根据条件加载 bean：即 @ConditionalOnXxx 的解析

（version: SpringBoot 2.2.2.RELEASE）

加载 @Configration 类，或者 @Configuration 类里面的 beanMethod 时，通常会用到条件加载，即：@CondtionalOnXxx

1. org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader#loadBeanDefinitionsForConfigurationClass()
    1.1 org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.TrackedConditionEvaluator#shouldSkip()

具体的实现代码如下：
org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.TrackedConditionEvaluator#shouldSkip()

public boolean shouldSkip(ConfigurationClass configClass) {
    Boolean skip = this.skipped.get(configClass);
    if (skip == null) {
        if (configClass.isImported()) {
            boolean allSkipped = true;
            for (ConfigurationClass importedBy : configClass.getImportedBy()) {
                if (!shouldSkip(importedBy)) {
                    allSkipped = false;
                    break;
                }
            }
            if (allSkipped) {
                // The config classes that imported this one were all skipped, therefore we are skipped...
                skip = true;
            }
        }
        if (skip == null) {
            // 通过表达式来判断是否要 skip：即 @ConditionalOnBean、@ConditionOnClass、@ConditionalOnMissingClass、@ConditionalOnExpression 等的处理
            skip = conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN);
        }
        this.skipped.put(configClass, skip);
    }
    return skip;
}