面試官：你經(jīng)常在SpringBoot中使用的條件注解底層是如何實(shí)現(xiàn)的才写？你了解過嗎葡兑？

SpringBoot內(nèi)部提供了特有的注解：條件注解(Conditional Annotation)。比如@ConditionalOnBean赞草、@ConditionalOnClass讹堤、@ConditionalOnExpression、@ConditionalOnMissingBean等厨疙。

條件注解存在的意義在于動態(tài)識別(也可以說是代碼自動化執(zhí)行)洲守。比如@ConditionalOnClass會檢查類加載器中是否存在對應(yīng)的類，如果有的話被注解修飾的類就有資格被Spring容器所注冊沾凄，否則會被skip梗醇。

比如FreemarkerAutoConfiguration這個(gè)自動化配置類的定義如下：

@Configuration@ConditionalOnClass({ freemarker.template.Configuration.class, ???FreeMarkerConfigurationFactory.class })@AutoConfigureAfter(WebMvcAutoConfiguration.class)@EnableConfigurationProperties(FreeMarkerProperties.class)public class FreeMarkerAutoConfiguration ???

這個(gè)自動化配置類被@ConditionalOnClass條件注解修飾，這個(gè)條件注解存在的意義在于判斷類加載器中是否存在freemarker.template.Configuration和FreeMarkerConfigurationFactory這兩個(gè)類撒蟀，如果都存在的話會在Spring容器中加載這個(gè)FreeMarkerAutoConfiguration配置類叙谨；否則不會加載。

# 條件注解內(nèi)部的一些基礎(chǔ)

在分析條件注解的底層實(shí)現(xiàn)之前保屯，我們先來看一下這些條件注解的定義手负。以@ConditionalOnClass注解為例，它的定義如下：

@Target({ ElementType.TYPE, ElementType.METHOD })@Retention(RetentionPolicy.RUNTIME)@Documented@Conditional(OnClassCondition.class)public @interface ConditionalOnClass { ?Class<?>[] value() default {}; // 需要匹配的類 ?String[] name() default {}; // 需要匹配的類名} ???

它有2個(gè)屬性姑尺，分別是類數(shù)組和字符串?dāng)?shù)組(作用一樣竟终，類型不一樣)，而且被@Conditional注解所修飾股缸，這個(gè)@Conditional注解有個(gè)名為values的Class[]類型的屬性。 這個(gè)Condition是個(gè)接口吱雏，用于匹配組件是否有資格被容器注冊敦姻，定義如下：

public interface Condition { ?// ConditionContext內(nèi)部會存儲Spring容器、應(yīng)用程序環(huán)境信息歧杏、資源加載器镰惦、類加載器 ?boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);} ???

也就是說@Conditional注解屬性中可以持有多個(gè)Condition接口的實(shí)現(xiàn)類，所有的Condition接口需要全部匹配成功后這個(gè)@Conditional修飾的組件才有資格被注冊犬绒。

Condition接口有個(gè)子接口ConfigurationCondition：

public interface ConfigurationCondition extends Condition {??ConfigurationPhase getConfigurationPhase();??public static enum ConfigurationPhase {????PARSE_CONFIGURATION,????REGISTER_BEAN ?}} ???

這個(gè)子接口是一種特殊的條件接口旺入，多了一個(gè)getConfigurationPhase方法，也就是條件注解的生效階段凯力。只有在ConfigurationPhase中定義的兩種階段下才會生效茵瘾。

Condition接口有個(gè)實(shí)現(xiàn)抽象類SpringBootCondition，SpringBoot中所有條件注解對應(yīng)的條件類都繼承這個(gè)抽象類咐鹤。它實(shí)現(xiàn)了matches方法：

@Overridepublic final boolean matches(ConditionContext context, ???AnnotatedTypeMetadata metadata) { ?String classOrMethodName = getClassOrMethodName(metadata); // 得到類名或者方法名(條件注解可以作用的類或者方法上) ?try { ???ConditionOutcome outcome = getMatchOutcome(context, metadata); // 抽象方法拗秘，具體子類實(shí)現(xiàn)。ConditionOutcome記錄了匹配結(jié)果boolean和log信息 ???logOutcome(classOrMethodName, outcome); // log記錄一下匹配信息 ???recordEvaluation(context, classOrMethodName, outcome); // 報(bào)告記錄一下匹配信息 ???return outcome.isMatch(); // 返回是否匹配 ?} ?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); ?}} ???

