springmvc进阶（4）：mvc:annotation-driven详解

在spring中所有的自定义命名空间（像mvc，context等）下的标签解析都是由BeanDefinitionParser接口的子类来完成的。那么就找到标签的解析类，看看是如何解析的。

BeanDefinitionParser接口的实现类:

我们看到有多个AnnotationDrivenBeanDefinitionParser，他们是用来处理不同命名空间下的标签的，我们今天研究的是标签，所以我们找到对应的实现类是org.springframework.web.servlet.config.AnnotationDrivenBeanDefinitionParser

对于一个陌生的类，首先要先去看它的Javadoc:

 * A {@link BeanDefinitionParser} that provides the configuration for the
 * {@code } MVC namespace element.
 *  * 
This class registers the following {@link HandlerMapping}s:
 * 

 * 
{@link RequestMappingHandlerMapping}
 * ordered at 0 for mapping requests to annotated controller methods.
 * 
{@link BeanNameUrlHandlerMapping}
 * ordered at 2 to map URL paths to controller bean names.
 * 
 *  * 
Note: Additional HandlerMappings may be registered
 * as a result of using the {@code } or the
 * {@code } MVC namespace elements.
 *  * 
This class registers the following {@link HandlerAdapter}s:
 * 

 * 
{@link RequestMappingHandlerAdapter}
 * for processing requests with annotated controller methods.
 * 
{@link HttpRequestHandlerAdapter}
 * for processing requests with {@link HttpRequestHandler}s.
 * 
{@link SimpleControllerHandlerAdapter}
 * for processing requests with interface-based {@link Controller}s.
 * 
 *  * 
This class registers the following {@link HandlerExceptionResolver}s:
 * 

 * 
{@link ExceptionHandlerExceptionResolver} for handling exceptions
 * through @{@link ExceptionHandler} methods.
 * 
{@link ResponseStatusExceptionResolver} for exceptions annotated
 * with @{@link ResponseStatus}.
 * 
{@link DefaultHandlerExceptionResolver} for resolving known Spring
 * exception types
 * 
 *  * 
This class registers an {@link org.springframework.util.AntPathMatcher}
 * and a {@link org.springframework.web.util.UrlPathHelper} to be used by:
 * 

 * 
the {@link RequestMappingHandlerMapping},
 * 
the {@link HandlerMapping} for ViewControllers
 * 
and the {@link HandlerMapping} for serving resources
 * 
 * Note that those beans can be configured by using the {@code path-matching} MVC namespace element.
 *  * 
Both the {@link RequestMappingHandlerAdapter} and the
 * {@link ExceptionHandlerExceptionResolver} are configured with instances of
 * the following by default:
 * 

 * 
A {@link ContentNegotiationManager}
 * 
A {@link DefaultFormattingConversionService}
 * 
A {@link org.springframework.validation.beanvalidation.LocalValidatorFactoryBean}
 * if a JSR-303 implementation is available on the classpath
 * 
A range of {@link HttpMessageConverter}s depending on what 3rd party
 * libraries are available on the classpath.
 * 

该注释文档说明该解析器向容器中注入了如下组件：

• HandlerMapping ：请求映射
  1、RequestMappingHandlerMapping
       支持@RequestMapping注解

  2、BeanNameUrlHandlerMapping
       将controller类的名字映射为请求url

• HandlerAdapter : 请求处理
  1、RequestMappingHandlerAdapter
       处理@Controller和@RequestMapping注解的处理器
  2、HttpRequestHandlerAdapter
        处理继承了HttpRequestHandler创建的处理器
  3、SimpleControllerHandlerAdapter
       处理继承自Controller接口的处理器

• ExceptionResolver：处理异常的解析器
  1、ExceptionHandlerExceptionResolver
  2、ResponseStatusExceptionResolver
  3、DefaultHandlerExceptionResolver

代码如下：

@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
    ...
    parserContext.registerComponent(new BeanComponentDefinition(handlerMappingDef, methodMappingName));
    parserContext.registerComponent(new BeanComponentDefinition(handlerAdapterDef, handlerAdapterName));
    parserContext.registerComponent(new BeanComponentDefinition(uriCompContribDef, uriCompContribName));
    parserContext.registerComponent(new BeanComponentDefinition(exceptionHandlerExceptionResolver, methodExceptionResolverName));
    parserContext.registerComponent(new BeanComponentDefinition(responseStatusExceptionResolver, responseStatusExceptionResolverName));
    parserContext.registerComponent(new BeanComponentDefinition(defaultExceptionResolver, defaultExceptionResolverName));
    parserContext.registerComponent(new BeanComponentDefinition(mappedCsInterceptorDef, mappedInterceptorName));

    // Ensure BeanNameUrlHandlerMapping (SPR-8289) and default HandlerAdapters are not "turned off"
    MvcNamespaceUtils.registerDefaultComponents(parserContext, source);

    parserContext.popAndRegisterContainingComponent();

    ...
}

上面省略大量代码,这里只贴出部分注册组件的代码部分。
其中

// Ensure BeanNameUrlHandlerMapping (SPR-8289) and default HandlerAdapters are not "turned off"
MvcNamespaceUtils.registerDefaultComponents(parserContext, source);

这个将容器默认mapping和adapter加进来：

public static void registerDefaultComponents(ParserContext parserContext, Object source) {
    registerBeanNameUrlHandlerMapping(parserContext, source);
    registerHttpRequestHandlerAdapter(parserContext, source);
    registerSimpleControllerHandlerAdapter(parserContext, source);
}

总结
这里来说说配置开启的两个常用的组件，RequestMappingHandlerMapping和RequestMappingHandlerAdapter。

• RequestMappingHandlerMapping是HandlerMapping的实现类，它会在容器启动的时候，扫描容器内的bean,解析带有@RequestMapping
  注解的方法，并将其解析为url和handlerMethod键值对方式注册到请求映射表中。
• RequestMappingHandlerAdapter是HandlerAdapter的实现类，它是处理请求的适配器，说白了，就是确定调用哪个类的哪个方法，并且构造方法参数，返回值。

像其实标签是告诉Spring容器来扫描指定包下的类，并注册被@Component，@Controller，@Service，@Repository等注解标记的组件相似。
是告知Spring容器，我们启用注解驱动，支持@RequestMapping注解,这样我们就可以使用@RequestMapping来配置处理器。