javascript
@data注解不生效_面试官:你经常在SpringBoot中使用的条件注解底层是如何实现的?你了解过吗?...
SpringBoot內部提供了特有的注解:條件注解(Conditional Annotation)。比如@ConditionalOnBean、@ConditionalOnClass、@ConditionalOnExpression、@ConditionalOnMissingBean等。
條件注解存在的意義在于動態識別(也可以說是代碼自動化執行)。比如@ConditionalOnClass會檢查類加載器中是否存在對應的類,如果有的話被注解修飾的類就有資格被Spring容器所注冊,否則會被skip。
比如FreemarkerAutoConfiguration這個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這個自動化配置類被@ConditionalOnClass條件注解修飾,這個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這兩個類,如果都存在的話會在Spring容器中加載這個FreeMarkerAutoConfiguration配置類;否則不會加載。
# 條件注解內部的一些基礎
在分析條件注解的底層實現之前,我們先來看一下這些條件注解的定義。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個屬性,分別是類數組和字符串數組(作用一樣,類型不一樣),而且被@Conditional注解所修飾,這個@Conditional注解有個名為values的Class extends Condition>[]類型的屬性。 這個Condition是個接口,用于匹配組件是否有資格被容器注冊,定義如下:
public interface Condition { // ConditionContext內部會存儲Spring容器、應用程序環境信息、資源加載器、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Condition接口的實現類,所有的Condition接口需要全部匹配成功后這個@Conditional修飾的組件才有資格被注冊。
Condition接口有個子接口ConfigurationCondition:
public interface ConfigurationCondition extends Condition { ConfigurationPhase getConfigurationPhase(); public static enum ConfigurationPhase { PARSE_CONFIGURATION, REGISTER_BEAN }}這個子接口是一種特殊的條件接口,多了一個getConfigurationPhase方法,也就是條件注解的生效階段。只有在ConfigurationPhase中定義的兩種階段下才會生效。
Condition接口有個實現抽象類SpringBootCondition,SpringBoot中所有條件注解對應的條件類都繼承這個抽象類。它實現了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,具體子類實現。ConditionOutcome記錄了匹配結果boolean和log信息 logOutcome(classOrMethodName, outcome); // log記錄一下匹配信息 recordEvaluation(context, classOrMethodName, outcome); // 報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)。
@ConditionalOnClass或者@ConditionalOnMissingClass注解對應的條件類是OnClassCondition,定義如下:
@Order(Ordered.HIGHEST_PRECEDENCE) // 優先級、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的類,返回一個匹配失敗的ConditionalOutcome return ConditionOutcome .noMatch("required @ConditionalOnClass classes not found: " + StringUtils.collectionToCommaDelimitedString(missing)); } // 如果類加載器中存在對應的類的話,匹配信息進行記錄 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。用于查詢類名在對應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。在OnClassCondition執行過程中得到的最終ConditionalOutcome中的log message如下:
@ConditionalOnBean(Spring容器中存在指明的bean)、@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指明的bean)都是基于Bean的條件注解,它們對應的條件類是ConditionOnBean。
@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。提供CURRENT(只在當前容器中找)、PARENTS(只在所有的父容器中找;但是不包括當前容器)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); // 構造一個BeanSearchSpec,會從@ConditionalOnBean注解中獲取屬性,然后設置到BeanSearchSpec中 List matching = getMatchingBeans(context, spec); // 從BeanFactory中根據策略找出所有匹配的bean if (matching.isEmpty()) { // 如果沒有匹配的bean,返回一個沒有匹配成功的ConditionalOutcome return ConditionOutcome .noMatch("@ConditionalOnBean " + spec + " found no beans"); } // 如果找到匹配的bean,匹配信息進行記錄 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 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,判斷是否只有一個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,針對@ConditionalOnMissingBean注解 BeanSearchSpec spec = new BeanSearchSpec(context, metadata, ConditionalOnMissingBean.class); List 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、ConditionalOnWebApplication、ConditionalOnResource、ConditionalOnProperty、ConditionalOnExpression等條件注解,有興趣的讀者可以自行查看它們的底層處理邏輯。
# 各種條件注解的總結
# SpringBoot條件注解的激活機制
分析完了條件注解的執行邏輯之后,接下來的問題就是SpringBoot是如何讓這些條件注解生效的?
SpringBoot使用ConditionEvaluator這個內部類完成條件注解的解析和判斷。
在Spring容器的refresh過程中,只有跟解析或者注冊bean有關系的類都會使用ConditionEvaluator完成條件注解的判斷,這個過程中一些類不滿足條件的話就會被skip。這些類比如有AnnotatedBeanDefinitionReader、ConfigurationClassBeanDefinitionReader、ConfigurationClassParse、ClassPathScanningCandidateComponentProvider等。
比如ConfigurationClassParser的構造函數會初始化內部屬性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); // 構造ConditionEvaluator用于處理條件注解 this.conditionEvaluator = new ConditionEvaluator(registry, environment, resourceLoader);}ConfigurationClassParser對每個配置類進行解析的時候都會使用ConditionEvaluator:
if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) { return;}ConditionEvaluator的skip方法:
public boolean shouldSkip(AnnotatedTypeMetadata metadata, ConfigurationPhase phase) { // 如果這個類沒有被@Conditional注解所修飾,不會skip if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) { return false; } // 如果參數中沒有設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 conditions = new ArrayList(); // 獲取配置類的條件注解得到條件數據,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(); } // 沒有這個解析類不需要階段的判斷或者解析類和參數中的階段一致才會繼續進行 if (requiredPhase == null || requiredPhase == phase) { // 階段一致切不滿足條件的話,返回true并跳過這個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,可以通過BeanFactory獲取(BeanFactory是有父子關系的;每個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 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.......來源:http://fangjian0423.github.io/2017/05/16/springboot-condition-annotation/?往期推薦?
?
細細拆分了一下Spring容器的refresh過程,真難...
阿里面試官:給我描述一下緩存擊穿的現象,并說說你的解決思路?
要實現單點登錄,難嗎?
點擊
總結
以上是生活随笔為你收集整理的@data注解不生效_面试官:你经常在SpringBoot中使用的条件注解底层是如何实现的?你了解过吗?...的全部內容,希望文章能夠幫你解決所遇到的問題。
- 上一篇: python每天一个小程序_Python
- 下一篇: 编译原理什么是移进规约冲突_编译原理复习