@bean注解和@component注解的区别_通过源码查看 @Component，@Service 等注解是如何被解析的...
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來源：my.oschina.net/floor/blog/4325651
- 前言
- 1.@Component解析流程
- 找入口
- 找核心方法
- 概要分析
- 2.查文檔找思路
- 3. 探尋@Component派生性流程
- 1. 確定metadataReader
- 2.查看match方法找重點方法
- 逐步分析
- 總結
前言
@Component和@Service都是工作中常用的注解，Spring如何解析？
1.@Component解析流程
找入口
Spring Framework2.0開始，引入可擴展的XML編程機制，該機制要求XML Schema命名空間需要與Handler建立映射關系。
該關系配置在相對于classpath下的/META-INF/spring.handlers中。
如上圖所示 ContextNamespaceHandler對應context:... 分析的入口。
找核心方法
瀏覽ContextNamespaceHandler
在parse中有一個很重要的注釋
// Actually scan for bean definitions and register them.
ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
大意是：ClassPathBeanDefinitionScanner#doScan是掃描BeanDefinition并注冊的實現 。
ClassPathBeanDefinitionScanner 的源碼如下：
protected?Set?doScan(String...?basePackages)?{???Assert.notEmpty(basePackages,?"At?least?one?base?package?must?be?specified");
???Set?beanDefinitions?=?new?LinkedHashSet<>();for?(String?basePackage?:?basePackages)?{//findCandidateComponents?讀資源裝換為BeanDefinition
??????Set?candidates?=?findCandidateComponents(basePackage);for?(BeanDefinition?candidate?:?candidates)?{
?????????ScopeMetadata?scopeMetadata?=?this.scopeMetadataResolver.resolveScopeMetadata(candidate);
?????????candidate.setScope(scopeMetadata.getScopeName());
?????????String?beanName?=?this.beanNameGenerator.generateBeanName(candidate,?this.registry);if?(candidate?instanceof?AbstractBeanDefinition)?{
????????????postProcessBeanDefinition((AbstractBeanDefinition)?candidate,?beanName);
?????????}if?(candidate?instanceof?AnnotatedBeanDefinition)?{
????????????AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition)?candidate);
?????????}if?(checkCandidate(beanName,?candidate))?{
????????????BeanDefinitionHolder?definitionHolder?=?new?BeanDefinitionHolder(candidate,?beanName);
????????????definitionHolder?=
??????????????????AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata,?definitionHolder,?this.registry);
????????????beanDefinitions.add(definitionHolder);
????????????registerBeanDefinition(definitionHolder,?this.registry);
?????????}
??????}
???}return?beanDefinitions;
}
上邊的代碼，從方法名，猜測：
findCandidateComponents：從classPath掃描組件，并轉換為備選BeanDefinition，也就是要做的解析@Component的核心方法。
概要分析
findCandidateComponents在其父類ClassPathScanningCandidateComponentProvider 中。
public?class?ClassPathScanningCandidateComponentProvider?implements?EnvironmentCapable,?ResourceLoaderAware?{//省略其他代碼
public?Set?findCandidateComponents(String?basePackage)?{
???if?(this.componentsIndex?!=?null?&&?indexSupportsIncludeFilters())?{
??????return?addCandidateComponentsFromIndex(this.componentsIndex,?basePackage);
???}
???else?{
??????return?scanCandidateComponents(basePackage);
???}
}
private?Set?scanCandidateComponents(String?basePackage)?{
???Set?candidates?=?new?LinkedHashSet<>();try?{
??????String?packageSearchPath?=?ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX?+
????????????resolveBasePackage(basePackage)?+?'/'?+?this.resourcePattern;
??????Resource[]?resources?=?getResourcePatternResolver().getResources(packageSearchPath);//省略部分代碼for?(Resource?resource?:?resources)?{//省略部分代碼if?(resource.isReadable())?{try?{
???????????????MetadataReader?metadataReader?=?getMetadataReaderFactory().getMetadataReader(resource);if?(isCandidateComponent(metadataReader))?{
??????????????????ScannedGenericBeanDefinition?sbd?=?new?ScannedGenericBeanDefinition(metadataReader);
??????????????????sbd.setSource(resource);if?(isCandidateComponent(sbd))?{
?????????????????????candidates.add(sbd);//省略部分代碼
??????}
???}catch?(IOException?ex)?{//省略部分代碼?}return?candidates;
}
}
findCandidateComponents大體思路如下：
ClassPathScanningCandidateComponentProvider#isCandidateComponent其源碼如下：
protected?boolean?isCandidateComponent(MetadataReader?metadataReader)?throws?IOException?{????//省略部分代碼
