面试官:注解@Component，@Service是如何被解析的?
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 doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set beanDefinitions = new LinkedHashSet<>();
for (String basePackage : basePackages) {
//findCandidateComponents 读资源装换为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 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) 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。公众号搜索,[Java学习之道],回复'福利',3T资料等你来拿!
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 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 {
@Override
public 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> allMetaTypes = this.metaAnnotationMap.values();
for (Set 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 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);
}
}
}
}
}
}
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
将package转化为ClassLoader类资源搜索路径packageSearchPath
加载搜素路径下的资源。
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
添加到返回结果的list