# 基于Class的條件注解

SpringBoot提供了兩個(gè)基于Class的條件注解：@ConditionalOnClass(類加載器中存在指明的類)或者@ConditionalOnMissingClass(類加載器中不存在指明的類)祈惶。

@ConditionalOnClass或者@ConditionalOnMissingClass注解對應(yīng)的條件類是OnClassCondition雕旨，定義如下：

@Order(Ordered.HIGHEST_PRECEDENCE) // 優(yōu)先級扮匠、最高級別class OnClassCondition extends SpringBootCondition {??@Override ?public ConditionOutcome getMatchOutcome(ConditionContext context, ?????AnnotatedTypeMetadata metadata) {????StringBuffer matchMessage = new StringBuffer(); // 記錄匹配信息????MultiValueMap<String, Object> onClasses = getAttributes(metadata, ???????ConditionalOnClass.class); // 得到@ConditionalOnClass注解的屬性 ???if (onClasses != null) { // 如果屬性存在 ?????List<String> missing = getMatchingClasses(onClasses, MatchType.MISSING, ?????????context); // 得到在類加載器中不存在的類 ?????if (!missing.isEmpty()) { // 如果存在類加載器中不存在對應(yīng)的類，返回一個(gè)匹配失敗的ConditionalOutcome ???????return ConditionOutcome ???????????.noMatch("required @ConditionalOnClass classes not found: " ???????????????+ StringUtils.collectionToCommaDelimitedString(missing)); ?????} ???????????????// 如果類加載器中存在對應(yīng)的類的話凡涩，匹配信息進(jìn)行記錄 ?????matchMessage.append("@ConditionalOnClass classes found: " ?????????+ StringUtils.collectionToCommaDelimitedString( ?????????????getMatchingClasses(onClasses, MatchType.PRESENT, context))); ???} ???????// 對@ConditionalOnMissingClass注解做相同的邏輯處理(說明@ConditionalOnClass和@ConditionalOnMissingClass可以一起使用) ???MultiValueMap<String, Object> onMissingClasses = getAttributes(metadata, ???????ConditionalOnMissingClass.class); ???if (onMissingClasses != null) { ?????List<String> present = getMatchingClasses(onMissingClasses, MatchType.PRESENT, ?????????context); ?????if (!present.isEmpty()) { ???????return ConditionOutcome ???????????.noMatch("required @ConditionalOnMissing classes found: " ???????????????+ StringUtils.collectionToCommaDelimitedString(present)); ?????} ?????matchMessage.append(matchMessage.length() == 0 ? "" : " "); ?????matchMessage.append("@ConditionalOnMissing classes not found: " ?????????+ StringUtils.collectionToCommaDelimitedString(getMatchingClasses( ?????????????onMissingClasses, MatchType.MISSING, context))); ???} ???????// 返回全部匹配成功的ConditionalOutcome ???return ConditionOutcome.match(matchMessage.toString()); ?}??private enum MatchType { // 枚舉：匹配類型棒搜。用于查詢類名在對應(yīng)的類加載器中是否存在。????PRESENT { // 匹配成功 ?????@Override ?????public boolean matches(String className, ConditionContext context) { ???????return ClassUtils.isPresent(className, context.getClassLoader()); ?????} ???},????MISSING { // 匹配不成功 ?????@Override ?????public boolean matches(String className, ConditionContext context) { ???????return !ClassUtils.isPresent(className, context.getClassLoader()); ?????} ???};????public abstract boolean matches(String className, ConditionContext context);??}} ???

比如FreemarkerAutoConfiguration中的@ConditionalOnClass注解中有value屬性是freemarker.template.Configuration.class和FreeMarkerConfigurationFactory.class活箕。在OnClassCondition執(zhí)行過程中得到的最終ConditionalOutcome中的log message如下：

[if !supportLists]·?[endif]

1?@ConditionalOnClass?classes?found:?freemarker.template.Configuration,org.springframework.ui.freemarker.FreeMarkerConfigurationFactory

# 基于Bean的條件注解

@ConditionalOnBean(Spring容器中存在指明的bean)力麸、@ConditionalOnMissingBean(Spring容器中不存在指明的bean)以及ConditionalOnSingleCandidate(Spring容器中存在且只存在一個(gè)指明的bean)都是基于Bean的條件注解，它們對應(yīng)的條件類是ConditionOnBean讹蘑。

@ConditionOnBean注解定義如下：

@Target({ ElementType.TYPE, ElementType.METHOD })@Retention(RetentionPolicy.RUNTIME)@Documented@Conditional(OnBeanCondition.class)public @interface ConditionalOnBean { ?Class<?>[] value() default {}; // 匹配的bean類型 ?String[] type() default {}; // 匹配的bean類型的類名 ?Class<? extends Annotation>[] annotation() default {}; // 匹配的bean注解 ?String[] name() default {}; // 匹配的bean的名字 ?SearchStrategy search() default SearchStrategy.ALL; // 搜索策略末盔。提供CURRENT(只在當(dāng)前容器中找)、PARENTS(只在所有的父容器中找座慰；但是不包括當(dāng)前容器)和ALL(CURRENT和PARENTS的組合)} ???

