助力SpringBoot自动配置的条件注解ConditionalOnXXX分析--SpringBoot源码（三）

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {

	Class<?>[] value() default {};

	String[] name() default {};

}

/**
 * 实现spring 的Condition接口，并且重写matches()方法，如果@ConditionalOnLinux的注解属性environment是linux就返回true
 *
 */
public class LinuxCondition implements Condition {

	@Override
	public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
		// 获得注解@ConditionalOnLinux的所有属性
		List<AnnotationAttributes> allAnnotationAttributes = annotationAttributesFromMultiValueMap(
				metadata.getAllAnnotationAttributes(
						ConditionalOnLinux.class.getName()));
		for (AnnotationAttributes annotationAttributes : allAnnotationAttributes) {
			// 获得注解@ConditionalOnLinux的environment属性
			String environment = annotationAttributes.getString("environment");
			// 若environment等于linux，则返回true
			if ("linux".equals(environment)) {
				return true;
			}
		}
		return false;
	}
}

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(LinuxCondition.class)
public @interface ConditionalOnLinux {
	// 标注是哪个环境
	String environment() default "";

}

@Configuration
public class ConditionConfig {
        // 只有`@ConditionalOnLinux`的注解属性`environment`是"linux"时才会创建bean
	@Bean
	@ConditionalOnLinux(environment = "linux")
	public Environment linuxEnvironment() {
		return new LinuxEnvironment();
	}
}

@FunctionalInterface
public interface Condition {
	boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

public interface ConditionContext {
    BeanDefinitionRegistry getRegistry();
    @Nullable
    ConfigurableListableBeanFactory getBeanFactory();
    Environment getEnvironment();
    ResourceLoader getResourceLoader();
    @Nullable
    ClassLoader getClassLoader();
}

public interface AnnotatedTypeMetadata {
	boolean isAnnotated(String annotationName);
	Map<String, Object> getAnnotationAttributes(String annotationName);
	Map<String, Object> getAnnotationAttributes(String annotationName, boolean classValuesAsString);
	MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName);
	MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName, boolean classValuesAsString);
}

// 这个方法主要是如果是解析阶段则跳过，如果是注册阶段则不跳过
public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
	// 若没有被@Conditional或其派生注解所标注，则不会跳过
	if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
		return false;
	}
	// 没有指定phase，注意phase可以分为PARSE_CONFIGURATION或REGISTER_BEAN类型
	if (phase == null) {
		// 若标有@Component，@Import，@Bean或@Configuration等注解的话，则说明是PARSE_CONFIGURATION类型
		if (metadata instanceof AnnotationMetadata &&
				ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
			return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
		}
		// 否则是REGISTER_BEAN类型
		return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
	}

	List<Condition> conditions = new ArrayList<>();
	// TODO 获得所有标有@Conditional注解或其派生注解里面的Condition接口实现类并实例化成对象。
	// 比如@Conditional(OnBeanCondition.class)则获得OnBeanCondition.class，OnBeanCondition.class往往实现了Condition接口
	for (String[] conditionClasses : getConditionClasses(metadata)) {
		// 将类实例化成对象
		for (String conditionClass : conditionClasses) {
			Condition condition = getCondition(conditionClass, this.context.getClassLoader());
			conditions.add(condition);
		}
	}
	// 排序，即按照Condition的优先级进行排序
	AnnotationAwareOrderComparator.sort(conditions);

	for (Condition condition : conditions) {
		ConfigurationPhase requiredPhase = null;
		if (condition instanceof ConfigurationCondition) {
			// 从condition中获得对bean是解析还是注册
			requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
		}
		// 若requiredPhase为null或获取的阶段类型正是当前阶段类型且不符合condition的matches条件，则跳过
		if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
			return true;
		}
	}

	return false;
}

if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
    return true;
}

class ProfileCondition implements Condition {
	@Override
	public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
		MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName());
		if (attrs != null) {
			for (Object value : attrs.get("value")) {
				if (context.getEnvironment().acceptsProfiles(Profiles.of((String[]) value))) {
					return true;
				}
			}
			return false;
		}
		return true;
	}
}

