@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();
}

@FunctionalInterface
public interface Condition {
	//決定條件是否匹配
	boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

public interface ConditionContext {
	//返回BeanDefinitionRegistry注冊表，可以檢查Bean的定義
	BeanDefinitionRegistry getRegistry();
	//ConfigurableListableBeanFactory ，可以檢查Bean是否已經(jīng)存在，進(jìn)一步檢查Bean屬性
	@Nullable
	ConfigurableListableBeanFactory getBeanFactory();
	//獲取Envirment，獲取當(dāng)前環(huán)境變量，監(jiān)測當(dāng)前環(huán)境變量是否存在
	Environment getEnvironment();
	//ResourceLoader ,用于讀取或檢查所加載的資源
	ResourceLoader getResourceLoader();
	//返回ClassLoader ，用于檢查類是否存在
	@Nullable
	ClassLoader getClassLoader();
}

public interface AnnotatedTypeMetadata {
    MergedAnnotations getAnnotations();
    default boolean isAnnotated(String annotationName) {
        return this.getAnnotations().isPresent(annotationName);
    }
    @Nullable
    default Map<String, Object> getAnnotationAttributes(String annotationName) {
        return this.getAnnotationAttributes(annotationName, false);
    }
    @Nullable
    default Map<String, Object> getAnnotationAttributes(String annotationName, boolean classValuesAsString) {
        MergedAnnotation<Annotation> annotation = this.getAnnotations().get(annotationName, (Predicate)null, MergedAnnotationSelectors.firstDirectlyDeclared());
        return !annotation.isPresent() ? null : annotation.asAnnotationAttributes(Adapt.values(classValuesAsString, true));
    }
    @Nullable
    default MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName) {
        return this.getAllAnnotationAttributes(annotationName, false);
    }
    @Nullable
    default MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName, boolean classValuesAsString) {
        MergedAnnotation.Adapt[] adaptations = Adapt.values(classValuesAsString, true);
        return (MultiValueMap)this.getAnnotations().stream(annotationName).filter(MergedAnnotationPredicates.unique(MergedAnnotation::getMetaTypes)).map(MergedAnnotation::withNonMergedAttributes).collect(MergedAnnotationCollectors.toMultiValueMap((map) -> {
            return map.isEmpty() ? null : map;
        }, adaptations));
    }
}

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnWebApplicationCondition.class)
public @interface ConditionalOnWebApplication {
	//所需的web類型
	Type type() default Type.ANY;
	//可選應(yīng)用枚舉類
	enum Type {
		//任何類型
		ANY,
		//基于servlet的web應(yīng)用
		SERVLET,
		//基于reactive的web應(yīng)用
		REACTIVE
	}
}

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 {
			ConditionOutcome outcome = getMatchOutcome(context, metadata);
			logOutcome(classOrMethodName, outcome);
			recordEvaluation(context, classOrMethodName, outcome);
			return outcome.isMatch();
		}
		catch (NoClassDefFoundError ex) {
			throw new IllegalStateException("Could not evaluate condition on " + classOrMethodName + " due to "
					+ ex.getMessage() + " not found. Make sure your own configuration does not rely on "
					+ "that class. This can also happen if you are "
					+ "@ComponentScanning a springframework package (e.g. if you "
					+ "put a @ComponentScan in the default package by mistake)", ex);
		}
		catch (RuntimeException ex) {
			throw new IllegalStateException("Error processing condition on " + getName(metadata), ex);
		}
	}
	......
	/**
	 * Determine the outcome of the match along with suitable log output.
	 * @param context the condition context
	 * @param metadata the annotation metadata
	 * @return the condition outcome
	 */
	public abstract ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata);
......
}

@Order(Ordered.HIGHEST_PRECEDENCE + 20)
class OnWebApplicationCondition extends FilteringSpringBootCondition {
	......
	@Override
	public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
		boolean required = metadata.isAnnotated(ConditionalOnWebApplication.class.getName());
		ConditionOutcome outcome = isWebApplication(context, metadata, required);
		if (required && !outcome.isMatch()) {
			return ConditionOutcome.noMatch(outcome.getConditionMessage());
		}
		if (!required && outcome.isMatch()) {
			return ConditionOutcome.noMatch(outcome.getConditionMessage());
		}
		return ConditionOutcome.match(outcome.getConditionMessage());
	}
......
}

@Order(Ordered.HIGHEST_PRECEDENCE + 20)
class OnWebApplicationCondition extends FilteringSpringBootCondition {
	private static final String SERVLET_WEB_APPLICATION_CLASS = "org.springframework.web.context.support.GenericWebApplicationContext";
	......
	//推斷web應(yīng)用是否匹配
	private ConditionOutcome isWebApplication(ConditionContext context, AnnotatedTypeMetadata metadata,
			boolean required) {
		switch (deduceType(metadata)) {
		case SERVLET:
			return isServletWebApplication(context);
		case REACTIVE:
			return isReactiveWebApplication(context);
		default:
			return isAnyWebApplication(context, required);
		}
	}
	......
	private ConditionOutcome isServletWebApplication(ConditionContext context) {
		ConditionMessage.Builder message = ConditionMessage.forCondition("");
		//判斷常量定義是否存在
		if (!ClassNameFilter.isPresent(SERVLET_WEB_APPLICATION_CLASS, context.getClassLoader())) {
			return ConditionOutcome.noMatch(message.didNotFind("servlet web application classes").atAll());
		}
		//判斷BeanFactory是否存在
		if (context.getBeanFactory() != null) {
			String[] scopes = context.getBeanFactory().getRegisteredScopeNames();
			if (ObjectUtils.containsElement(scopes, "session")) {
				return ConditionOutcome.match(message.foundExactly("'session' scope"));
			}
		}
		//判斷Enviroment的類型是否為ConfigurableWebEnvironment類型
		if (context.getEnvironment() instanceof ConfigurableWebEnvironment) {
			return ConditionOutcome.match(message.foundExactly("ConfigurableWebEnvironment"));
		}
		//判斷ResourceLoader的類型是否為WebApplicationContext
		if (context.getResourceLoader() instanceof WebApplicationContext) {
			return ConditionOutcome.match(message.foundExactly("WebApplicationContext"));
		}
		return ConditionOutcome.noMatch(message.because("not a servlet web application"));
	}
	......
	//從AnnotateTypeMeatdata中獲取type值
	private Type deduceType(AnnotatedTypeMetadata metadata) {
		Map<String, Object> attributes = metadata.getAnnotationAttributes(ConditionalOnWebApplication.class.getName());
		if (attributes != null) {
			return (Type) attributes.get("type");
		}
		return Type.ANY;
	}
}