@ComponentScan

简介（What）

@ComponentScan注解相信大家再熟悉不过了，该注释可以将标记有@Component或其扩展注解如@Service、@Controller等类当成Spring的组件注册到Spring容器中，实现了类似<context:component-scan>标签的组件扫描指令，大大地减少了<xml>配置文件的数量。

功能（Features）

时机（When）

How

@ComponentScan由ComponentScanAnnotationParser类提供解析处理，方法为parse()。

package org.springframework.context.annotation;

class ComponentScanAnnotationParser {

    public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, final String declaringClass) {
        ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
                componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);

        Class<? extends BeanNameGenerator> generatorClass = componentScan.getClass("nameGenerator");
        boolean useInheritedGenerator = (BeanNameGenerator.class == generatorClass);
        scanner.setBeanNameGenerator(useInheritedGenerator ? this.beanNameGenerator :
                BeanUtils.instantiateClass(generatorClass));

        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));
        }

        scanner.setResourcePattern(componentScan.getString("resourcePattern"));

        for (AnnotationAttributes filter : componentScan.getAnnotationArray("includeFilters")) {
            for (TypeFilter typeFilter : typeFiltersFor(filter)) {
                scanner.addIncludeFilter(typeFilter);
            }
        }
        for (AnnotationAttributes filter : componentScan.getAnnotationArray("excludeFilters")) {
            for (TypeFilter typeFilter : typeFiltersFor(filter)) {
                scanner.addExcludeFilter(typeFilter);
            }
        }

        boolean lazyInit = componentScan.getBoolean("lazyInit");
        if (lazyInit) {
            scanner.getBeanDefinitionDefaults().setLazyInit(true);
        }

        // 解析要扫描的包，取 basePackages 和 basePackageClasses 指定的包的合集，如果未指定，则取 declaringClass 所在的包。
        Set<String> basePackages = new LinkedHashSet<>();
        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);
        }
        for (Class<?> clazz : componentScan.getClassArray("basePackageClasses")) {
            basePackages.add(ClassUtils.getPackageName(clazz));
        }
        if (basePackages.isEmpty()) {
            basePackages.add(ClassUtils.getPackageName(declaringClass));
        }

        scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {

            @Override
            protected boolean matchClassName(String className) {
                return declaringClass.equals(className);
            }
        });
        return scanner.doScan(StringUtils.toStringArray(basePackages));
    }

}

ClassPathBeanDefinitionScanner

第6-7行声明了一个ClassPathBeanDefinitionScanner实例，并将@ComponentScan的useDefaultFilters的值（默认为true）传入。

ClassPathBeanDefinitionScanner scanner=new ClassPathBeanDefinitionScanner(this.registry,
        componentScan.getBoolean("useDefaultFilters"),this.environment,this.resourceLoader);

ClassPathBeanDefinitionScanner通过registerDefaultFilters()方法注册默认的TypeFilter。

AnnotationTypeFilter(Component.class)覆盖了@Component及其扩展，如Repository、@Service、@Controller等。

public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {

    @SuppressWarnings("unchecked")
    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.
        }
    }

}

扫描包

41-53行解析扫描包的范围由属性basePackages和basePackageClasses指定的包的合集，如果未指定，则为该注解声明类所在的包。

doScan

Options

option 类型 说明 默认
value String[]   {}
basePackages String[]   {}
basePackageClasses Class[]   {}
nameGenerator      
scopeResolver      
scopedProxy      
resourcePattern String 资源表达式 **/*.class
useDefaultFilters boolean 使用默认过滤器 true
includeFilters @Filter 包含过滤器  
excludeFilters @Filter 排除过滤器  
lazyInit boolean 懒加载 false