mvc:annotation-driven

<mvc:annotation-driven />这个标签注册了Spring MVC分发请求到Controller控制器所必须的HandlerMapping和HandlerAdapter实例。

Spring 3.1 之前:

1. DefaultAnnotationHandlerMapping
2. AnnotationMethodHandlerAdapter
3. AnnotationMethodHandlerExceptionResolver

Spring 3.1+ ：

1. RequestMappingHandlerMapping
2. RequestMappingHandlerAdapter
3. ExceptionHandlerExceptionResolver

DefaultAnnotationHandlerMapping 根据url决定使用哪个controller，AnnotationMethodHandlerAdapter 选择处理请求的实际方法。RequestMappingHandlerMapping包涵上面2个作用，所以将请求直接映射到方法。

源码分析

DispatcherServlet初始化的时候，会调用一个方法如下：

protected void initStrategies(ApplicationContext context) {  
        initMultipartResolver(context);  
        initLocaleResolver(context);  
        initThemeResolver(context);  
        //初始化一些HandlerMapping  
        initHandlerMappings(context);  
        //初始化一些HandlerAdapter  
        initHandlerAdapters(context);  
        initHandlerExceptionResolvers(context);  
        initRequestToViewNameTranslator(context);  
        initViewResolvers(context);  
        initFlashMapManager(context);  
    }

它会初始化一些HandlerMapping和HandlerAdapter，这两个方法非常重要

• 初始化HandlerMapping

private void initHandlerMappings(ApplicationContext context) {  
        this.handlerMappings = null;  
  
        if (this.detectAllHandlerMappings) {  
            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.  
            Map<String, HandlerMapping> matchingBeans =  
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);  
            if (!matchingBeans.isEmpty()) {  
                this.handlerMappings = new ArrayList<HandlerMapping>(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");  
            }  
        }  
    }

detectAllHandlerMappings是DispatcherServlet的一个属性，可以在web.xml中配置，默认是true，如果为true,则会去从本工程mvc-servlet.xml文件中去探测所有实现了HandlerMapping的bean，如果有，则加入DispatcherServlet的handlerMappings中。如果detectAllHandlerMappings为false,则直接去容器中找id="handlerMapping"且实现了HandlerMapping的bean.如果以上都没找到，则会去加载默认的HandlerMapping。

/** Detect all HandlerMappings or just expect "handlerMapping" bean? */  
private boolean detectAllHandlerMappings = true;

如果没有配置HandlerMapping,所以它会去加载默认的，默认的配置如下：

protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {  
        String key = strategyInterface.getName();  
        //defaultStrategies存储了默认的配置  
        String value = defaultStrategies.getProperty(key);  
        if (value != null) {  
            String[] classNames = StringUtils.commaDelimitedListToStringArray(value);  
            List<T> strategies = new ArrayList<T>(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<T>();  
        }  
    }

看看defaultStrategies是如何初始化的：

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 {  
            //这里的DEFAULT_STRATEGIES_PATH就是DispatcherServlet.properties  
            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());  
        }  
    }

这里使用静态代码块来加载配置文件DispatcherServlet.properties，它所在位置就是和DispatcherServlet同一目录下面。默认的配置文件的内容如下：

#这里就是默认的HandlerMapping的配置  
org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\  
    org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping  
#这里就是默认的HandlerAdapter的配置  
org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\  
    org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\  
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter  
  
org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\  
    org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\  
    org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

也就是说，当你什么都没有配置时，默认会加载以上的配置

默认HandlerMapping:

BeanNameUrlHandlerMapping
DefaultAnnotationHandlerMapping

默认HandlerAdapter:

HttpRequestHandlerAdapter
SimpleControllerHandlerAdapter
AnnotationMethodHandlerAdapter

参考：

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/mvc.html

http://shouce.jb51.net/spring/mvc.html