基于springboot2.3.12.RELEASR版本分析
所有的ConditionalOnXXX注解上方都有一个Conditional注解，这个注解的属性是Condition类的实现类。

public void parse(Set<BeanDefinitionHolder> configCandidates) {
   for (BeanDefinitionHolder holder : configCandidates) {
      BeanDefinition bd = holder.getBeanDefinition();
      try {
         // 主启动类的beanDefinition是AnnotatedBeanDefinition的间接子类
         // 走这个分支
         if (bd instanceof AnnotatedBeanDefinition) {
            parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
         }
         else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
            parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
         }
         else {
            parse(bd.getBeanClassName(), holder.getBeanName());
         }
      }
      catch (BeanDefinitionStoreException ex) {
         throw ex;
      }
      catch (Throwable ex) {
         throw new BeanDefinitionStoreException(
               "Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
      }
   }

   this.deferredImportSelectorHandler.process();
}


protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException {
   processConfigurationClass(new ConfigurationClass(metadata, beanName), DEFAULT_EXCLUSION_FILTER);
}


protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException {
        // 这个就是根据配置类上面的Conditional注解判断是否解析这个配置类
		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.
        // 如果一个类的父类也是配置类，那么继续解析。
		SourceClass sourceClass = asSourceClass(configClass, filter);
		do {
			sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
		}
		while (sourceClass != null);

		this.configurationClasses.put(configClass, configClass);
	}

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
   // 如果配置类不含有Conditional注解，那么就需要被解析，直接return false
   if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
      return false;
   }
   // 刚开始传入的phase为PARSE_CONFIGURATION，此if不成立
   if (phase == null) {
      if (metadata instanceof AnnotationMetadata &&
            ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
         return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
      }
      return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
   }

   List<Condition> conditions = new ArrayList<>();
   // 遍历Conditional注解的属性(Condition接口的实现类)
   for (String[] conditionClasses : getConditionClasses(metadata)) {
      for (String conditionClass : conditionClasses) {
         // 实例化condition接口的实现类
         Condition condition = getCondition(conditionClass, this.context.getClassLoader());
         conditions.add(condition);
      }
   }
	
   AnnotationAwareOrderComparator.sort(conditions);

   for (Condition condition : conditions) {
      ConfigurationPhase requiredPhase = null;
      if (condition instanceof ConfigurationCondition) {
         requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
      }
      // 开始调用前面所有condition实现类的matches方法来决定是否跳过
      // Condition接口的一级实现类是SpringBootCondition，所有其他的Conditon实现类
      // 都要间接实现SpringBootCondition
      if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
         return true;
      }
   }

   return false;
}

@Order(Ordered.HIGHEST_PRECEDENCE + 20)
class OnJavaCondition extends SpringBootCondition {

   private static final JavaVersion JVM_VERSION = JavaVersion.getJavaVersion();

   @Override
   public ConditionOutcome getMatchOutcome(ConditionContext context,
         AnnotatedTypeMetadata metadata) {
      // 获取ConditionalOnJava注解的属性
      Map<String, Object> attributes = metadata
            .getAnnotationAttributes(ConditionalOnJava.class.getName());
      // range默认为EQUAL_OR_NEWER
      Range range = (Range) attributes.get("range");
      // value如果我们写的是JavaVersion.NINE,但是我们运行的java版本为1.8
      // 那么就不匹配，标注了@ConditionalOnJava的配置类就不会被解析
      JavaVersion version = (JavaVersion) attributes.get("value");
      return getMatchOutcome(range, JVM_VERSION, version);
   }

   protected ConditionOutcome getMatchOutcome(Range range, JavaVersion runningVersion,
         JavaVersion version) {
      boolean match = isWithin(runningVersion, range, version);
      String expected = String.format(
            (range != Range.EQUAL_OR_NEWER) ? "(older than %s)" : "(%s or newer)",
            version);
      ConditionMessage message = ConditionMessage
            .forCondition(ConditionalOnJava.class, expected)
            .foundExactly(runningVersion);
      return new ConditionOutcome(match, message);
   }

   /**
    * Determines if the {@code runningVersion} is within the specified range of versions.
    * @param runningVersion the current version.
    * @param range the range
    * @param version the bounds of the range
    * @return if this version is within the specified range
    */
   private boolean isWithin(JavaVersion runningVersion, Range range,
         JavaVersion version) {
      // range默认为EQUAL_OR_NEWER
      if (range == Range.EQUAL_OR_NEWER) {
         return runningVersion.isEqualOrNewerThan(version);
      }
      if (range == Range.OLDER_THAN) {
         return runningVersion.isOlderThan(version);
      }
      throw new IllegalStateException("Unknown range " + range);
   }

}

public abstract class SpringBootCondition implements Condition {

   private final Log logger = LogFactory.getLog(getClass());

   @Override
   public final boolean matches(ConditionContext context,
         AnnotatedTypeMetadata metadata) {
      String classOrMethodName = getClassOrMethodName(metadata);
      try {
         //这个方法的返回对象的isMatch布尔属性来解决是否需要解析当前配置类
         ConditionOutcome outcome = getMatchOutcome(context, metadata);
         logOutcome(classOrMethodName, outcome);
         recordEvaluation(context, classOrMethodName, outcome);
         return outcome.isMatch();
      }
       
      // ...
   }
   // 模板方法，所有SpringBootCondition继承类需要实现这个方法
   public abstract ConditionOutcome getMatchOutcome(ConditionContext context,
			AnnotatedTypeMetadata metadata);