作者：温安适

来源：my.oschina.net/floor/blog/4325651

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开始，引入可扩展的XML编程机制，该机制要求XML Schema命名空间需要与Handler建立映射关系。

该关系配置在相对于classpath下的/META-INF/spring.handlers中。

如上图所示 ContextNamespaceHandler对应context:... 分析的入口。

找核心方法

浏览ContextNamespaceHandler

在parse中有一个很重要的注释

// Actually scan for bean definitions and register them.

ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);

大意是：ClassPathBeanDefinitionScanner#doScan是扫描BeanDefinition并注册的实现 。

ClassPathBeanDefinitionScanner 的源码如下：

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {Assert.notEmpty(basePackages, "At least one base package must be specified");Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();for (String basePackage : basePackages) {//findCandidateComponents 读资源装换为BeanDefinitionSet<BeanDefinition> 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;
}

上边的代码，从方法名，猜测：

findCandidateComponents：从classPath扫描组件，并转换为备选BeanDefinition，也就是要做的解析@Component的核心方法。

概要分析

findCandidateComponents在其父类ClassPathScanningCandidateComponentProvider 中。

public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {
//省略其他代码
public Set<BeanDefinition> findCandidateComponents(String basePackage) {if (this.componentsIndex != null && indexSupportsIncludeFilters()) {return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);}else {return scanCandidateComponents(basePackage);}
}
private Set<BeanDefinition> scanCandidateComponents(String basePackage) {Set<BeanDefinition> candidates = new LinkedHashSet<>();try {String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +resolveBasePackage(basePackage) + '/' + this.resourcePattern;Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);//省略部分代码for (Resource resource : resources) {//省略部分代码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);//省略部分代码}}catch (IOException ex) {//省略部分代码 }return candidates;
}
}

findCandidateComponents大体思路如下：

• String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX resolveBasePackage(basePackage) + '/' + this.resourcePattern; 将package转化为ClassLoader类资源搜索路径packageSearchPath，例如：com.wl.spring.boot转化为classpath*:com/wl/spring/boot/**/*.class

• Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath); 加载搜素路径下的资源。

• isCandidateComponent 判断是否是备选组件

• candidates.add(sbd); 添加到返回结果的list

ClassPathScanningCandidateComponentProvider#isCandidateComponent其源码如下：

protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {//省略部分代码for (TypeFilter tf : this.includeFilters) {if (tf.match(metadataReader, getMetadataReaderFactory())) {return isConditionMatch(metadataReader);}}return false;
}

includeFilters由registerDefaultFilters()设置初始值，有@Component，没有@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如何处理@Service的注解的呢？？？？

2.查文档找思路

查阅官方文档，下面这话：

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管理的组件的通用原型。@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，在读取@Service，也读取了它的元注解，并将@Service作为@Component处理。

3. 探寻@Component派生性流程

回顾ClassPathScanningCandidateComponentProvider 中的关键的代码片段如下：

private Set<BeanDefinition> scanCandidateComponents(String basePackage) {//省略其他代码MetadataReader metadataReader   =getMetadataReaderFactory().getMetadataReader(resource);  if(isCandidateComponent(metadataReader)){//....}
}
public final MetadataReaderFactory getMetadataReaderFactory() {if (this.metadataReaderFactory == null) {this.metadataReaderFactory = new CachingMetadataReaderFactory();}return this.metadataReaderFactory;
}

1. 确定metadataReader

CachingMetadataReaderFactory继承自 SimpleMetadataReaderFactory，就是对SimpleMetadataReaderFactory加了一层缓存。

其内部的SimpleMetadataReaderFactory#getMetadataReader 为：

public class SimpleMetadataReaderFactory implements MetadataReaderFactory{@Overridepublic MetadataReader getMetadataReader(Resource resource) throws IOException {return new SimpleMetadataReader(resource, this.resourceLoader.getClassLoader());}
}

这里可以看出

MetadataReader metadataReader =new SimpleMetadataReader(...);

2.查看match方法找重点方法

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，从名称看是处理元注解，我们重点关注

逐步分析

找metadata.hasMetaAnnotation

metadata=metadataReader.getAnnotationMetadata();

metadataReader =new SimpleMetadataReader(...)

metadata= new SimpleMetadataReader#getAnnotationMetadata()

//SimpleMetadataReader 的构造方法
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（。。）

也就是说

metadata.hasMetaAnnotation=AnnotationMetadataReadingVisitor#hasMetaAnnotation

其方法如下：

public class AnnotationMetadataReadingVisitor{// 省略部分代码
@Override
public boolean hasMetaAnnotation(String metaAnnotationType) {Collection<Set<String>> allMetaTypes = this.metaAnnotationMap.values();for (Set<String> metaTypes : allMetaTypes) {if (metaTypes.contains(metaAnnotationType)) {return true;}}return false;
}
}

逻辑很简单，就是判断该注解的元注解在，在不在metaAnnotationMap中，如果在就返回true。

这里面核心就是metaAnnotationMap，搜索AnnotationMetadataReadingVisitor类，没有发现赋值的地方？？！。

查找metaAnnotationMap赋值

回到SimpleMetadataReader 的方法，

//这个accept方法，很可疑，在赋值之前执行
SimpleMetadataReader(Resource resource, @Nullable ClassLoader classLoader) throws IOException {
//省略其他代码
AnnotationMetadataReadingVisitor visitor = new AnnotationMetadataReadingVisitor(classLoader);
classReader.accept(visitor, ClassReader.SKIP_DEBUG);this.annotationMetadata = visitor;}

发现一个可疑的语句：classReader.accept。

查看accept方法

public class ClassReader {//省略其他代码
public void accept(..省略代码){//省略其他代码readElementValues(classVisitor.visitAnnotation(annotationDescriptor, /* visible = */ true),currentAnnotationOffset,true,charBuffer);
}
}

查看readElementValues方法

public class ClassReader{//省略其他代码
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;
}
}

这里面的核心就是  annotationVisitor.visitEnd();

确定annotationVisitor

这里的annotationVisitor=AnnotationMetadataReadingVisitor#visitAnnotation

源码如下，注意这里传递了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

查阅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<AnnotationAttributes> 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<Annotation> visited = new LinkedHashSet<>();for (Annotation metaAnnotation : metaAnnotations) {recursivelyCollectMetaAnnotations(visited, metaAnnotation);}if (!visited.isEmpty()) {Set<String> 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);}}}}
}
}

内部方法recursivelyCollectMetaAnnotations 递归的读取注解，与注解的元注解（读@Service，再读元注解@Component），并设置到metaAnnotationMap，也就是AnnotationMetadataReadingVisitor 中的metaAnnotationMap中。

总结

大致如下：

ClassPathScanningCandidateComponentProvider#findCandidateComponents

1.将package转化为ClassLoader类资源搜索路径packageSearchPath

2.加载搜素路径下的资源。

3.isCandidateComponent 判断是否是备选组件。

内部调用的TypeFilter的match方法：

• AnnotationTypeFilter#matchself中metadata.hasMetaAnnotation处理元注解

• metadata.hasMetaAnnotation=AnnotationMetadataReadingVisitor#hasMetaAnnotation

就是判断当前注解的元注解在不在metaAnnotationMap中。

AnnotationAttributesReadingVisitor#visitEnd()内部方法recursivelyCollectMetaAnnotations 递归的读取注解，与注解的元注解（读@Service，再读元注解@Component），并设置到metaAnnotationMap

4.添加到返回结果的list