在实际开发中，我们经常使用Spring的@Component、@Service、@Repository以及 @Controller等注解来实现bean托管给Spring容器管理。Spring是怎么样实现的呢？我们一起跟着源码看看整个过程吧！
照旧，先看调用时序图：
public AnnotationConfigApplicationContext(String... basePackages) {
this();
scan(basePackages);
refresh();
}
Spring启动时，会去扫描指定包下的文件。
public void scan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
this.scanner.scan(basePackages);
}
对应时序图方法1，ClassPathBeanDefinitionScanner#scan。交给ClassPathBeanDefinitionScanner处理。
ClassPathBeanDefinitionScanner 初始化时设置了注解过滤器
public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,Environment environment, @Nullable ResourceLoader resourceLoader) {
Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
this.registry = registry;
if (useDefaultFilters) {
// 注册注解过滤器
registerDefaultFilters();
}
setEnvironment(environment);
setResourceLoader(resourceLoader);
}
protected void registerDefaultFilters() {
// 添加Component类型
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));
}
catch (ClassNotFoundException ex) {
}
try {
this.includeFilters.add(new AnnotationTypeFilter(
((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false));
}
catch (ClassNotFoundException ex) {
}
}
在includeFilters添加了Component，ManagedBean两种注解类型。后面用来过滤加载到的class文件是否需要交给Spring容器管理。
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) {
// 扫描包下有Spring Component注解，并且生成BeanDefinition
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) {
// 设置scope，默认是singleton
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);
// 生成代理类信息
definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
beanDefinitions.add(definitionHolder);
// 注册到Spring容器
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
对应时序图方法2，ClassPathBeanDefinitionScanner#doScan。该方法对包下class文件解析，符合Spring容器管理的类生成BeanDefinition，并注册到容器中。
扫描包下的class文件，把有Component注解的封装BeanDefinition列表返回。
public Set<BeanDefinition> findCandidateComponents(String basePackage) {
if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
}
else {
return scanCandidateComponents(basePackage);
}
}
对应时序图方法3，ClassPathScanningCandidateComponentProvider#findCandidateComponents。
private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
Set<BeanDefinition> candidates = new LinkedHashSet<>();
try {
// classpath*:basePackage/**/*.class
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage(basePackage) + \'/\' + this.resourcePattern;
// 获取 basePackage 包下的 .class 文件资源
Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
for (Resource resource : resources) {
// 判断是否可读
if (resource.isReadable()) {
try {
// 获取.class文件类信息
MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
if (isCandidateComponent(metadataReader)) {
ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
sbd.setResource(resource);
sbd.setSource(resource);
if (isCandidateComponent(sbd)) {
candidates.add(sbd);
}
}
} catch (Throwable ex) {
throw new BeanDefinitionStoreException("Failed to read candidate component class: " + resource, ex);
}
}
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
}
return candidates;
}
对应时序图方法4，ClassPathScanningCandidateComponentProvider#scanCandidateComponents。
public MetadataReader getMetadataReader(Resource resource) throws IOException {
if (this.metadataReaderCache instanceof ConcurrentMap) {
// No synchronization necessary...
MetadataReader metadataReader = this.metadataReaderCache.get(resource);
if (metadataReader == null) {
// 获取.class类元信息
metadataReader = super.getMetadataReader(resource);
this.metadataReaderCache.put(resource, metadataReader);
}
return metadataReader;
}
else if (this.metadataReaderCache != null) {
synchronized (this.metadataReaderCache) {
MetadataReader metadataReader = this.metadataReaderCache.get(resource);
if (metadataReader == null) {
metadataReader = super.getMetadataReader(resource);
this.metadataReaderCache.put(resource, metadataReader);
}
return metadataReader;
}
}
else {
return super.getMetadataReader(resource);
}
}
对应时序图方法5，CachingMetadataReaderFactory#getMetadataReader。 super.getMetadataReader(resource) 调用的是 SimpleMetadataReaderFactory#getMetadataReader。
public MetadataReader getMetadataReader(Resource resource) throws IOException {
// 默认是SimpleMetadataReader实例
return new SimpleMetadataReader(resource, this.resourceLoader.getClassLoader());
}
SimpleMetadataReader(Resource resource, @Nullable ClassLoader classLoader) throws IOException {
// 加载.class文件
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);
// 解析.class元信息
classReader.accept(visitor, ClassReader.SKIP_DEBUG);
this.annotationMetadata = visitor;
this.classMetadata = visitor;
this.resource = resource;
}
对应时序图方法6，SimpleMetadataReader#SimpleMetadataReader。 组装SimpleMetadataReader。
public void accept(
final ClassVisitor classVisitor,
final Attribute[] attributePrototypes,
final int parsingOptions) {
Context context = new Context();
context.attributePrototypes = attributePrototypes;
context.parsingOptions = parsingOptions;
context.charBuffer = new char[maxStringLength];
... 省略代码
// Visit the RuntimeVisibleAnnotations attribute.
