Spring framework MVC源码阅读笔记（一）

org.springframework.web.servlet.DispatcherServlet

下图来自Spring官方文档，MVC对请求的处理流程



上图中Front controller由DispatcherServlet充当，负责拦截和响应请求。实际请求的处理和响应的生成由Controller完成，这里的Controller具体就是平时用@Controller注解配置的类，这些类中由@RequestMapping配置的方法对应Handler，Spring MVC提供了HandlerMapping接口用于保存请求与Handler之间的对应的关系，请求最终会交给最匹配的Handler来处理。（View template部分暂缺，还说不清）

下图为DispatcherServlet继承结构



该类根据配置拦截请求，从图片和类名可知该类就是是一个Servlet，那么普通的Servlet所用有的特性该类自然也有。

下图为Servlet配置信息，该信息来自Servlet规范（该文档非常值得一看）



先看DispatcherServlet的一般配置

<web-app>

<servlet>
<servlet-name>example</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
<servlet-name>example</servlet-name>
<url-pattern>/example/*</url-pattern>
</servlet-mapping>

</web-app>

该类由容器负责实例化，load-on-startup为1应该是在容器启动是就加载该类。Servlet配置信息比较关注init-param节点，该节点可以为Servlet提供一些参数，通过复写init方法可以获得，DispatcherServlet的属性也是通过这个方式配置的，其具体实现在祖先类HttpServletBean中找到，不过之前平时使用时从来没配过。

自定义的Servlet通常会继承javax.servlet.http.HttpServlet然后根据需要复写不同的方法，DispatcherServlet只复写了doService方法，其他的都交由祖先类处理。

以下是doService的具体实现

@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.
//总而言之先保存下request携带的属性，从代码可以看出只保留了MVC相关的属性
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<>();
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.
//为request添加一些属性，在真正负责处理请求的代码使用(？)
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());

//Flash相关，暂时不管
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 {
//实际转发请求的代码
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);
}
}
}
}

DispatcherServlet中维护了一个（或不止一个Map）用于保存处理者和请求之间的对于关系，从中获取最匹配的Handler来处理请求，如果找不到大概就会返回404的错误。以下为挑选Handler及实际处理请求实现：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
//看字面意思，为处理异步请求做准备，到目前为止没怎么用过异步Servlet
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

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

try {
//是否为上传文件的请求，是的话先处理下
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);

// Determine handler for the current request.
//选择具体的Handler
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.
//关于HTTP协议中缓存的部分，对于动态生成的内容使用缓存不熟
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;
}
catch (Throwable err) {
// As of 4.3, we're processing Errors thrown from handler methods as well,
// making them available for @ExceptionHandler methods and other scenarios.
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", 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);
}
}
}
}

//逻辑不复杂，就是从Map中获取Handler，至于该Map是如何维护的还要再看
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
for (HandlerMapping hm : this.handlerMappings) {
if (logger.isTraceEnabled()) {
logger.trace(
"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
}
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}

未完，待续……