Spring MVC学习之二 DispatcherServlet如何处理请求

上一节我们知道了DispatcherServlet的初始化方法怎么调用后，接下来我们先看看下初始化时在initStrategies()都做了什么事情。

其内部调用了9个init*()方法，这些方法基本上就是从context中取出对应的bean并赋值给DispatcherServlet对应的属性，例如

此处只是简单的看下初始化，后续会进行详细的学习，请期待。

Servlet的请求处理都是在doService()方法中，下面是DispatcherServlet的doService()方法：

       /**
* Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
* for the actual dispatching.
*/
@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()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}

}

该方法做了三项工作：1.将request中的属性保存了一个快照；

                                  2.将框架中的对象保存到request中；

                                  3.调用doDispatch(request, response)分发请求；

处理请求的主要工作是在doDispatch(request, response)，下面我们重点分析doDispatch(request, response)。

       /**
* 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 {
1 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;
}
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, 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
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}

局部变量asyncManager是一个异步的web管理器，我们暂且不考虑它，只分析业务相关的代码。

1）processedRequest = checkMultipart(request);这行代码是与上传文件相关的处理，解析给定的请求为文件和参数并将其包装到一个MultipartHttpServletRequest中，MultipartHttpServletRequest提供了在标准的servletRequest方法中可以访问文件和容器参数。

2）multipartRequestParsed = (processedRequest != request);根据上一步返回processedRequest与入参对比，设置是否为文件上传的请求

3）mappedHandler = getHandler(processedRequest);从当前的handlerMappings获取第一个HandlerExecutionChain，HandlerExecutionChain中有处理器和拦截器。后续我们会详细学习此对象。

4）if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;

}

    没有找到handler则处理异常并返回结束方法的执行

5）HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

    获取给定handler的HandlerAdapter

6）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;
}

}

    此处代码做了修改性校验，判断内容是否被修改，不做过多就学习。HEAD请求，是HTTP1.0协议中的请求方法，用来测试超链接的有效性，可用性和最近修改。

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

}

    对请求做拦截器的前置拦截处理，如果被拦截则返回

8）// Actually invoke the handler.

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

    handlerAdapter调用handler处理请求返回modelAndView

9）applyDefaultViewName(processedRequest, mv);如果当前返回的view为空则根据配置设置默认的view

10）mappedHandler.applyPostHandle(processedRequest, response, mv); 调用拦截器做后置处理

11）processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);处理分发后的请求结果，如果上步处理有异常则生成异常view,接着对存在并没有被清空的view进行渲染，渲染结束后如果存在拦截器则进行完成后拦截处理。

方法如下：

/**
* Handle the result of handler selection and handler invocation, which is
* either a ModelAndView or an Exception to be resolved to a ModelAndView.
*/
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {


boolean errorView = false;


if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}


// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
"': assuming HandlerAdapter completed request handling");
}
}


if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}


if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}

}

经过以上主要的11步骤，基本完成了请求的处理。流程图如下