???for?(TypeFilter?tf?:?this.includeFilters)?{
??????if?(tf.match(metadataReader,?getMetadataReaderFactory()))?{
?????????return?isConditionMatch(metadataReader);
??????}
???}
???return?false;
}
includeFilters由registerDefaultFilters()設置初始值，有@Component，沒有@Service啊？
protected?void?registerDefaultFilters()?{???this.includeFilters.add(new?AnnotationTypeFilter(Component.class));
???ClassLoader?cl?=?ClassPathScanningCandidateComponentProvider.class.getClassLoader();
???try?{
??????this.includeFilters.add(new?AnnotationTypeFilter(
????????????((Class?extends?Annotation>)?ClassUtils.forName("javax.annotation.ManagedBean",?cl)),?false));
??????logger.trace("JSR-250?'javax.annotation.ManagedBean'?found?and?supported?for?component?scanning");
???}
???catch?(ClassNotFoundException?ex)?{
??????//?JSR-250?1.1?API?(as?included?in?Java?EE?6)?not?available?-?simply?skip.
???}
???try?{
??????this.includeFilters.add(new?AnnotationTypeFilter(
????????????((Class?extends?Annotation>)?ClassUtils.forName("javax.inject.Named",?cl)),?false));
??????logger.trace("JSR-330?'javax.inject.Named'?annotation?found?and?supported?for?component?scanning");
???}
???catch?(ClassNotFoundException?ex)?{
??????//?JSR-330?API?not?available?-?simply?skip.
???}
}
Spring如何處理@Service的注解的呢？？？？
2.查文檔找思路
查閱官方文檔，下面這話：
https://docs.spring.io/spring/docs/5.0.17.RELEASE/spring-framework-reference/core.html#beans-meta-annotations
@Component is a generic stereotype for any Spring-managed component. @Repository, @Service, and @Controller are specializations of @Component
大意如下：
@Component是任何Spring管理的組件的通用原型。@Repository、@Service和@Controller是派生自@Component。
@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)
@Documented
//?@Service?派生自@Component
@Component
public?@interface?Service?{
???/**
????*?The?value?may?indicate?a?suggestion?for?a?logical?component?name,
????*?to?be?turned?into?a?Spring?bean?in?case?of?an?autodetected?component.
????*?@return?the?suggested?component?name,?if?any?(or?empty?String?otherwise)
????*/
???@AliasFor(annotation?=?Component.class)
???String?value()?default?"";
}
@Component是@Service的元注解，Spring 大概率，在讀取@Service，也讀取了它的元注解，并將@Service作為@Component處理。
3. 探尋@Component派生性流程
回顧ClassPathScanningCandidateComponentProvider 中的關鍵的代碼片段如下：
private?Set?scanCandidateComponents(String?basePackage)?{?//省略其他代碼
?MetadataReader?metadataReader
?????????????=getMetadataReaderFactory().getMetadataReader(resource);
???if(isCandidateComponent(metadataReader)){
???????//....
???}
}
public?final?MetadataReaderFactory?getMetadataReaderFactory()?{
???if?(this.metadataReaderFactory?==?null)?{
??????this.metadataReaderFactory?=?new?CachingMetadataReaderFactory();
???}
???return?this.metadataReaderFactory;
}
1. 確定metadataReader
CachingMetadataReaderFactory繼承自 SimpleMetadataReaderFactory，就是對SimpleMetadataReaderFactory加了一層緩存。
其內部的SimpleMetadataReaderFactory#getMetadataReader 為：
public?class?SimpleMetadataReaderFactory?implements?MetadataReaderFactory?{????@Override
public?MetadataReader?getMetadataReader(Resource?resource)?throws?IOException?{
???return?new?SimpleMetadataReader(resource,?this.resourceLoader.getClassLoader());
}
????}
這里可以看出
MetadataReader metadataReader =new SimpleMetadataReader(...);
2.查看match方法找重點方法
AnnotationTypeFilter#matchself方法如下：
@Overrideprotected?boolean?matchSelf(MetadataReader?metadataReader)?{
???AnnotationMetadata?metadata?=?metadataReader.getAnnotationMetadata();
???return?metadata.hasAnnotation(this.annotationType.getName())?||
?????????(this.considerMetaAnnotations?&&?metadata.hasMetaAnnotation(this.annotationType.getName()));
}
是metadata.hasMetaAnnotation法，從名稱看是處理元注解，我們重點關注
逐步分析
找metadata.hasMetaAnnotation
metadata=metadataReader.getAnnotationMetadata();
metadataReader =new SimpleMetadataReader(...)
