spring mvc DispatcherServlet详解之一---处理请求深入解析

要深入理解spring mvc的工作流程，就需要先了解spring mvc的架构：



从上图可以看到 前端控制器DispatcherServlet在其中起着主导作用，理解了DispatcherServlet 就完全可以说弄清楚了spring mvc。

为了加深对spring mvc的整个工作流程的理解，本文从分析DispatcherServlet的工作过程来一窥spring mvc的整个面貌。

1. 初始化



protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context); //文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；
initLocaleResolver(context); //本地化解析
initThemeResolver(context);　　　//主题解析

initHandlerMappings(context); //通过HandlerMapping，将请求映射到处理器

initHandlerAdapters(context); //通过HandlerAdapter支持多种类型的处理器

initHandlerExceptionResolvers(context); //如果执行过程中遇到异常将交给HandlerExceptionResolver来解析
initRequestToViewNameTranslator(context); //直接解析请求到视图名

initViewResolvers(context); //通过ViewResolver解析逻辑视图名到具体视图实现
initFlashMapManager(context); //flash映射管理器

}

单个resolver
initMultipartResolver，initLocaleResolver，initThemeResolver，initRequestToViewNameTranslator，initFlashMapManager 这五个初始化方法流程相同，都是使用
context.getBean(String name, Class<FlashMapManager> requiredType)的方式获取到相应的Resolver。以initMultipartResolver为例，见如下：

/**
* Initialize the MultipartResolver used by this class.
* <p>If no bean is defined with the given name in the BeanFactory for this namespace,
* no multipart handling is provided.
*/
private void initMultipartResolver(ApplicationContext context) {
try {
this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
if (logger.isDebugEnabled()) {
logger.debug("Using MultipartResolver [" + this.multipartResolver + "]");
}
}
catch (NoSuchBeanDefinitionException ex) {
// Default is no multipart resolver.
this.multipartResolver = null;
if (logger.isDebugEnabled()) {
logger.debug("Unable to locate MultipartResolver with name '" + MULTIPART_RESOLVER_BEAN_NAME +
"': no multipart request handling provided");
}
}
}

多个resolver

initHandlerMappings，initHandlerAdapters，initHandlerExceptionResolvers，initViewResolvers 获取方式相同，使用：
BeanFactoryUtils.beansOfTypeIncludingAncestors(ListableBeanFactory lbf, Class<HandlerMapping> type, boolean includeNonSingletons, boolean allowEagerInit)

的方式获取到相应的Resolver。以initHandlerMappings为例，见如下：

/**
* Initialize the HandlerMappings used by this class.
* <p>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<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");
}
}
}

那么深入看一下BeanFactoryUtils.beansOfTypeIncludingAncestors 究竟做了什么？返回指定类型和子类型的所有bean，若该bean factory 是一个继承类型的beanFactory，这个方法也会获取祖宗factory中定义的指定类型的bean。

/**
* Return all beans of the given type or subtypes, also picking up beans defined in
* ancestor bean factories if the current bean factory is a HierarchicalBeanFactory.
* The returned Map will only contain beans of this type.
* <p>Does consider objects created by FactoryBeans if the "allowEagerInit" flag is set,
* which means that FactoryBeans will get initialized. If the object created by the
* FactoryBean doesn't match, the raw FactoryBean itself will be matched against the
* type. If "allowEagerInit" is not set, only raw FactoryBeans will be checked
* (which doesn't require initialization of each FactoryBean).
* <p><b>Note: Beans of the same name will take precedence at the 'lowest' factory level,
* i.e. such beans will be returned from the lowest factory that they are being found in,
* hiding corresponding beans in ancestor factories.</b> This feature allows for
* 'replacing' beans by explicitly choosing the same bean name in a child factory;
* the bean in the ancestor factory won't be visible then, not even for by-type lookups.
* @param lbf the bean factory
* @param type type of bean to match
* @param includeNonSingletons whether to include prototype or scoped beans too
* or just singletons (also applies to FactoryBeans)
* @param allowEagerInit whether to initialize <i>lazy-init singletons</i> and
* <i>objects created by FactoryBeans</i> (or by factory methods with a
* "factory-bean" reference) for the type check. Note that FactoryBeans need to be
* eagerly initialized to determine their type: So be aware that passing in "true"
* for this flag will initialize FactoryBeans and "factory-bean" references.
* @return the Map of matching bean instances, or an empty Map if none
* @throws BeansException if a bean could not be created
*/
public static <T> Map<String, T> beansOfTypeIncludingAncestors(
ListableBeanFactory lbf, Class<T> type, boolean includeNonSingletons, boolean allowEagerInit)
throws BeansException {

Assert.notNull(lbf, "ListableBeanFactory must not be null");
Map<String, T> result = new LinkedHashMap<String, T>(4);
result.putAll(lbf.getBeansOfType(type, includeNonSingletons, allowEagerInit));
if (lbf instanceof HierarchicalBeanFactory) {
HierarchicalBeanFactory hbf = (HierarchicalBeanFactory) lbf;
if (hbf.getParentBeanFactory() instanceof ListableBeanFactory) {
Map<String, T> parentResult = beansOfTypeIncludingAncestors(
(ListableBeanFactory) hbf.getParentBeanFactory(), type, includeNonSingletons, allowEagerInit);
for (Map.Entry<String, T> entry : parentResult.entrySet()) {
String beanName = entry.getKey();
if (!result.containsKey(beanName) && !hbf.containsLocalBean(beanName)) {
result.put(beanName, entry.getValue());
}
}
}
}
return result;
}

 2. 提供服务

     我们来看看这个servlet是如何提供服务的？

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (logger.isDebugEnabled()) {
String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
}

// Keep a snapshot of the request attributes in case of an include,
// to be able to restore the original attributes after the include.
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<String, Object>();
Enumeration<?> attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}

// Make framework objects available to handlers and view objects.
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

try {
doDispatch(request, response);
}
finally {
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
return;
}
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}

   从上面我们可以看到，提供服务只要分4步：

  1. 保存现场。保存request 熟悉的快照，以便能在必要时恢复。

  2. 将框架需要的对象放入request中，以便view和handler使用。

  3. 请求分发服务.

  4. 恢复现场。

其中最重要的是请求分发服务：

/**
* Process the actual dispatching to the handler.
* <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
* The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
* to find the first that supports the handler class.
* <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
* themselves to decide which methods are acceptable.
* @param request current HTTP request
* @param response current HTTP response
* @throws Exception in case of any kind of processing failure
*/
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;

WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

try {
ModelAndView mv = null;
Exception dispatchException = null;

try {
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);

// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}

// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (logger.isDebugEnabled()) {
logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
}
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}

if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

try {
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
}

applyDefaultViewName(request, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Error err) {
triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
return;
}
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}

分发过程如下：

1. 判断是否设置了multipart resolver，设置的话转换为multipart request，没有的话则继续下面的步骤。

2. 根据当前request，获取hangdler。

3. 根据当前request，获取HandlerAdapter。

4. 如果支持http请求头，处理 last-modified header请求头。

5. 应用已注册interceptor的preHandle方法

6. HandlerAdapter处理请求。

7. 设置默认视图。

8. 应用已注册interceptor的postHandle方法。

9. 处理异常或者视图渲染。

小结：

   DispatherServlet整个过程的细节一章之内很难描述的面面俱到，只能分析部分流程，想了解更具体的实现需要从源代码中去寻找。