spring mvc 的工作原理（1）DispatcherServlet 请求处理流程

摘要: 上一篇我简单的介绍了spring mvc的原理、流程，下面我要深入源码了解下。

类关系

DispatcherServlet 继承 FrameworkServlet 继承 HttpServletBean，HttpServletBean 是一个HttpServlet和EnvironmentCapable, EnvironmentAware接口的实现。

本章关注点在DispatcherServlet对请求的分派调用流程上，这些类暂不关注。

成员变量

1、静态变量主要是一些bean名称，以及request attribute 的key。如

[code=language-java]/** Well-known name for the MultipartResolver object in the bean factory for this namespace. */
public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";

/** Well-known name for the LocaleResolver object in the bean factory for this namespace. */
public static final String LOCALE_RESOLVER_BEAN_NAME = "localeResolver";

/** Well-known name for the ThemeResolver object in the bean factory for this namespace. */
public static final String THEME_RESOLVER_BEAN_NAME = "themeResolver";

2、具体的特殊的bean，上一章中提到的特殊bean。如

[code=language-java]/** MultipartResolver used by this servlet */
private MultipartResolver multipartResolver;

/** LocaleResolver used by this servlet */
private LocaleResolver localeResolver;

/** ThemeResolver used by this servlet */
private ThemeResolver themeResolver;

/** List of HandlerMappings used by this servlet */
private List<HandlerMapping> handlerMappings;

方法

方法重点关注doService，doDispatch这两个方法，其他的方法是get/set 和初始化方法，不是本章的关注对象。下面的这两个方法源码和注释。

[code=language-java]    //重写HttpServlet 的请求处理方法
@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.
//保留一个原请求属性的备份。
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
//是内部发出的请求 forward
attributesSnapshot = new HashMap<String, Object>();
Enumeration<?> attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
//如果是DispatcherServlet cleanupAfterInclude(是include 请求就清理)属性为true，或者request的属性是DispatcherServlet附加的，保存备份
if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}

//附加webApplicationContext 的特殊bean
// Make framework objects available to handlers and view objects.
//web application context
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
//国际化
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
//主题
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
//？？？
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

//闪存，重定向时可以不携带参数
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 {
//分派到handler，进行业务逻辑处理
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);
}
}
}
}

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 {
//检查是否 multipart request
processedRequest = checkMultipart(request);
//request 不等于 processedRequest 表示检查到multi ，并构建返回 multi request
multipartRequestParsed = (processedRequest != request);

// Determine handler for the current request.
//决定、判断、获取 handler
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
//找不到相应的handler ，返回并提示 noHandlerFound
noHandlerFound(processedRequest, response);
return;
}

// Determine handler adapter for the current request.
//查找handler 的适配器（不同的方法细节多样、参数等，通过adapter 封装）
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

// Process last-modified header, if supported by the handler.
//判断是否支持last-modified
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;
}
}

//执行前拦截器
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

// Actually invoke the handler.
//调用实际业务方法逻辑
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//解析modelview 中的名称映射对应的视图模板。
applyDefaultViewName(processedRequest, mv);
//执行后拦截器
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
//处理执行的结果，异常的话，跳转处理异常的代码。
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);
}
}
}
}

注释不正确的地方，请指出。

从源码我们可以看出，基本上请求的出来流程正如第一篇中所描述的。

第一步是绑定各种各样的web context bean。

第二步是搜索获取对应请求的处理handler，以及对应的adapter。

第三步是执行找出的handler，前拦截器，handler， 后拦截器。

第四步是将结果或异常进行处理，如将string 映射到具体的view 或者输出json等。捕获异常的调用Exception handler 处理。