if (runtimeVisibleAnnotationsOffset != 0) {
int numAnnotations = readUnsignedShort(runtimeVisibleAnnotationsOffset);
int currentAnnotationOffset = runtimeVisibleAnnotationsOffset + 2;
while (numAnnotations-- > 0) {
// Parse the type_index field.
String annotationDescriptor = readUTF8(currentAnnotationOffset, charBuffer);
currentAnnotationOffset += 2;
// 这里面封装Spring Component注解
currentAnnotationOffset =
readElementValues(classVisitor.visitAnnotation(annotationDescriptor,true),
currentAnnotationOffset,true,charBuffer);
}
}
... 省略代码
}
对应时序图方法7，ClassReader#accept。该方法把二进制的.class文件解析组装到AnnotationMetadataReadingVisitor
private int readElementValues(
final AnnotationVisitor annotationVisitor,
final int annotationOffset,
final boolean named,
final char[] charBuffer) {
... 省略代码
if (annotationVisitor != null) {
// 主要逻辑还在这里面
annotationVisitor.visitEnd();
}
return currentOffset;
}
对应时序图方法8，ClassReader#readElementValues。
public void visitEnd() {
super.visitEnd();
Class<? extends Annotation> annotationClass = this.attributes.annotationType();
if (annotationClass != null) {
... 省略代码
// 过滤java.lang.annotation包下的注解，及保留Spring注解
if (!AnnotationUtils.isInJavaLangAnnotationPackage(annotationClass.getName())) {
try {
// 获取该类上的所有注解
Annotation[] metaAnnotations = annotationClass.getAnnotations();
if (!ObjectUtils.isEmpty(metaAnnotations)) {
Set<Annotation> visited = new LinkedHashSet<>();
for (Annotation metaAnnotation : metaAnnotations) {
// 过滤java.lang.annotation包下的注解，及保留Spring注解
recursivelyCollectMetaAnnotations(visited, metaAnnotation);
}
// 封装需要的注解
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) {
}
}
}
}
对应时序图方法9，AnnotationAttributesReadingVisitor#visitEnd。过滤掉 java.lang.annotation 包下的注解，然后把剩下的注解放到metaAnnotationMap。
protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {
for (TypeFilter tf : this.excludeFilters) {
if (tf.match(metadataReader, getMetadataReaderFactory())) {
return false;
}
}
for (TypeFilter tf : this.includeFilters) {
if (tf.match(metadataReader, getMetadataReaderFactory())) {
return isConditionMatch(metadataReader);
}
}
return false;
}
对应时序图方法10，ClassPathScanningCandidateComponentProvider#isCandidateComponent。使用前面提过的ClassPathBeanDefinitionScanner初始化时设置的注解类型过滤器，includeFilters 包含ManagedBean和Component类型。
public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory)
throws IOException {
if (matchSelf(metadataReader)) {
return true;
}
... 省略代码
return false;
}
对应时序图方法11，AbstractTypeHierarchyTraversingFilter#match。
protected boolean matchSelf(MetadataReader metadataReader) {
AnnotationMetadata metadata = metadataReader.getAnnotationMetadata();
return metadata.hasAnnotation(this.annotationType.getName()) ||
(this.considerMetaAnnotations && metadata.hasMetaAnnotation(this.annotationType.getName()));
}
对应时序图方法12，AnnotationTypeFilter#matchSelf。判断类的metadata中是否包含Component。
总结@Component到Spring bean容器管理过程。第一步，初始化时设置了Component类型过滤器；第二步，根据指定扫描包扫描.class文件，生成Resource对象；第三步、解析.class文件并注解归类，生成MetadataReader对象；第四步、使用第一步的注解过滤器过滤出有@Component类；第五步、生成BeanDefinition对象；第六步、把BeanDefinition注册到Spring容器。以上是@Component注解原理，@Service、@Controller和@Repository上都有@Component修饰，所以原理是一样的。