spring mvc DispatcherServlet之前端控制器架构详解

spring mvc DispatcherServlet之前端控制器架构详解

前端控制器是整个MVC框架中最为核心的一块，它主要用来拦截符合要求的外部请求，并把请求分发到不同的控制器去处理，根据控制器处理后的结果，生成相应的响应发送到客户端。前端控制器既可以使用Filter实现（Struts2采用这种方式），也可以使用Servlet来实现(spring MVC框架)。

DispatcherServlet 作为前置控制器是web服务器的入口，是spring mvc最重要的一个类，通过它的生命周期可以加深对web服务器的理解。

servlet的生命周期

首先我们回忆一下servlet的生命周期：

Servlet生命周期分为三个阶段：【Servlet生命周期与工作原理详解】

1.初始化阶段  调用init()方法。Servlet被装载后，Servlet容器创建一个Servlet实例并且调用Servlet的init()方法进行初始化。在Servlet的整个生命周期内，init()方法只被调用一次。

2.响应客户请求阶段　　调用service()方法

3.终止阶段　　调用destroy()方法

Servlet初始化阶段

在下列时刻Servlet容器装载Servlet：

1.Servlet容器启动时自动装载某些Servlet，实现它只需要在web.XML文件中的之间添加如下代码：

1

　　2.在Servlet容器启动后，客户首次向Servlet发送请求

3.Servlet类文件被更新后，重新装载Servlet

DispatcherServlet的结构

复习了上述知识后我们来看看DispatcherServlet的结构：

DispatcherServlet继承自抽象类：FrameworkServlet，间接继承了HttpServlet （FrameworkServlet继承自HttpServletBean，而HttpServletBean继承自HttpServlet ）

Servlet的初始化

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

}

servlet如何处理请求：

servlet的service方法处理http请求。

FrameworkServlet.java 定义了servlet的service和destroy方法，如下所示：

/**

* Override the parent class implementation in order to intercept PATCH

* requests.

*/

@Override

protected void service(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

String method = request.getMethod();

if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {

processRequest(request, response);

}

else {

super.service(request, response);

}

}

我们知道http请求类型有七种（外加一个option选项），定义如下：

public enum RequestMethod {

GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE

}

FrameworkServlet的service()处理不同的请求，我们以常见的post来说明：

/**

* Process this request, publishing an event regardless of the outcome.

*

The actual event handling is performed by the abstract

* {@link #doService} template method.

*/

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

long startTime = System.currentTimeMillis();

Throwable failureCause = null;

LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();

LocaleContext localeContext = buildLocaleContext(request);

RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();

ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

initContextHolders(request, localeContext, requestAttributes);

try {

doService(request, response);

}

catch (ServletException ex) {

failureCause = ex;

throw ex;

}

catch (IOException ex) {

failureCause = ex;

throw ex;

}

catch (Throwable ex) {

failureCause = ex;

throw new NestedServletException("Request processing failed", ex);

}

finally {

resetContextHolders(request, previousLocaleContext, previousAttributes);

if (requestAttributes != null) {

requestAttributes.requestCompleted();

}

if (logger.isDebugEnabled()) {

if (failureCause != null) {

this.logger.debug("Could not complete request", failureCause);

}

else {

if (asyncManager.isConcurrentHandlingStarted()) {

logger.debug("Leaving response open for concurrent processing");

}

else {

this.logger.debug("Successfully completed request");

}

}

}

publishRequestHandledEvent(request, startTime, failureCause);

}

}

FrameworkServlet 抽象定义了处理流程，留待子类来实现该方法，完成具体的请求处理。

/**

* Subclasses must implement this method to do the work of request handling,

* receiving a centralized callback for GET, POST, PUT and DELETE.

*

The contract is essentially the same as that for the commonly overridden

* {@code doGet} or {@code doPost} methods of HttpServlet.

*

This class intercepts calls to ensure that exception handling and

* event publication takes place.

* @param request current HTTP request

* @param response current HTTP response

* @throws Exception in case of any kind of processing failure

* @see javax.servlet.http.HttpServlet#doGet

* @see javax.servlet.http.HttpServlet#doPost

*/

protected abstract void doService(HttpServletRequest request, HttpServletResponse response)

throws Exception;

具体实现如下：

/**

* 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 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.

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);

}

}

}

重头戏，作为请求分发器的实现：

功能：1. 把请求分发到handler（按照配置顺序获取servlet的映射关系获取handler）；2. 根据servlet已安装的  HandlerAdapters 去查询第一个能处理的handler；3. handler激发处理请求

/**

* Process the actual dispatching to the handler.

*

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.

*

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);

}

}

}

servlet销毁

/**

* Close the WebApplicationContext of this servlet.

* @see org.springframework.context.ConfigurableApplicationContext#close()

*/

@Override

public void destroy() {

getServletContext().log("Destroying Spring FrameworkServlet '" + getServletName() + "'");

// Only call close() on WebApplicationContext if locally managed...

if (this.webApplicationContext instanceof ConfigurableApplicationContext && !this.webApplicationContextInjected) {

((ConfigurableApplicationContext) this.webApplicationContext).close();

}

}

小结：

本文因篇章限制，仅仅介绍了请求处理的流程，没有对代码进行深入的分析，接下来的文章将从细微处着手，分析spring的代码之美。

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