SpringMVC的DispatcherServlet的默认策略

在使用SpringMVC的时候，我们知道需要HandlerMapping定义请求路径与处理器之间的映射，需要HandlerAdapter来调用处理器方法并返回一个ModelAndView对象，需要ViewResolver来解析视图。这些是SpringMVC中最基本的接口。通常我们都需要在SpringMVC的配置文件中定义好需要使用的HandlerMapping、HandlerAdapter和ViewResolver，基于注解的SpringMVC配置也是类似的。所以一般我们的SpringMVC配置文件会是如下这样：

mvc命名空间是Spring提供的一个简化SpringMVC配置的命名空间，<mvc:annotation-driven/>会自动在Spring的bean容器中注册HandlerMapping接口实现RequestMappingHandlerMapping类型的bean和HandlerAdapter接口实现类RequestMappingHandlerAdapter类型的bean。<mvc:view-resolvers/>是用来快速定义ViewResolver实现的，其中<mvc:jsp/>会自动定义一个InternalResourceViewResolver类型的ViewResolver。

针对这些比较基本接口，即使我们不定义它们的实现，SpringMVC内部也给了默认的定义，它管这些叫策略。SpringMVC把这些默认策略都定义在一个叫DispatcherServlet.properties的文件中，它与DispatcherServlet在同一个包中。以下是笔者在使用的4.1.0版本中的DispatcherServlet.properties文件中的定义。

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolverorg.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolverorg.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\	org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMappingorg.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\	org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\	org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapterorg.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\	org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\	org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolverorg.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslatororg.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolverorg.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

从上面的定义中我们可以看出来，在使用SpringMVC时，即使你不定义HandlerMapping，SpringMVC默认也会给你定义一个BeanNameUrlHandlerMapping和DefaultAnnotationHandlerMapping。HandlerAdapter和ViewResolver也是一样的。还有一些其它的默认策略也请参考DispatcherServlet.properties中。如果默认策略不能满足你的要求，那么我们可以在bean容器中定义我们自己对应的实现，这个时候就会应用我们自己的实现了。我们拿初始化HandlerMapping为例，来看一下DispatcherServlet的源码。

/** * Initialize the HandlerMappings used by this class. * 
If no HandlerMapping beans are defined in the BeanFactory for this namespace, * we default to BeanNameUrlHandlerMapping. */private void initHandlerMappings(ApplicationContext context) {    this.handlerMappings = null;    if (this.detectAllHandlerMappings) {        // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.        Map
      
        matchingBeans =                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);        if (!matchingBeans.isEmpty()) {            this.handlerMappings = new ArrayList
       
        (matchingBeans.values());            // We keep HandlerMappings in sorted order.            OrderComparator.sort(this.handlerMappings);        }    }    else {        try {            HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);            this.handlerMappings = Collections.singletonList(hm);        }        catch (NoSuchBeanDefinitionException ex) {            // Ignore, we'll add a default HandlerMapping later.        }    }    // Ensure we have at least one HandlerMapping, by registering    // a default HandlerMapping if no other mappings are found.    if (this.handlerMappings == null) {        this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);        if (logger.isDebugEnabled()) {            logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");        }    }}
       
      

从源码中我们可以看到，SpringMVC先会从绑定的ApplicationContext中获取对应的HandlerMapping定义，如果没有取到就会调用getDefaultStrategies(context, HandlerMapping.class)从默认策略中获取。它的代码如下所示。

protected 
     
       List
      
        getDefaultStrategies(ApplicationContext context, Class
       
         strategyInterface) {    String key = strategyInterface.getName();    String value = defaultStrategies.getProperty(key);    if (value != null) {        String[] classNames = StringUtils.commaDelimitedListToStringArray(value);        List
        
          strategies = new ArrayList
         
          (classNames.length);        for (String className : classNames) {            try {                Class
           clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());                Object strategy = createDefaultStrategy(context, clazz);                strategies.add((T) strategy);            }            catch (ClassNotFoundException ex) {                throw new BeanInitializationException(                        "Could not find DispatcherServlet's default strategy class [" + className +                                "] for interface [" + key + "]", ex);            }            catch (LinkageError err) {                throw new BeanInitializationException(                        "Error loading DispatcherServlet's default strategy class [" + className +                                "] for interface [" + key + "]: problem with class file or dependent class", err);            }        }        return strategies;    }    else {        return new LinkedList
          
           (); }}
          
         
        
       
      
     

其中的defaultStrategies就是对应的DispatcherServlet.properties文件中的内容。

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";private static final Properties defaultStrategies;static {    // Load default strategy implementations from properties file.    // This is currently strictly internal and not meant to be customized    // by application developers.    try {        ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);        defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);    }    catch (IOException ex) {        throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());    }}

（注：本文是基于SpringMVC4.1.0所写）