ClassPathBeanDefinitionScanner继承自ClassPathScanningCandidateComponentProvider，构造时要求指定一个BeanDefinitionRegistry对象，其扩展了一个scan方法，可以同时指定多个要扫描的包。底层在扫描bean定义时还是使用的父类的findCandidateComponents方法，但是扫描后会自动利用持有的BeanDefinitionRegistry自动对bean定义进行注册。注册bean定义的bean名称会使用持有的BeanNameGenerator生成，默认是AnnotationBeanNameGenerator；如果对应的bean定义是AnnotatedBeanDefinition类型的，还会处理对应的一些注解定义。而使用ClassPathScanningCandidateComponentProvider时我们只能获取到对应的bean定义，另外的bean注册等还需要我们来做。
ClassPathBeanDefinitionScanner的核心代码如下：
public int scan(String... basePackages) {
int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
doScan(basePackages);
// Register annotation config processors, if necessary.
if (this.includeAnnotationConfig) {
AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}
return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
for (String basePackage : basePackages) {
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;
}
假设现有HelloOne和HelloTwo两个类，分别位于包com.elim.learn.spring.bean.registry.one和com.elim.learn.spring.bean.registry.two下，它们类上都标注了HelloAnnotation注解，如果现在需要只扫描包com.elim.learn.spring.bean.registry.one和com.elim.learn.spring.bean.registry.two下类上拥有HelloAnnotation注解的类作为bean，则可以定义如下BeanDefinitionRegistryPostProcessor进行bean扫描。
public class CustomBeanDefinitionRegistry implements BeanDefinitionRegistryPostProcessor {
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
}
@Override
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(registry);
TypeFilter includeFilter = new AnnotationTypeFilter(HelloAnnotation.class);
scanner.addIncludeFilter(includeFilter);
String[] basePackages = {"com.elim.learn.spring.bean.registry.one", "com.elim.learn.spring.bean.registry.two"};
scanner.scan(basePackages);
}
}
测试代码如下：
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes={CustomBeanDefinitionRegistry.class})
public class CustomBeanDefinitionRegistryTest {
@Autowired
private ApplicationContext applicationContext;
@Test
public void assertBean() {
Assert.assertNotNull(this.applicationContext.getBean(HelloOne.class));
Assert.assertNotNull(this.applicationContext.getBean(HelloTwo.class));
}
}
关于ClassPathBeanDefinitionScanner的更多信息请参考相应的API文档或源码。
（本文由Elim写于2017年9月29日）