metadata= new SimpleMetadataReader#getAnnotationMetadata()
//SimpleMetadataReader?的構造方法SimpleMetadataReader(Resource?resource,?@Nullable?ClassLoader?classLoader)?throws?IOException?{
???InputStream?is?=?new?BufferedInputStream(resource.getInputStream());
???ClassReader?classReader;
???try?{
??????classReader?=?new?ClassReader(is);
???}
???catch?(IllegalArgumentException?ex)?{
??????throw?new?NestedIOException("ASM?ClassReader?failed?to?parse?class?file?-?"?+
????????????"probably?due?to?a?new?Java?class?file?version?that?isn't?supported?yet:?"?+?resource,?ex);
???}
???finally?{
??????is.close();
???}
???AnnotationMetadataReadingVisitor?visitor?=
????????????new?AnnotationMetadataReadingVisitor(classLoader);
???classReader.accept(visitor,?ClassReader.SKIP_DEBUG);
???this.annotationMetadata?=?visitor;
???//?(since?AnnotationMetadataReadingVisitor?extends?ClassMetadataReadingVisitor)
???this.classMetadata?=?visitor;
???this.resource?=?resource;
}
metadata=new SimpleMetadataReader(...)**.**getAnnotationMetadata()= new AnnotationMetadataReadingVisitor(。。)
也就是說
metadata.hasMetaAnnotation=AnnotationMetadataReadingVisitor#hasMetaAnnotation
其方法如下：
public?class?AnnotationMetadataReadingVisitor{????//?省略部分代碼
@Override
public?boolean?hasMetaAnnotation(String?metaAnnotationType)?{
???Collection>?allMetaTypes?=?this.metaAnnotationMap.values();for?(Set?metaTypes?:?allMetaTypes)?{if?(metaTypes.contains(metaAnnotationType))?{return?true;
??????}
???}return?false;
}
}
邏輯很簡單，就是判斷該注解的元注解在，在不在metaAnnotationMap中，如果在就返回true。
這里面核心就是metaAnnotationMap，搜索AnnotationMetadataReadingVisitor類，沒有發現賦值的地方？？！。
查找metaAnnotationMap賦值
回到SimpleMetadataReader 的方法，
//這個accept方法，很可疑，在賦值之前執行SimpleMetadataReader(Resource?resource,?@Nullable?ClassLoader?classLoader)?throws?IOException?{
//省略其他代碼
AnnotationMetadataReadingVisitor?visitor?=?new?AnnotationMetadataReadingVisitor(classLoader);
classReader.accept(visitor,?ClassReader.SKIP_DEBUG);
?this.annotationMetadata?=?visitor;
?}
發現一個可疑的語句：classReader.accept。
查看accept方法
public?class?ClassReader?{????????//省略其他代碼
public?void?accept(..省略代碼){
????//省略其他代碼
????readElementValues(
????classVisitor.visitAnnotation(annotationDescriptor,?/*?visible?=?*/?true),
????currentAnnotationOffset,
?????true,
????charBuffer);
}
}
查看readElementValues方法
public?class?ClassReader{????//省略其他代碼
private?int?readElementValues(final?AnnotationVisitor?annotationVisitor,final?int?annotationOffset,final?boolean?named,final?char[]?charBuffer)?{
??int?currentOffset?=?annotationOffset;
??//?Read?the?num_element_value_pairs?field?(or?num_values?field?for?an?array_value).
??int?numElementValuePairs?=?readUnsignedShort(currentOffset);
??currentOffset?+=?2;
??if?(named)?{
????//?Parse?the?element_value_pairs?array.
