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...
路漫漫其修远兮，吾将上下而求索！
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？ ConfigurationClassPostProcessor实现了BeanDefinitionRegistryPostProcessor，BeanDefinitionRegistryPostProcessor又实现了BeanFactoryPostProcessor，关于BeanFactoryPostProcessor，大家可以看看这篇文章：Spring拓展接口之BeanFactoryPostProcessor，占位符与敏感信息解密原理
概念介绍与应用
@Configuration
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Component public @interface Configuration { /** * 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. * <p>The custom name applies only if the Configuration class is picked up via * component scanning or supplied directly to a {@link AnnotationConfigApplicationContext}. * If the Configuration class is registered as a traditional XML bean definition, * the name/id of the bean element will take precedence. * @return the suggested component name, if any (or empty String otherwise) * @see org.springframework.beans.factory.support.DefaultBeanNameGenerator */ @AliasFor(annotation = Component.class) String value() default ""; }
@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<beans>，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<bean>，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
@Configuration public class MyConfiguration { @Bean public Cat mycat() { return new Cat(); } }
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
@FunctionalInterface public interface Condition { /** * Determine if the condition matches. * @param context the condition context * @param metadata metadata of the {@link org.springframework.core.type.AnnotationMetadata class} * or {@link org.springframework.core.type.MethodMetadata method} being checked * @return {@code true} if the condition matches and the component can be registered, * or {@code false} to veto the annotated component's registration */ boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata); }
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) @Documented public @interface Conditional { /** * All {@link Condition}s that must {@linkplain Condition#matches match} * in order for the component to be registered. */ Class<? extends Condition>[] value(); }
Spring的条件注解，其value是一个Class<? extends Condition>[]，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.
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) throws IOException { // 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); }
其中shouldSkip方法如下
/** * Determine if an item should be skipped based on {@code @Conditional} annotations. * @param metadata the meta data * @param phase the phase of the call * @return if the item should be skipped */ public boolean shouldSkip(@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)) { return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION); } return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN); } // 要解析的配置类的条件集合,即@Conditional的value List<Condition> 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 instanceof ConfigurationCondition) { 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; }
我们再回过头去看processConfigBeanDefinitions方法
/** * Build and validate a configuration model based on the registry of * {@link Configuration} classes. * 验证@Configuration修饰的类，满足条件则构建成configuration model */ 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 // 检测自定义的bean生成策略 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 // 解析每一个被@Configuration修饰的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); // 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<ConfigurationClass> 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 = new ConfigurationClassBeanDefinitionReader( 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<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(); } }
@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由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<Beans>、<Bean>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）。