當我們需要使用BeanPostProcessor時，直接在Spring配置文件中定義這些Bean顯得比較笨拙，例如：
使用@Autowired注解，必須事先在Spring容器中聲明AutowiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor "/>
使用 @Required注解，就必須聲明RequiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>
類似地，使用@Resource、@PostConstruct、@PreDestroy等注解就必須聲明 CommonAnnotationBeanPostProcessor；使用@PersistenceContext注解，就必須聲明 PersistenceAnnotationBeanPostProcessor的Bean。
這樣的聲明未免太不優雅，而Spring為我們提供了一種極為方便注冊這些BeanPostProcessor的方式，即使用<context:annotation- config/>隱式地向 Spring容器注冊AutowiredAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor以及PersistenceAnnotationBeanPostProcessor這4個BeanPostProcessor。如下：
<context:annotation-config/>
另，在我們使用注解時一般都會配置掃描包路徑選項：
<context:component-scan base-package="pack.pack"/>
該配置項其實也包含了自動注入上述processor的功能，因此當使用<context:component-scan/>后，即可將<context:annotation-config/>省去。
<mvc:annotation-driven/>
相當於注冊了DefaultAnnotationHandlerMapping和AnnotationMethodHandlerAdapter兩個bean，配置一些messageconverter。即解決了@Controller注解的使用前提配置。
<mvc:annotation-driven/> is a tag added in Spring 3.0 which does the following:
1. Configures the Spring 3 Type ConversionService (alternative to PropertyEditors)
2. Adds support for formatting Number fields with @NumberFormat
3. Adds support for formatting Date, Calendar, and Joda Time fields with @DateTimeFormat, if Joda Time is on the classpath
4. Adds support for validating @Controller inputs with @Valid, if a JSR-303 Provider is on the classpath
5. Adds support for support for reading and writing XML, if JAXB is on the classpath (HTTP message conversion with @RequestBody/@ResponseBody)
6. Adds support for reading and writing JSON, if Jackson is o n the classpath (along the same lines as #5)