OnBeanCondition條件類的匹配代碼如下：

@Overridepublic ConditionOutcome getMatchOutcome(ConditionContext context, ???AnnotatedTypeMetadata metadata) { ?StringBuffer matchMessage = new StringBuffer(); // 記錄匹配信息 ?if (metadata.isAnnotated(ConditionalOnBean.class.getName())) { ???BeanSearchSpec spec = new BeanSearchSpec(context, metadata, ???????ConditionalOnBean.class); // 構(gòu)造一個(gè)BeanSearchSpec陨舱，會從@ConditionalOnBean注解中獲取屬性，然后設(shè)置到BeanSearchSpec中 ???List<String> matching = getMatchingBeans(context, spec); // 從BeanFactory中根據(jù)策略找出所有匹配的bean ???if (matching.isEmpty()) { // 如果沒有匹配的bean版仔，返回一個(gè)沒有匹配成功的ConditionalOutcome ?????return ConditionOutcome ?????????.noMatch("@ConditionalOnBean " + spec + " found no beans"); ???} ???// 如果找到匹配的bean游盲，匹配信息進(jìn)行記錄 ???matchMessage.append( ???????"@ConditionalOnBean " + spec + " found the following " + matching); ?} ?if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) { // 相同的邏輯，針對@ConditionalOnSingleCandidate注解 ???BeanSearchSpec spec = new SingleCandidateBeanSearchSpec(context, metadata, ???????ConditionalOnSingleCandidate.class); ???List<String> matching = getMatchingBeans(context, spec); ???if (matching.isEmpty()) { ?????return ConditionOutcome.noMatch( ?????????"@ConditionalOnSingleCandidate " + spec + " found no beans"); ???} ???else if (!hasSingleAutowireCandidate(context.getBeanFactory(), matching)) { // 多了一層判斷蛮粮，判斷是否只有一個(gè)bean ?????return ConditionOutcome.noMatch("@ConditionalOnSingleCandidate " + spec ?????????+ " found no primary candidate amongst the" + " following " ?????????+ matching); ???} ???matchMessage.append("@ConditionalOnSingleCandidate " + spec + " found " ???????+ "a primary candidate amongst the following " + matching); ?} ?if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) { // 相同的邏輯益缎，針對@ConditionalOnMissingBean注解 ???BeanSearchSpec spec = new BeanSearchSpec(context, metadata, ???????ConditionalOnMissingBean.class); ???List<String> matching = getMatchingBeans(context, spec); ???if (!matching.isEmpty()) { ?????return ConditionOutcome.noMatch("@ConditionalOnMissingBean " + spec ?????????+ " found the following " + matching); ???} ???matchMessage.append(matchMessage.length() == 0 ? "" : " "); ???matchMessage.append("@ConditionalOnMissingBean " + spec + " found no beans"); ?} ?return ConditionOutcome.match(matchMessage.toString()); //返回匹配成功的ConditonalOutcome} ???

SpringBoot還提供了其他比如ConditionalOnJava、ConditionalOnNotWebApplication然想、ConditionalOnWebApplication莺奔、ConditionalOnResource、ConditionalOnProperty变泄、ConditionalOnExpression等條件注解令哟，有興趣的讀者可以自行查看它們的底層處理邏輯。

# SpringBoot條件注解的激活機(jī)制

分析完了條件注解的執(zhí)行邏輯之后妨蛹，接下來的問題就是SpringBoot是如何讓這些條件注解生效的屏富？

SpringBoot使用ConditionEvaluator這個(gè)內(nèi)部類完成條件注解的解析和判斷。

在Spring容器的refresh過程中蛙卤，只有跟解析或者注冊bean有關(guān)系的類都會使用ConditionEvaluator完成條件注解的判斷狠半，這個(gè)過程中一些類不滿足條件的話就會被skip。這些類比如有AnnotatedBeanDefinitionReader颤难、ConfigurationClassBeanDefinitionReader神年、ConfigurationClassParse、ClassPathScanningCandidateComponentProvider等行嗤。

比如ConfigurationClassParser的構(gòu)造函數(shù)會初始化內(nèi)部屬性conditionEvaluator：

public ConfigurationClassParser(MetadataReaderFactory metadataReaderFactory, ???ProblemReporter problemReporter, Environment environment, ResourceLoader resourceLoader, ???BeanNameGenerator componentScanBeanNameGenerator, BeanDefinitionRegistry registry) {??this.metadataReaderFactory = metadataReaderFactory; ?this.problemReporter = problemReporter; ?this.environment = environment; ?this.resourceLoader = resourceLoader; ?this.registry = registry; ?this.componentScanParser = new ComponentScanAnnotationParser( ?????resourceLoader, environment, componentScanBeanNameGenerator, registry); ?// 構(gòu)造ConditionEvaluator用于處理?xiàng)l件注解 ?this.conditionEvaluator = new ConditionEvaluator(registry, environment, resourceLoader);} ???

