SpringMVC之细说HandlerMapping

简单来说就是用来存储所有URL与处理类的map关系，以及在请求过来的时候根据输入URL
匹配到对应的处理类

第一步：

DefaultAnnotationHandlerMapping继承了AbstractDetectingUrlHandlerMapping，
在AbstractDetectingUrlHandlerMapping中遍历所有的DispatcherServlet配置文件中的
所有bean，找到相应的URL并进行缓存

protected void detectHandlers() throws BeansException {

        if (logger.isDebugEnabled()) {

        logger.debug("Looking for URL mappings in application context: " 
     
                            + getApplicationContext());

        }

        //获取了所有的bean，值得注意的是此处仅取出所有DispatcherServlet相关配置
              //文件涉及的所有类

        String[] beanNames = (this.detectHandlersInAncestorContexts ?

        BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), 
                                                            Object.class) :

        getApplicationContext().getBeanNamesForType(Object.class));

        // 遍历

        for (String beanName : beanNames) {

        //拿到Controller 上配置的所有URL

        String[] urls = determineUrlsForHandler(beanName);

        if (!ObjectUtils.isEmpty(urls)) {

        // 将所有的 url与controller对应关系存储下来

        registerHandler(urls, beanName);

        }else {

        if (logger.isDebugEnabled()) {

        logger.debug("Rejected bean name '" + beanName + "': no URL paths identified");

        }

        }

        }

        }

第二步：

看一下这个类：DefaultAnnotationHandlerMapping，其中

determineUrlsForHandler(String beanName)和
determineUrlsForHandlerMethods(Class<?> handlerType, 
final boolean hasTypeLevelMapping)

解析bean的@RequestMapping，

protected String[] determineUrlsForHandler(String beanName) {

        ApplicationContext context = getApplicationContext();

        Class<?> handlerType = context.getType(beanName);

        //找到对应bean 类级别标注的所有的 @RequestMapping

        RequestMapping mapping = context.findAnnotationOnBean(beanName, RequestMapping.class);

        if (mapping != null) {

        this.cachedMappings.put(handlerType, mapping);

        Set<String> urls = new LinkedHashSet<String>();

        String[] typeLevelPatterns = mapping.value();

        if (typeLevelPatterns.length > 0) {

        // 找到对应bean 方法级别的所有 @RequestMapping 

        String[] methodLevelPatterns = determineUrlsForHandlerMethods(handlerType, true);

        for (String typeLevelPattern : typeLevelPatterns) {

        if (!typeLevelPattern.startsWith("/")) {

        typeLevelPattern = "/" + typeLevelPattern;

        }

        boolean hasEmptyMethodLevelMappings = false;

        for (String methodLevelPattern : methodLevelPatterns) {

        if (methodLevelPattern == null) {

        hasEmptyMethodLevelMappings = true;

        }else {

        String combinedPattern = getPathMatcher().combine(typeLevelPattern, methodLevelPattern);

        //将类级别@RequestMapping和方法级别的@RequestMapping组合放入list 

        addUrlsForPath(urls, combinedPattern);

        }

        }

        if (hasEmptyMethodLevelMappings ||

        org.springframework.web.servlet.mvc.Controller.class.isAssignableFrom(handlerType)) {

        addUrlsForPath(urls, typeLevelPattern);

        }

        }

        return StringUtils.toStringArray(urls);

        }else {

        // actual paths specified by @RequestMapping at method level

        return determineUrlsForHandlerMethods(handlerType, false);

        }

        }

        else if (AnnotationUtils.findAnnotation(handlerType, Controller.class) != null) {

        // @RequestMapping to be introspected at method level

        return determineUrlsForHandlerMethods(handlerType, false);

        }else {

        return null;

        }

        }

第三步：

AbstractUrlHandlerMapping.registerHandler(String urlPath, Object handler)

将url 和 controller放入缓存，后续处理url请求时将会用到。

protected void registerHandler(String urlPath, Object handler) 
                   throws BeansException, IllegalStateException {

    Assert.notNull(urlPath, "URL path must not be null");

    Assert.notNull(handler, "Handler object must not be null");

    Object resolvedHandler = handler;

    if (!this.lazyInitHandlers && handler instanceof String) {

        String handlerName = (String)handler;

        if (this.getApplicationContext().isSingleton(handlerName)) {

            resolvedHandler = this.getApplicationContext().getBean(handlerName);

        }

    }

    Object mappedHandler = this.handlerMap.get(urlPath);

    if (mappedHandler != null) {

        if (mappedHandler != resolvedHandler) {

            throw new IllegalStateException("Cannot map "  
                       this.getHandlerDescription(handler) + 
                       " to URL path [" + urlPath + "]: There is already " +
                       this.getHandlerDescription(mappedHandler) + " mapped.");

        }

    } else if (urlPath.equals("/")) {

        if (this.logger.isInfoEnabled()) {

            this.logger.info("Root mapping to " + this.getHandlerDescription(handler));

        }

        this.setRootHandler(resolvedHandler);

    } else if (urlPath.equals("/*")) {

        if (this.logger.isInfoEnabled()) {

            this.logger.info("Default mapping to " + this.getHandlerDescription(handler));

        }

        this.setDefaultHandler(resolvedHandler);

    } else {

        this.handlerMap.put(urlPath, resolvedHandler);

        if (this.logger.isInfoEnabled()) {

            this.logger.info("Mapped URL path [" + urlPath + "] onto " + 
                                       this.getHandlerDescription(handler));

        }

    }

}