SpringMVC学习笔记 --- 映射处理器(续)

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SpringMVC - 基础篇

如何根据http请求选择合适的controller是MVC中一项十分关键的功能，在Spring MVC中，HandlerMapping接口是这一活动的抽象。同时，Spring MVC也提供了一系列HandlerMapping的实现，根据一定的规则选择controller。如果当前的HandlerMappign实现中没有能够满足你所需要的规则是，可以通过实现HandlerMapping接口进行扩展。下面，就来看一下HandlerMapping相关的类图：

在Spring MVC中，关于HandlerMapping的使用，主要包括两个部分：注册和查找。在HandlerMapping的实现中，持有一个handlerMap这样一个HashMap<String, Object>，其中key是http请求的path信息，value可以是一个字符串，或者是一个处理请求的HandlerExecutionChain，如果是String类型，则会将其视为Spring的bean名称。在HandlerMapping对象的创建中，IoC容器执行了一个容器回调方法setApplicationContext，在这个方法中调用initApplicationContext方法进行初始化，各个子类可以根据需求的不同覆写这个方法。关于handlerMap信息的注册就是在initApplicationContext方法中被执行的。下面就来看一下注册url到controller映射信息的实现：

SimpleUrlHandlerMapping中的注册实现代码：

protected void registerHandlers(Map<String, Object> urlMap) throws BeansException {  
        if (urlMap.isEmpty()) {  
            logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");  
        }  
        else {  
//urlMap信息是根据配置文件注入进来的   
            for (Map.Entry<String, Object> entry : urlMap.entrySet()) {  
                String url = entry.getKey();  
                Object handler = entry.getValue();  
                // Prepend with slash if not already present.   
                if (!url.startsWith("/")) {  
                    url = "/" + url;  
                }  
                // Remove whitespace from handler bean name.   
                if (handler instanceof String) {  
                    handler = ((String) handler).trim();  
                }  
                registerHandler(url, handler);  
            }  
        }  
    }

AbstractDetectingUrlHandlerMapping中的注册实现代码：

protected void detectHandlers() throws BeansException {  
        if (logger.isDebugEnabled()) {  
            logger.debug("Looking for URL mappings in application context: " + getApplicationContext());  
        }  
        String[] beanNames = (this.detectHandlersInAncestorContexts ?  
                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :  
                getApplicationContext().getBeanNamesForType(Object.class));  
  
        // Take any bean name that we can determine URLs for.   
        for (String beanName : beanNames) {  
            String[] urls = determineUrlsForHandler(beanName);  
            if (!ObjectUtils.isEmpty(urls)) {  
                // URL paths found: Let's consider it a handler.   
                registerHandler(urls, beanName);  
            }  
            else {  
                if (logger.isDebugEnabled()) {  
                    logger.debug("Rejected bean name '" + beanName + "': no URL paths identified");  
                }  
            }  
        }  
    }

单个的映射关系注册是在registerHandler方法中执行的：

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;  
  
        // Eagerly resolve handler if referencing singleton via name.   
//如果给定的handeler是字符串，则认为是bean name，直接到IoC容器中取得bean instance   
        if (!this.lazyInitHandlers && handler instanceof String) {  
            String handlerName = (String) handler;  
            if (getApplicationContext().isSingleton(handlerName)) {  
                resolvedHandler = getApplicationContext().getBean(handlerName);  
            }  
        }  
  
        Object mappedHandler = this.handlerMap.get(urlPath);  
//同一path不能对应多个处理对象   
        if (mappedHandler != null) {  
            if (mappedHandler != resolvedHandler) {  
                throw new IllegalStateException(  
                        "Cannot map handler [" + handler + "] to URL path [" + urlPath +  
                        "]: There is already handler [" + resolvedHandler + "] mapped.");  
            }  
        }  
        else {  
            if (urlPath.equals("/")) {  
                if (logger.isInfoEnabled()) {  
                    logger.info("Root mapping to handler [" + resolvedHandler + "]");  
                }  
                setRootHandler(resolvedHandler);  
            }  
            else if (urlPath.equals("/*")) {  
                if (logger.isInfoEnabled()) {  
                    logger.info("Default mapping to handler [" + resolvedHandler + "]");  
                }  
                setDefaultHandler(resolvedHandler);  
            }  
            else {  
//将path和handler放在handlerMap中，保存了它们之间的映射关系   
                this.handlerMap.put(urlPath, resolvedHandler);  
                if (logger.isInfoEnabled()) {  
                    logger.info("Mapped URL path [" + urlPath + "] onto handler [" + resolvedHandler + "]");  
                }  
            }  
        }  
    }

通过以上代码，在HandlerMapping对象被创建的时候，即完成了url到handler之间的映射关系的注册。handlerMap已经被赋值，可以被使用了。

在完成url到handler映射关系的注册后，就可以使用getHandler方法，根据http请求获得handler对象了。下面，首先看一下getHandler方法的调用时序：

