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 <https://github.com/youzhibing>

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

前情回顾

　　估摸着大家已经忘记了createApplicationContext
<https://www.cnblogs.com/youzhibing/p/9686969.html>
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 @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 ""; } 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<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); }
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 @interface Conditional {
/** * All {@link Condition}s that must {@linkplain Condition#matches match} *
in order for the component to be registered.*/ Class<? extends Condition>[]
value(); } View Code
　　　　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
<https://gitee.com/youzhibing/spring-boot-2.0.3/tree/master/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) 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); } View Code
　　　　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; } View Code
　　　　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(); } }
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<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）。

参考

　　SpringBoot源码分析之条件注解的底层实现
<https://fangjian0423.github.io/2017/05/16/springboot-condition-annotation/>

　　Spring 工具类 ConfigurationClassParser 分析得到
<https://blog.csdn.net/andy_zhang2007/article/details/78549773>

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