通过源码查看 @Component,@Service 等注解是如何被解析的

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来源: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.查文档找思路

  • 3. 探寻@Component派生性流程

    • 1. 确定metadataReader

    • 2.查看match方法找重点方法

    • 逐步分析

  • 总结

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BeanDefinition
Set<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:

  1. 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

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

  3. isCandidateComponent 判断是否是备选组件

  4. 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 {
@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<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

END

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