Spring 中 @Component、@Service 等注解如何被解析？
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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(kāi)始，引入可擴(kuò)展的XML編程機(jī)制，該機(jī)制要求XML Schema命名空間需要與Handler建立映射關(guān)系。
該關(guān)系配置在相對(duì)于classpath下的/META-INF/spring.handlers中。
如上圖所示?ContextNamespaceHandler對(duì)應(yīng)context:... 分析的入口。
找核心方法
瀏覽ContextNamespaceHandler
在parse中有一個(gè)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(cè)的實(shí)現(xiàn)。
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(cè)：
findCandidateComponents：從classPath掃描組件，并轉(zhuǎn)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(lèi)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：
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX resolveBasePackage(basePackage) + '/' + this.resourcePattern; ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?將package轉(zhuǎn)化為ClassLoader類(lèi)資源搜索路徑packageSearchPath，例如：com.wl.spring.boot轉(zhuǎn)化為classpath*:com/wl/spring/boot/**/*.class Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath); ?加載搜素路徑下的資源。 isCandidateComponent 判斷是否是備選組件 candidates.add(sbd); 添加到返回結(jié)果的list
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(shè)置初始值，有@Component，沒(méi)有@Service?。?/p>
protected?void?registerDefaultFilters()?{
???this.includeFilters.add(new?AnnotationTypeFilter(Component.class));
???ClassLoader?cl?=?ClassPathScanningCandidateComponentProvider.class.getClassLoader();
???try?{
??????this.includeFilters.add(new?AnnotationTypeFilter(
????????????((Class)?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)?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C(jī)lassPathScanningCandidateComponentProvider 中的關(guān)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(duì)SimpleMetadataReaderFactory加了一層緩存。
其內(nèi)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(diǎn)方法
AnnotationTypeFilter#matchself方法如下：
@Override
protected?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(chēng)看是處理元注解，我們重點(diǎn)關(guān)注
逐步分析
找metadata.hasMetaAnnotation
metadata=metadataReader.getAnnotationMetadata();
metadataReader =new SimpleMetadataReader(...)
metadata= new SimpleMetadataReader#getAnnotationMetadata()
//SimpleMetadataReader?的構(gòu)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(shuō)
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(jiǎn)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(lèi)，沒(méi)有發(fā)現(xiàn)賦值的地方？？！。
查找metaAnnotationMap賦值
回到SimpleMetadataReader 的方法，
//這個(gè)accept方法，很可疑，在賦值之前執(zhí)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(fā)現(xiàn)一個(gè)可疑的語(yǔ)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?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(nèi)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(shè)置到metaAnnotationMap，也就是AnnotationMetadataReadingVisitor 中的metaAnnotationMap中。
總結(jié)
大致如下：
ClassPathScanningCandidateComponentProvider#findCandidateComponents
將package轉(zhuǎn)化為ClassLoader類(lèi)資源搜索路徑packageSearchPath
加載搜素路徑下的資源。
isCandidateComponent 判斷是否是備選組件。
內(nèi)部調(diào)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(dāng)前注解的元注解在不在metaAnnotationMap中。
AnnotationAttributesReadingVisitor#visitEnd()內(nèi)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(shè)置到metaAnnotationMap
添加到返回結(jié)果的list
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