spring-boot-2.0.3不一样系列之番外篇 - @Configuration、Condition与@Conditional

前言

开心一刻

一名劫匪慌忙中窜上了一辆车的后座，上车后发现主驾和副驾的一男一女疑惑地回头看着他，他立即拔出枪威胁到：“赶快开车，甩掉后面的警车，否则老子一枪崩了你！”，于是副驾上的男人转过脸对那女的说：“大姐，别慌，听我口令把刚才的动作再练习一遍，挂一档，轻松离合，轻踩油门，走...走，哎 走...哎，哎，对，走走... 最后，三人都躺到了医院，劫匪的手上还戴上了铐子...

劫匪的内心路漫漫其修远兮，吾将上下而求索！

github：https://github.com/youzhibing

码云(gitee)：https://gitee.com/youzhibing

前情回顾

估摸着大家已经忘记了createApplicationContext的内容，本文不做过多的回顾，只是提醒大家：在AnnotationConfigServletWebServerApplicationContext的实例化过程中，实例化了AnnotatedBeanDefinitionReader，另外也将ConfigurationClassPostProcessor定义注册到了beanFactory中，如下图所示

看着AnnotatedBeanDefinitionReader、ConfigurationClassPostProcessor是不是隐约感觉到了什么？

概念介绍与应用

@Configuration

@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Componentpublic @interfaceConfiguration {//// Explicitly specify the name of the Spring bean definition associated / with this Configuration class. If left unspecified (the common case), / a bean name will be automatically generated. /

The custom name applies only if the Configuration class is picked up via / component scanning or supplied directly to a {@linkAnnotationConfigApplicationContext}. / If the Configuration class is registered as a traditional XML bean definition, / the name/id of the bean element will take precedence. /@returnthe suggested component name, if any (or empty String otherwise) /@seeorg.springframework.beans.factory.support.DefaultBeanNameGenerator//@AliasFor(annotation= Component.class) String value()default ""; }View Code

@Configuration能够修饰Class、interface和enum，用的最多的还是标注在类上，相当于把该类作为spring的xml配置文件中的，用于配置spring容器；@Configuration往往会结合@Bean来使用，@Bean等价于spring的xml配置文件中的，用于注册bean对象。@Configuration和@Bean组成了基于java类的配置，是spring的推荐配置方式。最简单的使用如下

@Configurationpublic classMyConfiguration { @BeanpublicCat mycat() {return newCat(); } }

如上代码就会在spring容器中注册一个名叫mycat的Cat类型的Bean

Condition

@FunctionalInterfacepublic interfaceCondition {//// Determine if the condition matches. /@paramcontext the condition context /@parammetadata metadata of the {@linkorg.springframework.core.type.AnnotationMetadata class} / or {@linkorg.springframework.core.type.MethodMetadata method} being checked /@return{@codetrue} if the condition matches and the component can be registered, / or {@codefalse} to veto the annotated component's registration/*/ booleanmatches(ConditionContext context, AnnotatedTypeMetadata metadata); }View Code

Spring的条件化配置，当我们向spring注册bean时，可以对这个bean添加一定的自定义条件，当满足这个条件时注册这个bean，否则不注册。springboot中部分实现子类如下

springboot更多实现请查看org.springframework.boot.autoconfigure.condition包。Condition一般配合@Conditional使用，更多信息往下看

@Conditional

@Target({ElementType.TYPE, ElementType.METHOD}) @Retention(RetentionPolicy.RUNTIME) @Documentedpublic @interfaceConditional {//// All {@linkCondition}s that must {@linkplainCondition/#matches match} / in order for the component to be registered./*/Class[] value(); }View Code

Spring的条件注解，其value是一个Class[]，只有数组中的全部Condition全部匹配成功时，被@Conditional修饰的组件才会被注册到Spring容器中。@Conditional只是一个标志，标示需要进行条件判断，而具体的判断规则则由具体的Condition来实现。

在SpringBoot源码中很容易看到被@Conditional注解的组合注解，例如：@ConditionalOnBean、@ConditionalOnMissingBean、@ConditionalOnClass、@ConditionalOnMissingClass等，具体如下

springboot还提供了AutoConfigureAfter、AutoConfigureBefore、AutoConfigureOrder，看名字基本知道其作用，具体细节需要大家自己去跟了。

完整应用案例

接口都能访问通，数据返回也都正确，非常完美

完整工程代码：spring-boot-condition

当我们把MyConfiguration中的myCat方法注释掉（ConditionWeb中的cat相关也注释掉），再启动应用的时候，应用报错启动不起来，提示如下信息：

Description: Field dog in com.lee.condition.web.ConditionWeb required a bean of type'com.lee.condition.model.Dog'that could not be found.- Bean method 'myDog' in 'MyConfiguration'not loaded because @ConditionalOnBean (types: com.lee.condition.model.Cat; SearchStrategy: all) did not find any beans of type com.lee.condition.model.Cat Action: Consider revisiting the conditions above or defining a bean of type'com.lee.condition.model.Dog' in your configuration.View Code

ConditionWeb中需要Dog类型的bean，而Dog实例化又依赖Cat实例，而我们没有实例化Cat，所以应用启动报错，提示如上信息

源码探究

我们要探究什么了？不探究太细，就探究@Configuration修饰的配置类是何时解析的，@Conditional是何时生效、如何生效的

@Configuration修饰的配置类是何时解析的

ConfigurationClassPostProcessor是一个BeanFactoryPostProcessor（可以查看ConfigurationClassPostProcessor的类继承结构图），那么我们从AbstractApplicationContext的refresh方法调用的invokeBeanFactoryPostProcessors(beanFactory)方法开始

