionRegistrar 实现，从而决定是否执行 ImportSelector 或 ImportBeanDefinitionRegistrar 解决，其中 importingClassMetadata 就是以后元注解 @Import 的 AnnotationMetadata 对象。

综上所述，ConfigurationClassPostProcessor 负责筛选 @Component Class、@Configuration Class 及 @Bean 办法的 Bean 定义（BeanDefinition），ConfigurationClassPrser 则从候选的 Bean 定义中解析除 Configuration 汇合，随后被 ConfigurationClassBeanDefinitionReader 转化并注册 BeanDefinition。

小结

@Enable“模块驱动”有两种实现形式

• 注解驱动
• 接口编程

Spring Framework 装载 @Configuration Class、ImportSelector 实现 以及 ImportBeanDefinitionRegistrar 实现 时，需引入 @Import 或 @ComponentScan。

但 Spring Framework 并不具备主动拆卸的能力。下节将说说 SpringBoot 主动拆卸。