SpringBoot进阶之道-@Enable模块驱动

草莓君_
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今天来说说@Enable模块驱动。Spring Framework是从3.1版本开始支持“@Enable模块驱动”的。所谓“模块”是指具备相同领域的功能组件集合，组合形成一个独立的单元。

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

框架实现 @Enable注解模块 激活模块
Spring Framework @EnableWebMvc Web Mvc模块
@EnableTransactionManagement 事务管理模块
@EnableCaching Caching模块
@EnableMBeanExport JMX模块
@EnableAsync 异步处理模块
@EnableWebFlux Web Flux 模块
@EnableAspectJAutoProxy AspectJ模块
Spring Boot @EnableAutoConfiguration 自动装配模块
@EnableManagementContext Actuator模块
@EnableConfigurationProperties 配置属性绑定模块
@EnableOAuth2Sso OAuth2 单点登陆模块
spring Cloud @EnableEurekaServer Eureka服务模块
@EnableConfigServer 配置服务器模块
@EnableFeignCliens Feign客户端模块
@EnableZuulProxy 服务网关Zuul模块
@EnableCircuitBreaker 服务熔断模块
  • 引入@Enable模块驱动的意义在于能简化装配步骤，实现了“按需装配”，同时屏蔽组件集合装配的细节。

1、剖析@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。这里需要注意的是:

  • Spring Framework3.0中存在一定的限制，仅支持@Configuration标注的类
  • 在Spring Framework3.1中，@Import则扩大了指责范围，还可以用于声明至少一个@Bean方法的类，以及ImportSelector或ImportBeanDefinitionRegistrar的实现类

估计有小伙伴已经发现了，Spring Framework提供了两类关于@Enable模块驱动的实现方式:

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

1）首先，我们举个栗子，拿@EnableAsync注解来说：

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

我们发现上面@EnableAsync的源码标注了@Import({AsyncConfigurationSelector.class})。@Import的作用我已经提及过了，接下来点开AsyncConfigurationSelector类，看看是啥子东西:

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

好，我们发现AsyncConfigurationSelector继承了AdviceModeImportSelector类，那么我们在点开AdviceModeImportSelector类，看看有什么秘密：

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

是不是很眼熟，没错，AdviceModeImportSelector实现了ImportSelector类，那么我们是不是可以推断，@EnableAysnc是以“接口编程”的方式实现@Enbale模块的。

2）那么，接下来我们再举个栗子，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 {
}

以上源码相信小伙伴们已经猜到了，@EnableWebMvc标注了@Import({DelegatingWebMvcConfiguration.class})。好，我们打开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。所以我们一样可以推断，@EnableWebMvc是以“注解驱动”实现@Enbale模块的。

相信看到这里，不少小伙伴已经可以自定义@Enable模块驱动了。

2、@Enable模块驱动原理

前面提到过@Enable模块驱动使用@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。导入类主要为@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，因此配套的导入注解是@Import。
  • 尽管Spring Framework3.0提供了注解驱动上下文实现AnnotationConfigApplicationContext，但与@Import配合比较繁琐，仅支持Spring组件类的逐个导入，如这样@Import({A.class,B.class,...})，因此当时还是无法完全替代XML元素<context:component-scan/>。
  • 即使Spring应用上下文与<context:component-scan/>结合使用，@Import的处理依旧无法执行。因此我们以前在开发spring项目时，同时配置XML元素<context:component-scan/>与<context:annotation-config/>。

下面我们分析<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;
    }
}

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

该方法从Spring Framework3.0开始，新增了@Configuration Class的处理实现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;
    }
}

从源码我可以看出，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”。

那么我们脑洞下，所以在Spring Framework中:

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

其实ConfigurationClassPostProcessor不仅在XML配置驱动下才可装配，Spring Framework3.0注解驱动上下文实现AnnotationConfigApplicationContext也可以装配：

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

我们继续观察成员变量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);
    }
}

我们分析以上源码，可以发现AnnotationConfigApplicationContext的AnnotatedBeanDefinitionReader类型成员reader在构造时，也显式的调用了AnnotationConfigUtils#registerAnnotationConfigProcessors(BeanDefinitionRegistry, Object)方法。

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

我们先来看看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;
}

可以看出ConfigurationClassPostProcessor实现了BeanFactoryPostProcessor并重写了postProcessBeanFactory方法。postProcessBeanFactory方法被调用后，随之处理@Configuration类和@Bean方法：

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

由于ImportSelector和ImportBeanDefinitionRegistrar从Spring Frameeork3.1才开始引入，所以3.0版本中不会出现两者的实现。由于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扩展了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中并没有太多变化，该版本的主要变化还是集中在ConfigurationClassParser的实现上，在其doProcessConfigurationClass(ConfigurationClass,AnnotationMetadata)方法中，增加了@PropertySource和@ComponentScan注解处理，并且更新了processImport(ConfigurationClass,String[],boolean)方法的实现：

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处理，其中importingClassMetadata就是当前元注解@Import的AnnotationMetadata对象。

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

小结

@Enable “模块驱动”有两种实现方式

  • 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并不具备自动装配的能力。下节将说说SpringBoot自动装配。

【参考小马哥SpringBoot编程思想】

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