SpringBoot之@Component注解源码
@Component注解的作用
@Component注解标识的bean会注入到SpringBoot中，托管给SpringBoot。
使用@Component注解需要注意
@Component需要搭配@ComponentScan注解才可以生效。
@Component注解标识的bean什么时候注入SpringBoot？
我们知道@SpringBootApplication注解，这个组合注解就是使用了@ComponentScan注解，
1. 使用自动装配@ComponentScan注解，入口方法为invokeBeanFactoryPostProcessors()；
2. @ComponentScan注解扫描
3. 扫描出来的bean
Set<BeanDefinitionHolder> scannedBeanDefinitions =
this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
4. ComponentScanAnnotationParser.parse();方法
// 解析@ComponentScan注解，扫描注入bean
public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, String declaringClass) {
ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);
// bean名称生成器,在解析注册BeanDefinition的时候用到
Class<? extends BeanNameGenerator> generatorClass = componentScan.getClass("nameGenerator");
boolean useInheritedGenerator = (BeanNameGenerator.class == generatorClass);
scanner.setBeanNameGenerator(useInheritedGenerator ? this.beanNameGenerator :
BeanUtils.instantiateClass(generatorClass));
// 扫描到@Component组件是是否生成代理以及生成代理方式
ScopedProxyMode scopedProxyMode = componentScan.getEnum("scopedProxy");
if (scopedProxyMode != ScopedProxyMode.DEFAULT) {
scanner.setScopedProxyMode(scopedProxyMode);
}
else {
Class<? extends ScopeMetadataResolver> resolverClass = componentScan.getClass("scopeResolver");
scanner.setScopeMetadataResolver(BeanUtils.instantiateClass(resolverClass));
}
// 扫描路径时规则
scanner.setResourcePattern(componentScan.getString("resourcePattern"));
// 如果一个对象如果不加上@Component注解，但是在扫描注解上加上该类的名称，那么也会被扫描加载
for (AnnotationAttributes includeFilterAttributes : componentScan.getAnnotationArray("includeFilters")) {
List<TypeFilter> typeFilters = TypeFilterUtils.createTypeFiltersFor(includeFilterAttributes, this.environment,
this.resourceLoader, this.registry);
for (TypeFilter typeFilter : typeFilters) {
scanner.addIncludeFilter(typeFilter);
}
}
// 扫描到某个类时需要忽略它
for (AnnotationAttributes excludeFilterAttributes : componentScan.getAnnotationArray("excludeFilters")) {
List<TypeFilter> typeFilters = TypeFilterUtils.createTypeFiltersFor(excludeFilterAttributes, this.environment,
this.resourceLoader, this.registry);
for (TypeFilter typeFilter : typeFilters) {
scanner.addExcludeFilter(typeFilter);
}
}
// 标识扫描注册BeanDefinition后是否延迟初始化,默认false
boolean lazyInit = componentScan.getBoolean("lazyInit");
if (lazyInit) {
scanner.getBeanDefinitionDefaults().setLazyInit(true);
}
Set<String> basePackages = new LinkedHashSet<>();
// 要扫描的路径,如果为空,解析的时候会解析被@ComponentScan标注类的包路径
String[] basePackagesArray = componentScan.getStringArray("basePackages");
for (String pkg : basePackagesArray) {
String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
Collections.addAll(basePackages, tokenized);
}
// 包路径类,与basePackages互斥
for (Class<?> clazz : componentScan.getClassArray("basePackageClasses")) {
basePackages.add(ClassUtils.getPackageName(clazz));
}
// 获取启动类所在的包
if (basePackages.isEmpty()) {
basePackages.add(ClassUtils.getPackageName(declaringClass));
}
scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {
@Override
protected boolean matchClassName(String className) {
return declaringClass.equals(className);
}
});
// 在指定的基本包中执行扫描，返回注册的bean定义。
return scanner.doScan(StringUtils.toStringArray(basePackages));
}
5. ClassPathBeanDefinitionScanner.doScan方法，扫描basePackages路径下的bean
因为我没有配置@ComponentScan注解中的basePackages属性，所以默认扫描使用启动类所在的包下的bean
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basePackage包，获取所有配置了@Component注解的bean
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);
// 注册@Component注解标识的bean，将其放到beanDefinitionMap容器中
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
问题1，如何扫描到basePackage包下面的@Component注解的类？
这块的代码逻辑封装在findCandidateComponents(basePackage);方法中
public Set<BeanDefinition> findCandidateComponents(String basePackage) {
if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
}
else {
// 扫描@Component注解的类
return scanCandidateComponents(basePackage);
}
}
private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
// 搜集合适BeanDefinition的集合
Set<BeanDefinition> candidates = new LinkedHashSet<>();
try {
// 包路径 classpath*:com/sunpeiyu/visualweb/**/*.class
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage(basePackage) + '/' + this.resourcePattern;
// 获取包路径下面的所有文件
Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
boolean traceEnabled = logger.isTraceEnabled();
boolean debugEnabled = logger.isDebugEnabled();
// 遍历文件资源
for (Resource resource : resources) {
if (traceEnabled) {
logger.trace("Scanning " + resource);
}
try {
// 读取资源的元信息，本身资源信息resource，元注解信息annotationMetadata
MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
if (isCandidateComponent(metadataReader)) {
ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
sbd.setSource(resource);
// 判断当前资源是否为@Component注解标识的类
if (isCandidateComponent(sbd)) {
if (debugEnabled) {
logger.debug("Identified candidate component class: " + resource);
}
// 添加到搜集@Component注解的类的容器
candidates.add(sbd);
}
else {
if (debugEnabled) {
logger.debug("Ignored because not a concrete top-level class: " + resource);
}
}
}
else {
if (traceEnabled) {
logger.trace("Ignored because not matching any filter: " + resource);
}
}
}
catch (FileNotFoundException ex) {
if (traceEnabled) {
logger.trace("Ignored non-readable " + resource + ": " + ex.getMessage());
}
}
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;
}
问题2，如果根据启动类获取到当前启动类所在的包？
这块的代码逻辑封装在ClassUtils.getPackageName(declaringClass)方法中，拿到全路径启动类，然后字符串截取前面的包
类似@Component的其他注解
@Service、@Controller、@Repository注解
这些注解本身还是@Component注解，所以本身这些注解会走上面一样的方法进行注入bean到beanDefinitionMap中。
@Repository
@Service
@Controller