// SpringBootCondition.java

public final boolean matches(ConditionContext context,
			AnnotatedTypeMetadata metadata) {
		// 得到metadata的类名或方法名
		String classOrMethodName = getClassOrMethodName(metadata);
		try {
			// 判断每个配置类的每个条件注解@ConditionalOnXXX是否满足条件，然后记录到ConditionOutcome结果中
			// 注意getMatchOutcome是一个抽象模板方法，交给OnXXXCondition子类去实现
			ConditionOutcome outcome = getMatchOutcome(context, metadata);
			// 打印condition评估的日志，哪些条件注解@ConditionalOnXXX是满足条件的，哪些是不满足条件的，这些日志都打印出来
			logOutcome(classOrMethodName, outcome);
			// 除了打印日志外，这些是否匹配的信息还要记录到ConditionEvaluationReport中
			recordEvaluation(context, classOrMethodName, outcome);
			// 最后返回@ConditionalOnXXX是否满足条件
			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);
		}
	}

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnResourceCondition.class)
public @interface ConditionalOnResource {

	/**
	 * The resources that must be present.
	 * @return the resource paths that must be present.
	 */
	String[] resources() default {};

}

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

	private final ResourceLoader defaultResourceLoader = new DefaultResourceLoader();

	@Override
	public ConditionOutcome getMatchOutcome(ConditionContext context,
			AnnotatedTypeMetadata metadata) {
		// 获得@ConditionalOnResource注解的属性元数据
		MultiValueMap<String, Object> attributes = metadata
				.getAllAnnotationAttributes(ConditionalOnResource.class.getName(), true);
		// 获得资源加载器，若ConditionContext中有ResourceLoader则用ConditionContext中的，没有则用默认的
		ResourceLoader loader = (context.getResourceLoader() != null)
				? context.getResourceLoader() : this.defaultResourceLoader;
		List<String> locations = new ArrayList<>();
		// 将@ConditionalOnResource中定义的resources属性值取出来装进locations集合
		collectValues(locations, attributes.get("resources"));
		Assert.isTrue(!locations.isEmpty(),
				"@ConditionalOnResource annotations must specify at "
						+ "least one resource location");
		// missing集合是装不存在指定资源的资源路径的
		List<String> missing = new ArrayList<>();
		// 遍历所有的资源路径，若指定的路径的资源不存在则将其资源路径存进missing集合中
		for (String location : locations) {
			// 这里针对有些资源路径是Placeholders的情况，即处理${}
			String resource = context.getEnvironment().resolvePlaceholders(location);
			if (!loader.getResource(resource).exists()) {
				missing.add(location);
			}
		}
		// 如果存在某个资源不存在，那么则报错
		if (!missing.isEmpty()) {
			return ConditionOutcome.noMatch(ConditionMessage
					.forCondition(ConditionalOnResource.class)
					.didNotFind("resource", "resources").items(Style.QUOTE, missing));
		}
		// 所有资源都存在，那么则返回能找到就提的资源
		return ConditionOutcome
				.match(ConditionMessage.forCondition(ConditionalOnResource.class)
						.found("location", "locations").items(locations));
	}
	
	// 将@ConditionalOnResource中定义的resources属性值取出来装进locations集合
	private void collectValues(List<String> names, List<Object> values) {
		for (Object value : values) {
			for (Object item : (Object[]) value) {
				names.add((String) item);
			}
		}
	}
}