????while?(numElementValuePairs--?>?0)?{
??????String?elementName?=?readUTF8(currentOffset,?charBuffer);
??????currentOffset?=
??????????readElementValue(annotationVisitor,?currentOffset?+?2,?elementName,?charBuffer);
????}
??}?else?{
????//?Parse?the?array_value?array.
????while?(numElementValuePairs--?>?0)?{
??????currentOffset?=
??????????readElementValue(annotationVisitor,?currentOffset,?/*?named?=?*/?null,?charBuffer);
????}
??}
??if?(annotationVisitor?!=?null)?{
????annotationVisitor.visitEnd();
??}
??return?currentOffset;
}
}
這里面的核心就是 annotationVisitor.visitEnd();
確定annotationVisitor
這里的annotationVisitor=AnnotationMetadataReadingVisitor#visitAnnotation
源碼如下，注意這里傳遞了metaAnnotationMap！！
public?class?AnnotationMetadataReadingVisitor{@Override
public?AnnotationVisitor?visitAnnotation(String?desc,?boolean?visible)?{
???String?className?=?Type.getType(desc).getClassName();
???this.annotationSet.add(className);
???return?new?AnnotationAttributesReadingVisitor(
?????????className,?this.attributesMap,
??????????????this.metaAnnotationMap,?this.classLoader);
}
}
annotationVisitor=AnnotationAttributesReadingVisitor
查閱annotationVisitor.visitEnd()
annotationVisitor=AnnotationAttributesReadingVisitor#visitEnd()
public?class?AnnotationAttributesReadingVisitor{@Override
public?void?visitEnd()?{
???super.visitEnd();
???Class?extends?Annotation>?annotationClass?=?this.attributes.annotationType();
???if?(annotationClass?!=?null)?{
??????List?attributeList?=?this.attributesMap.get(this.annotationType);if?(attributeList?==?null)?{this.attributesMap.add(this.annotationType,?this.attributes);
??????}else?{
?????????attributeList.add(0,?this.attributes);
??????}if?(!AnnotationUtils.isInJavaLangAnnotationPackage(annotationClass.getName()))?{try?{
????????????Annotation[]?metaAnnotations?=?annotationClass.getAnnotations();if?(!ObjectUtils.isEmpty(metaAnnotations))?{
???????????????Set?visited?=?new?LinkedHashSet<>();for?(Annotation?metaAnnotation?:?metaAnnotations)?{
??????????????????recursivelyCollectMetaAnnotations(visited,?metaAnnotation);
???????????????}if?(!visited.isEmpty())?{
??????????????????Set?metaAnnotationTypeNames?=?new?LinkedHashSet<>(visited.size());for?(Annotation?ann?:?visited)?{
?????????????????????metaAnnotationTypeNames.add(ann.annotationType().getName());
??????????????????}this.metaAnnotationMap.put(annotationClass.getName(),?metaAnnotationTypeNames);
???????????????}
????????????}
?????????}catch?(Throwable?ex)?{if?(logger.isDebugEnabled())?{
???????????????logger.debug("Failed?to?introspect?meta-annotations?on?"?+?annotationClass?+?":?"?+?ex);
????????????}
?????????}
??????}
???}
}
}
內部方法recursivelyCollectMetaAnnotations 遞歸的讀取注解，與注解的元注解(讀@Service，再讀元注解@Component)，并設置到metaAnnotationMap，也就是AnnotationMetadataReadingVisitor 中的metaAnnotationMap中。
總結
大致如下：
ClassPathScanningCandidateComponentProvider#findCandidateComponents
1、將package轉化為ClassLoader類資源搜索路徑packageSearchPath
2、加載搜素路徑下的資源。
3、isCandidateComponent 判斷是否是備選組件。
內部調用的TypeFilter的match方法：
AnnotationTypeFilter#matchself中metadata.hasMetaAnnotation處理元注解
metadata.hasMetaAnnotation=AnnotationMetadataReadingVisitor#hasMetaAnnotation
就是判斷當前注解的元注解在不在metaAnnotationMap中。
AnnotationAttributesReadingVisitor#visitEnd()內部方法recursivelyCollectMetaAnnotations 遞歸的讀取注解，與注解的元注解(讀@Service，再讀元注解@Component)，并設置到metaAnnotationMap
4、添加到返回結果的list
END
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