spring-boot-2.0.3源码篇 - @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！

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）。

参考

　　SpringBoot源码分析之条件注解的底层实现

　　Spring 工具类 ConfigurationClassParser 分析得得到配置类