@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
 
	/**
	 * All {@link Condition Conditions} that must {@linkplain Condition#matches match}
	 * in order for the component to be registered.
	 */
	Class<? extends Condition>[] value();
 
}

/**
 * @since 4.0
 * @see ConfigurationCondition
 * @see Conditional
 * @see ConditionContext
 */
@FunctionalInterface
public interface Condition {
 
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

public interface ConfigurationCondition extends Condition {
 
    // 定義了子類必須實現，返回下面枚舉的一種
    ConfigurationPhase getConfigurationPhase();
 
    // 判斷階段
	enum ConfigurationPhase {
            
        // Conponent階段，即將@Component加入到BeanFactory
        PARSE_CONFIGURATION,
 
        // 只有通過getBean才能正在將Bean注冊到Ioc容器中。前提是要將BeanDefinition添加到 
        // BeanFactory中
        REGISTER_BEAN
    }
}

protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
    if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(),  
           ConfigurationPhase.PARSE_CONFIGURATION)) {
 
        return;
    }
 
    // 省略其余代碼
}

this.conditionEvaluator = new ConditionEvaluator(registry, environment, resourceLoader);

class ConditionEvaluator {
 
    private final ConditionContextImpl context;
 
    public ConditionEvaluator(@Nullable BeanDefinitionRegistry registry,
			@Nullable Environment environment, @Nullable ResourceLoader resourceLoader) {
 
		this.context = new ConditionContextImpl(registry, environment, resourceLoader);
	}
    
    // 省略其他方法
 
    private static class ConditionContextImpl implements ConditionContext {
 
        private final BeanDefinitionRegistry registry;
 
        private final ConfigurableListableBeanFactory beanFactory;
 
        private final Environment environment;
 
        private final ResourceLoader resourceLoader;
 
        private final ClassLoader classLoader;
 
        public ConditionContextImpl(@Nullable BeanDefinitionRegistry registry,
                                    @Nullable Environment environment, @Nullable ResourceLoader resourceLoader) {
 
            this.registry = registry;
            this.beanFactory = deduceBeanFactory(registry);
            this.environment = (environment != null ? environment : deduceEnvironment(registry));
            this.resourceLoader = (resourceLoader != null ? resourceLoader : deduceResourceLoader(registry));
            this.classLoader = deduceClassLoader(resourceLoader, this.beanFactory);
        }
    }
}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedTomcat {
    // 省略其他代碼
}

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
    // 注解信息不能為空， 并且必須有Conditional或者其子類
    if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
        return false;
    }
    // 如果判斷階段為空，進行類型判斷再遞歸調用該方法
    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<>();
    // 獲取多有的Condition類型，如上面的EmbeddedTomcat同時需要多個條件成立
    for (String[] conditionClasses : getConditionClasses(metadata)) {
        for (String conditionClass : conditionClasses) {
            Condition condition = getCondition(conditionClass, this.context.getClassLoader());
            conditions.add(condition);
        }
    }
    // 條件排序（根據Spring的那三個排序方式定義）
    AnnotationAwareOrderComparator.sort(conditions);
 
    for (Condition condition : conditions) {
        ConfigurationPhase requiredPhase = null;
        if (condition instanceof ConfigurationCondition) {
            requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
        }
        // 判斷階段為空（非ConfigurationCondition的子類）、不需要判斷階段，則直接返回true
        // 否則才調用matches接口判斷
        if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
            return true;
        }
    }
    return false;
}