@Override
	public ConditionOutcome getMatchOutcome(ConditionContext context,
			AnnotatedTypeMetadata metadata) {
		ConditionMessage matchMessage = ConditionMessage.empty();
		// (1)，配置类（metadata）标注@ConditionalOnBean注解的情况
		if (metadata.isAnnotated(ConditionalOnBean.class.getName())) {
			// 将@ConditionalOnBean注解属性封装进BeanSearchSpec对象中
			// 注意BeanSearchSpec是一个静态内部类，用来存储@ConditionalOnBean和@ConditionalOnMissingBean注解的属性值
			BeanSearchSpec spec = new BeanSearchSpec(context, metadata,
					ConditionalOnBean.class);
			// 调用getMatchingBeans得到符合条件的bean
			MatchResult matchResult = getMatchingBeans(context, spec);
			// 如果不匹配
			if (!matchResult.isAllMatched()) {
				String reason = createOnBeanNoMatchReason(matchResult);
				return ConditionOutcome.noMatch(ConditionMessage
						.forCondition(ConditionalOnBean.class, spec).because(reason));
			}
			// 如果匹配
			matchMessage = matchMessage.andCondition(ConditionalOnBean.class, spec)
					.found("bean", "beans")
					.items(Style.QUOTE, matchResult.getNamesOfAllMatches());
		}
		// (2)，配置类（metadata）标注@ConditionalOnSingleCandidate注解的情况
		if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
			BeanSearchSpec spec = new SingleCandidateBeanSearchSpec(context, metadata,
					ConditionalOnSingleCandidate.class);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (!matchResult.isAllMatched()) {
				return ConditionOutcome.noMatch(ConditionMessage
						.forCondition(ConditionalOnSingleCandidate.class, spec)
						.didNotFind("any beans").atAll());
			}
			else if (!hasSingleAutowireCandidate(context.getBeanFactory(),
					matchResult.getNamesOfAllMatches(),
					spec.getStrategy() == SearchStrategy.ALL)) {
				return ConditionOutcome.noMatch(ConditionMessage
						.forCondition(ConditionalOnSingleCandidate.class, spec)
						.didNotFind("a primary bean from beans")
						.items(Style.QUOTE, matchResult.getNamesOfAllMatches()));
			}
			matchMessage = matchMessage
					.andCondition(ConditionalOnSingleCandidate.class, spec)
					.found("a primary bean from beans")
					.items(Style.QUOTE, matchResult.getNamesOfAllMatches());
		}
		// (3)，配置类（metadata）标注@ConditionalOnMissingBean注解的情况
		if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
			BeanSearchSpec spec = new BeanSearchSpec(context, metadata,
					ConditionalOnMissingBean.class);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (matchResult.isAnyMatched()) {
				String reason = createOnMissingBeanNoMatchReason(matchResult);
				return ConditionOutcome.noMatch(ConditionMessage
						.forCondition(ConditionalOnMissingBean.class, spec)
						.because(reason));
			}
			matchMessage = matchMessage.andCondition(ConditionalOnMissingBean.class, spec)
					.didNotFind("any beans").atAll();
		}
		// 最终返回matchMessage
		return ConditionOutcome.match(matchMessage);
	}

protected final MatchResult getMatchingBeans(ConditionContext context,
			BeanSearchSpec beans) {
		// 获得Spring容器的beanFactory
		ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
		// 判断bean的搜索策略是否是SearchStrategy.ANCESTORS策略
		if (beans.getStrategy() == SearchStrategy.ANCESTORS) {
			BeanFactory parent = beanFactory.getParentBeanFactory();
			Assert.isInstanceOf(ConfigurableListableBeanFactory.class, parent,
					"Unable to use SearchStrategy.PARENTS");
			beanFactory = (ConfigurableListableBeanFactory) parent;
		}
		// MatchResult用来存储bean的匹配结果
		MatchResult matchResult = new MatchResult();
		// 如果bean的搜索策略不是SearchStrategy.CURRENT的话，则置considerHierarchy为true
		boolean considerHierarchy = beans.getStrategy() != SearchStrategy.CURRENT;
		// 获取TypeExtractor，TypeExtractor是用来判断bean的类型的
		TypeExtractor typeExtractor = beans.getTypeExtractor(context.getClassLoader());
		// 获取是否有被忽略bean类型，若有的话将该bean类型的名称装进beansIgnoredByType集合
		// 这里主要是针对@ConditionalOnMissingBean的ignored属性
		List<String> beansIgnoredByType = getNamesOfBeansIgnoredByType(
				beans.getIgnoredTypes(), typeExtractor, beanFactory, context,
				considerHierarchy);
		// 遍历bean的所有类型
		for (String type : beans.getTypes()) {
			// 调用getBeanNamesForType方法根据bean类型得到所有符合条件的bean类型，并放到typeMatches集合
			Collection<String> typeMatches = getBeanNamesForType(beanFactory, type,
					typeExtractor, context.getClassLoader(), considerHierarchy);
			// 移除掉Ignored的类型
			typeMatches.removeAll(beansIgnoredByType);
			// 若typeMatches为空，那么则说明正在遍历的这个type类型不符合匹配条件，此时用matchResult记录一下这个不符合条件的类型
			if (typeMatches.isEmpty()) {
				matchResult.recordUnmatchedType(type);
			}
			// 若typeMatches不为空，那么则说明正在遍历的这个type类型符合匹配条件，此时用matchResult记录一下这个符合条件的类型
			else {
				matchResult.recordMatchedType(type, typeMatches);
			}
		}
		// 这里针对@ConditionalOnBean等注解的annotation属性的处理
		for (String annotation : beans.getAnnotations()) {
			List<String> annotationMatches = Arrays
					.asList(getBeanNamesForAnnotation(beanFactory, annotation,
							context.getClassLoader(), considerHierarchy));
			annotationMatches.removeAll(beansIgnoredByType);
			if (annotationMatches.isEmpty()) {
				matchResult.recordUnmatchedAnnotation(annotation);
			}
			else {
				matchResult.recordMatchedAnnotation(annotation, annotationMatches);
			}
		}
		// 这里针对@ConditionalOnBean等注解的name属性的处理
		for (String beanName : beans.getNames()) {
			// beansIgnoredByType集合不包含beanName且beanFactory包含这个bean，则匹配
			if (!beansIgnoredByType.contains(beanName)
					&& containsBean(beanFactory, beanName, considerHierarchy)) {
				matchResult.recordMatchedName(beanName);
			}
			// 否则，不匹配
			else {
				matchResult.recordUnmatchedName(beanName);
			}
		}
		// 最后返回匹配结果
		return matchResult;
	}

