spring mvc annotation-driven

使用spring mvc时，我们会在配置文件里添加<mvc:annotation-driven /><beans>
这个配置文件都加载了那些类呢？

1. Among others, registers: 
	* RequestMappingHandlerMapping 
	* RequestMappingHandlerAdapter 
	* ExceptionHandlerExceptionResolver 
in support of processing requests with annotated controller methods using annotations such as @RequestMapping , @ExceptionHandler, etc. 

2. Enables Spring 3 style type conversion through a ConversionService instance in addition to the JavaBeans PropertyEditors used for Data Binding. 

3. Enables support for formatting Number fields using the @NumberFormat annotation through the ConversionService. 

4. Enables support for formatting Date, Calendar, Long, and Joda Time fields using the @DateTimeFormat annotation, if Joda Time 1.3 or higher is present on the classpath. 

5. Enables support for validating @Controller inputs with @Valid, if a JSR-303 Provider is present on the classpath. 

6. Enables HttpMessageConverter support for @RequestBody method parameters and @ResponseBody method return values from @RequestMapping or @ExceptionHandler methods. 

This is the complete list of HttpMessageConverters set up by mvc:annotation-driven: 
* ByteArrayHttpMessageConverter converts byte arrays. 
* StringHttpMessageConverter converts strings. 
* ResourceHttpMessageConverter converts to/from org.springframework.core.io.Resource for all media types. 
* SourceHttpMessageConverter converts to/from a javax.xml.transform.Source. 
* FormHttpMessageConverter converts form data to/from a MultiValueMap<String, String>. 
* Jaxb2RootElementHttpMessageConverter converts Java objects to/from XML — added if JAXB2 is present on the classpath. This converter can read classes annotated with XmlRootElement and XmlType, and write classes annotated with withXmlRootElement, or subclasses thereof. 
* MappingJacksonHttpMessageConverter converts to/from JSON — added if Jackson is present on the classpath. 
* AtomFeedHttpMessageConverter converts Atom feeds — added if Rome is present on the classpath. 
* RssChannelHttpMessageConverter converts RSS feeds — added if Rome is present on the classpath. 

类似controller中ajax请求直接返回对象被组装成json格式，就是由于该配置文件注册了MappingJacksonHttpMessageConverter对象，当我们的工程引入了jackson相关的jar包，spring就可以自动将我们的返回对象封装成json对象输出给客户端。