@Component

What

@Component注解用于将指定的类标记为组件。

Usage

How

ClassPathBeanDefinitionScanner类提供了对@Component注释的解析支持。

在此之前，有必要先引入一下TypeFilter的概念，那么什么是TypeFilter呢？

TypeFilter

TypeFilter是一个类型过滤器接口，用于判定一个类的元数据是否满足条件，基定义如下：

package org.springframework.core.type.filter;

@FunctionalInterface
public interface TypeFilter {

    boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory) throws IOException;

}

几个重要的内置过滤器：

了解了TypeFilter的概念，现在，来看一下ClassPathBeanDefinitionScanner

ClassPathBeanDefinitionScanner

在ClassPathBeanDefinitionScanner构造方法中，当参数useDefaultFilters为true时，会调用registerDefaultFilters()方法，在ClassPathBeanDefinitionScanner (BeanDefinitionRegistry registry)构造方法中，useDefaultFilters也默认为true。

package org.springframework.context.annotation;

public class ClassPathBeanDefinitionScanner extends ClassPathScanningCandidateComponentProvider {

    public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry) {
        this(registry, true);
    }

    public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters) {
        this(registry, useDefaultFilters, getOrCreateEnvironment(registry));
    }

    public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
        Environment environment) {

        this(registry, useDefaultFilters, environment,
            (registry instanceof ResourceLoader ? (ResourceLoader) registry : null));
    }

    public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
        Environment environment, @Nullable ResourceLoader resourceLoader) {

        Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
        this.registry = registry;

        if (useDefaultFilters) {
            //注册默认的 TypeFilter's
            registerDefaultFilters();
        }
        setEnvironment(environment);
        setResourceLoader(resourceLoader);
    }

}

registerDefaultFilters()

在父类的registerDefaultFilters()方法中，注册了几个基于注释的过滤器AnnotationTypeFilter。 除了Spring的@Component注释，与其同时，还尝试注册JSR-250的@ManagedBean和JSR-330 的@Named。

package org.springframework.context.annotation;

public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {

    protected void registerDefaultFilters() {
        // 注册 @Component 注释过滤器
        this.includeFilters.add(new AnnotationTypeFilter(Component.class));
        ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
        try {
            // 注册 @ManagedBean 注释过滤器
            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 {
            // 注册 @Named 注释过滤器
            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.
        }
    }

}

scan(basePackages)