@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 "";
}

@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);
}

@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();
}

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.

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);
}

/**
 * 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;
}

/**
 * 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();
    }
}