ConfigurationClassParser對每個(gè)配置類進(jìn)行解析的時(shí)候都會使用ConditionEvaluator：

if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) { ?return;} ???

ConditionEvaluator的skip方法：

public boolean shouldSkip(AnnotatedTypeMetadata metadata, ConfigurationPhase phase) { ?// 如果這個(gè)類沒有被@Conditional注解所修飾瘤袖，不會skip ?if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) { ???return false; ?} ?// 如果參數(shù)中沒有設(shè)置條件注解的生效階段 ?if (phase == null) { ???// 是配置類的話直接使用PARSE_CONFIGURATION階段 ???if (metadata instanceof AnnotationMetadata && ???????ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) { ?????return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION); ???} ???// 否則使用REGISTER_BEAN階段 ???return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN); ?} ?// 要解析的配置類的條件集合 ?List<Condition> conditions = new ArrayList<Condition>(); ?// 獲取配置類的條件注解得到條件數(shù)據(jù)，并添加到集合中 ?for (String[] conditionClasses : getConditionClasses(metadata)) { ???for (String conditionClass : conditionClasses) { ?????Condition condition = getCondition(conditionClass, this.context.getClassLoader()); ?????conditions.add(condition); ???} ?}??// 對條件集合做個(gè)排序 ?AnnotationAwareOrderComparator.sort(conditions); ?// 遍歷條件集合 ?for (Condition condition : conditions) { ???ConfigurationPhase requiredPhase = null; ???if (condition instanceof ConfigurationCondition) { ?????requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase(); ???} ???// 沒有這個(gè)解析類不需要階段的判斷或者解析類和參數(shù)中的階段一致才會繼續(xù)進(jìn)行 ???if (requiredPhase == null || requiredPhase == phase) { ?????// 階段一致切不滿足條件的話昂验，返回true并跳過這個(gè)bean的解析 ?????if (!condition.matches(this.context, metadata)) { ???????return true; ?????} ???} ?}??return false;} ???

SpringBoot在條件注解的解析log記錄在了ConditionEvaluationReport類中捂敌，可以通過BeanFactory獲取(BeanFactory是有父子關(guān)系的艾扮；每個(gè)BeanFactory都存有一份ConditionEvaluationReport，互不相干)：

ConditionEvaluationReport conditionEvaluationReport = beanFactory.getBean("autoConfigurationReport", ConditionEvaluationReport.class);Map<String, ConditionEvaluationReport.ConditionAndOutcomes> result = conditionEvaluationReport.getConditionAndOutcomesBySource();for(String key : result.keySet()) { ???ConditionEvaluationReport.ConditionAndOutcomes conditionAndOutcomes = result.get(key); ???Iterator<ConditionEvaluationReport.ConditionAndOutcome> iterator = conditionAndOutcomes.iterator(); ???while(iterator.hasNext()) { ???????ConditionEvaluationReport.ConditionAndOutcome conditionAndOutcome = iterator.next(); ???????System.out.println(key + " -- " + conditionAndOutcome.getCondition().getClass().getSimpleName() + " -- " + conditionAndOutcome.getOutcome()); ???}} ???

打印出條件注解下的類加載信息：

.......org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration -- OnClassCondition -- required @ConditionalOnClass classes not found: freemarker.template.Configuration,org.springframework.ui.freemarker.FreeMarkerConfigurationFactoryorg.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration -- OnClassCondition -- required @ConditionalOnClass classes not found: groovy.text.markup.MarkupTemplateEngineorg.springframework.boot.autoconfigure.gson.GsonAutoConfiguration -- OnClassCondition -- required @ConditionalOnClass classes not found: com.google.gson.Gsonorg.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration -- OnClassCondition -- required @ConditionalOnClass classes not found: org.h2.server.web.WebServletorg.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration -- OnClassCondition -- required @ConditionalOnClass classes not found: org.springframework.hateoas.Resource,org.springframework.plugin.core.Pluginorg.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration -- OnClassCondition -- required @ConditionalOnClass classes not found: com.hazelcast.core.HazelcastInstance.......

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