下面就主要关注一下上图中涉及到的方法的代码实现。

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {  
    Object handler = getHandlerInternal(request);  
               //如果没有找到匹配的handler，则使用默认handler   
    if (handler == null) {  
        handler = getDefaultHandler();  
    }  
    if (handler == null) {  
        return null;  
    }  
    // Bean name or resolved handler?   
               //如果给定的handler是字符串类型，则认为是bean name   
    if (handler instanceof String) {  
        String handlerName = (String) handler;  
        handler = getApplicationContext().getBean(handlerName);  
    }  
               //如果Handler是HandlerExecutionChain类型，则只是添加拦截器，否则会创建一个新的HandlerExecutionChain   
    return getHandlerExecutionChain(handler, request);  
}

将url信息与handler进行匹配查找的操作是在lookupHandler方法中执行的，下面就来看一下lookupHandler方法的实现：

//这个方法可能的返回值是HandlerExecutionChain对象或者是null   
//在HandlerExecutionChain对象中的handler，是根据handlerMap中取出来的bean name获得到的bean instance   
protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {  
        // Direct match?   
                //直接匹配   
        Object handler = this.handlerMap.get(urlPath);  
        if (handler != null) {  
            // Bean name or resolved handler?   
            if (handler instanceof String) {  
                String handlerName = (String) handler;  
                                //从IoC容器中取出handler   
                handler = getApplicationContext().getBean(handlerName);  
            }  
            validateHandler(handler, request);  
                        //创建一个HandlerExecutionChain对象并返回   
            return buildPathExposingHandler(handler, urlPath, urlPath, null);  
        }  
        // Pattern match?   
                //根据一定的模式匹配规则   
        List<String> matchingPatterns = new ArrayList<String>();  
        for (String registeredPattern : this.handlerMap.keySet()) {  
            if (getPathMatcher().match(registeredPattern, urlPath)) {  
                matchingPatterns.add(registeredPattern);  
            }  
        }  
        String bestPatternMatch = null;  
        if (!matchingPatterns.isEmpty()) {  
            Collections.sort(matchingPatterns, getPathMatcher().getPatternComparator(urlPath));  
            if (logger.isDebugEnabled()) {  
                logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);  
            }  
            bestPatternMatch = matchingPatterns.get(0);  
        }  
        if (bestPatternMatch != null) {  
                        //处理最佳匹配   
            handler = this.handlerMap.get(bestPatternMatch);  
            // Bean name or resolved handler?   
            if (handler instanceof String) {  
                String handlerName = (String) handler;  
                handler = getApplicationContext().getBean(handlerName);  
            }  
            validateHandler(handler, request);  
            String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestPatternMatch, urlPath);  
            Map<String, String> uriTemplateVariables =  
                    getPathMatcher().extractUriTemplateVariables(bestPatternMatch, urlPath);  
                        //返回一个HandlerExecutionChain对象   
            return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);  
        }  
        // No handler found...   
        return null;  
    }

最后在关注一下getHandlerInternal的代码实现

protected Object getHandlerInternal(HttpServletRequest request) throws Exception {  
        String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);  
                //查找符合匹配规则的handler。可能的结果是HandlerExecutionChain对象或者是null   
        Object handler = lookupHandler(lookupPath, request);  
                //如果没有找到匹配的handler，则需要处理下default handler   
        if (handler == null) {  
            // We need to care for the default handler directly, since we need to   
            // expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.   
            Object rawHandler = null;  
            if ("/".equals(lookupPath)) {  
                rawHandler = getRootHandler();  
            }  
            if (rawHandler == null) {  
                rawHandler = getDefaultHandler();  
            }  
                        //在getRootHandler和getDefaultHandler方法中，可能持有的是bean name。   
            if (rawHandler != null) {  
                // Bean name or resolved handler?   
                if (rawHandler instanceof String) {  
                    String handlerName = (String) rawHandler;  
                    rawHandler = getApplicationContext().getBean(handlerName);  
                }  
                validateHandler(rawHandler, request);  
                handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);  
            }  
        }  
                //如果handler还是为空，则抛出错误。   
        if (handler != null && this.mappedInterceptors != null) {  
            Set<HandlerInterceptor> mappedInterceptors =  
                    this.mappedInterceptors.getInterceptors(lookupPath, this.pathMatcher);  
            if (!mappedInterceptors.isEmpty()) {  
                HandlerExecutionChain chain;  
                if (handler instanceof HandlerExecutionChain) {  
                    chain = (HandlerExecutionChain) handler;  
                } else {  
                    chain = new HandlerExecutionChain(handler);  
                }  
                chain.addInterceptors(mappedInterceptors.toArray(new HandlerInterceptor[mappedInterceptors.size()]));  
            }  
        }  
        if (handler != null && logger.isDebugEnabled()) {  
            logger.debug("Mapping [" + lookupPath + "] to handler '" + handler + "'");  
        }  
        else if (handler == null && logger.isTraceEnabled()) {  
            logger.trace("No handler mapping found for [" + lookupPath + "]");  
        }  
        return handler;  
    }

经过上述代码的处理，就可以使用getHandler方法获得到HandlerExecutionChain对象了。DispatcherServlet获得到HandlerExecutionChain对象后，便可以获得真正的handler，调用相应的Controller了。

这篇文章主要包括两个方面的内容：HandlerMapping中url到handler映射关系的注册和根据url查找handler。第一部分是在HandlerMapping对象创建后，使用IoC容器的回调方法触发的，第二部分是在DispatchServlet中，调用getHandler方法触发的。

装载：http://blog.csdn.net/prince2270/article/details/5894456