public ConditionOutcome getMatchOutcome(ConditionContext context,
			AnnotatedTypeMetadata metadata) {
		// 配置类是否标注有@ConditionalOnWebApplication注解
		boolean required = metadata
				.isAnnotated(ConditionalOnWebApplication.class.getName());
		// 调用isWebApplication方法返回匹配结果
		ConditionOutcome outcome = isWebApplication(context, metadata, required);
		// 若有标注@ConditionalOnWebApplication但不符合条件，则返回不匹配
		if (required && !outcome.isMatch()) {
			return ConditionOutcome.noMatch(outcome.getConditionMessage());
		}
		// 若没有标注@ConditionalOnWebApplication但符合条件，则返回不匹配
		if (!required && outcome.isMatch()) {
			return ConditionOutcome.noMatch(outcome.getConditionMessage());
		}
		// 这里返回匹配的情况，TODO 不过有个疑问：如果没有标注@ConditionalOnWebApplication注解，又不符合条件的话，也会执行到这里，返回匹配？
		return ConditionOutcome.match(outcome.getConditionMessage());
	}

private ConditionOutcome isWebApplication(ConditionContext context,
			AnnotatedTypeMetadata metadata, boolean required) {
		// 调用deduceType方法判断是哪种类型，其中有SERVLET，REACTIVE和ANY类型，其中ANY表示了SERVLET或REACTIVE类型
		switch (deduceType(metadata)) {
		// SERVLET类型
		case SERVLET:
			return isServletWebApplication(context);
		// REACTIVE类型
		case REACTIVE:
			return isReactiveWebApplication(context);
		default:
			return isAnyWebApplication(context, required);
		}
	}

private ConditionOutcome isServletWebApplication(ConditionContext context) {
		ConditionMessage.Builder message = ConditionMessage.forCondition("");
		// 若classpath中不存在org.springframework.web.context.support.GenericWebApplicationContext.class，则返回不匹配
		if (!ClassNameFilter.isPresent(SERVLET_WEB_APPLICATION_CLASS,
				context.getClassLoader())) {
			return ConditionOutcome.noMatch(
					message.didNotFind("servlet web application classes").atAll());
		}
		// 若classpath中存在org.springframework.web.context.support.GenericWebApplicationContext.class，那么又分为以下几种匹配的情况
		// session
		if (context.getBeanFactory() != null) {
			String[] scopes = context.getBeanFactory().getRegisteredScopeNames();
			if (ObjectUtils.containsElement(scopes, "session")) {
				return ConditionOutcome.match(message.foundExactly("'session' scope"));
			}
		}
		// ConfigurableWebEnvironment
		if (context.getEnvironment() instanceof ConfigurableWebEnvironment) {
			return ConditionOutcome
					.match(message.foundExactly("ConfigurableWebEnvironment"));
		}
		// WebApplicationContext
		if (context.getResourceLoader() instanceof WebApplicationContext) {
			return ConditionOutcome.match(message.foundExactly("WebApplicationContext"));
		}
		// 若以上三种都不匹配的话，则说明不是一个servlet web application
		return ConditionOutcome.noMatch(message.because("not a servlet web application"));
	}