当我们需要使用BeanPostProcessor时，直接在Spring配置文件中定义这些Bean显得比较笨拙，例如：
使用@Autowired注解，必须事先在Spring容器中声明AutowiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor "/>
使用 @Required注解，就必须声明RequiredAnnotationBeanPostProcessor的Bean：
<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>
类似地，使用@Resource、@PostConstruct、@PreDestroy等注解就必须声明 CommonAnnotationBeanPostProcessor；使用@PersistenceContext注解，就必须声明 PersistenceAnnotationBeanPostProcessor的Bean。
这样的声明未免太不优雅，而Spring为我们提供了一种极为方便注册这些BeanPostProcessor的方式，即使用<context:annotation- config/>隐式地向 Spring容器注册AutowiredAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor以及PersistenceAnnotationBeanPostProcessor这4个BeanPostProcessor。如下：
<context:annotation-config/>
另，在我们使用注解时一般都会配置扫描包路径选项：
<context:component-scan base-package="pack.pack"/>
该配置项其实也包含了自动注入上述processor的功能，因此当使用<context:component-scan/>后，即可将<context:annotation-config/>省去。
<mvc:annotation-driven/>
相当于注册了DefaultAnnotationHandlerMapping和AnnotationMethodHandlerAdapter两个bean，配置一些messageconverter。即解决了@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)