来到了processConfigurationClass方法，其详细代码如下

protected void processConfigurationClass(ConfigurationClass configClass) throwsIOException {//ConfigurationClass是否应该被skip if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {return; } ConfigurationClass existingClass= this.configurationClasses.get(configClass);if (existingClass != null) {if(configClass.isImported()) {if(existingClass.isImported()) { existingClass.mergeImportedBy(configClass); }//Otherwise ignore new imported config class; existing non-imported class overrides it. return; }else{//Explicit bean definition found, probably replacing an import.//Let's remove the old one and go with the new one. this.configurationClasses.remove(configClass);this.knownSuperclasses.values().removeIf(configClass::equals); } }//Recursively process the configuration class and its superclass hierarchy. 递归处理configuration class和它的父级类//也就说会递归处理我们的应用入口类：ConditionApplication.class，以及ConditionApplication.class的父级类 SourceClass sourceClass =asSourceClass(configClass);do{ sourceClass=doProcessConfigurationClass(configClass, sourceClass); }while (sourceClass != null);//将满足条件的ConfigurationClass都放入configurationClasses集合中//后续会加载configurationClasses集合中所有的ConfigurationClass中配置的bean定义 this.configurationClasses.put(configClass, configClass); }View Code

其中shouldSkip方法如下

//// Determine if an item should be skipped based on {@code@Conditional} annotations. /@parammetadata the meta data /@paramphase the phase of the call /@returnif the item should be skipped/*/ public booleanshouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {//如果这个类没有注解修饰，或者没有被@Conditional注解（包括Conditional系列）所修饰，不会skip if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {return false; }//如果参数中沒有设置条件注解的生效阶段 if (phase == null) {if (metadata instanceof AnnotationMetadata &&ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {returnshouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION); }returnshouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN); }//要解析的配置类的条件集合,即@Conditional的value List conditions = new ArrayList<>();for(String[] conditionClasses : getConditionClasses(metadata)) {for(String conditionClass : conditionClasses) { Condition condition= getCondition(conditionClass, this.context.getClassLoader()); conditions.add(condition); } }//对条件进行排序 AnnotationAwareOrderComparator.sort(conditions);//遍历条件，逐个匹配 for(Condition condition : conditions) { ConfigurationPhase requiredPhase= null;if (condition instanceofConfigurationCondition) { requiredPhase=((ConfigurationCondition) condition).getConfigurationPhase(); }//条件注解的生效阶段满足，一旦有条件匹配不成功，则返回true，skip此类 if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {return true; } }return false; }View Code

我们再回过头去看processConfigBeanDefinitions方法

//// Build and validate a configuration model based on the registry of / {@linkConfiguration} classes. / 验证@Configuration修饰的类，满足条件则构建成configuration model// public voidprocessConfigBeanDefinitions(BeanDefinitionRegistry registry) { List 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(newBeanDefinitionHolder(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());returnInteger.compare(i1, i2); });//Detect any custom bean name generation strategy supplied through the enclosing application context//检测自定义的bean生成策略 SingletonBeanRegistry sbr = null;if (registry instanceofSingletonBeanRegistry) { 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 = newStandardEnvironment(); }//Parse each @Configuration class//解析每一个被@Configuration修饰的class ConfigurationClassParser parser = newConfigurationClassParser(this.metadataReaderFactory, this.problemReporter, this.environment,this.resourceLoader, this.componentScanBeanNameGenerator, registry); Set candidates = new LinkedHashSet<>(configCandidates); Set alreadyParsed = new HashSet<>(configCandidates.size());do{ parser.parse(candidates);//解析过程中会将满足条件的@Configuration class存放到configurationClasses中 parser.validate();//满足条件的@Configuration class 都存放在了parser的configurationClasses中 Set configClasses = new LinkedHashSet<>(parser.getConfigurationClasses()); configClasses.removeAll(alreadyParsed);//Read the model and create bean definitions based on its content//读取@Configuration class中的配置（各个@Bean），并创建对应的bean definition（后续创建bean实例会用到bean定义） if (this.reader == null) {this.reader = newConfigurationClassBeanDefinitionReader( registry,this.sourceExtractor, this.resourceLoader, this.environment,this.importBeanNameGenerator, parser.getImportRegistry()); }this.reader.loadBeanDefinitions(configClasses); //加载全部@Configuration class中的配置 alreadyParsed.addAll(configClasses); candidates.clear();if (registry.getBeanDefinitionCount() >candidateNames.length) { String[] newCandidateNames=registry.getBeanDefinitionNames(); Set oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames)); Set 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(newBeanDefinitionHolder(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 instanceofCachingMetadataReaderFactory) {//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(); } }View Code

@Conditional是何时生效、如何生效的

这个问题再上面已经全部得到体现，Spring不会无脑的加载所有的@Configuration class，只会加载满足条件的@Configuration class，而@Conditional就是条件标志，至于条件匹配规则这有Condition提供；shouldSkip方法中用到Conditional和Condition，完成条件的匹配处理。

总结

1、@Configuration和@Bean组成了基于java类的配置，与xml中的、功能一致，Spring推荐java类的配置；

2、Condition与@Conditional实现了条件配置，只有满足了条件的@Configuration class和@Bean才会被注册到Spring容器；

3、Spring以我们的应用启动类为基础来递归扫描配置类，包括我们应用中的配置类、Spring自己的以及第三方的配置类（springboot集成的各种配置类(spring-boot-autoconfigure-xxx.RELEASE.jar下的spring.factories文件中的Auto Configure)，还有pageHelper的自动配置，等等）；前提是需要开启自动配置（@EnableAutoConfiguration）。

参考

SpringBoot源码分析之条件注解的底层实现

Spring 工具类 ConfigurationClassParser 分析得到