Spring Web框架架构的主要部分是DispatcherServlet。也就是本文中重点介绍的对象。
在本文的第一部分中，我们将看到基于Spring的DispatcherServlet的主要概念：前端控制器模式。第二部分将专门介绍Spring应用程序中的执行链。接下来是DispatcherServlet类的解释。在最后一部分，我们将尝试开发一个自定义的dispatcher servlet。
请注意，本文分析的DispatcherServlet来自Spring的5.0.0.RC3版本。如果使用不同的版本，则可能需要进行几个调整，其实因为分析的都是比较固定的东西，很少有改的。
什么是前端控制器模式？
在进入DispatcherServlet之前，我们需要了解一些关于它的概念基础。DispatcherServlet所隐含的关键概念其实就是前端控制器模式。
此模式为Web应用程序提供了一个中心入口点。该集中入口点将系统组件的共同特征进行重新组合。我们可以在那里找到安全资源，语言切换，会话管理，缓存或输入过滤的处理程序。这样做的一个很大的好处是:这个共同的入口点有助于避免代码重复。
因此，从技术上讲，前端控制器模式由一个捕获所有传入请求的类组成。之后，分析每个请求以知道哪个控制器以及哪个方法应该来处理该请求。
前端控制器模式有助于对以下询问做出最佳响应:
- 如何集中授权和认证？
- 如何处理正确的视图渲染？
- 如何使用URL重写映射将请求发送到适当的控制器？
这个前台控制器模式包含5名参与者：
- 客户端：发送请求。
- 控制器：应用程序的中心点，捕获所有请求。
- 调度员：管理视图的选择，以呈现给客户端。
- 视图：表示呈现给客户端的内容。
- 帮助：帮助查看和/或控制器完成请求处理。
什么是DispatcherServlet的执行链？
由标题可以看到，前端控制器模式有自己的执行链。这意味着它有自己的逻辑来处理请求并将视图返回给客户端：
请求由客户端发送。它到达作为Spring的默认前端控制器的
DispatcherServlet类。-
DispatcherServlet使用请求处理程序映射来发现将分析请求的控制器(controller)。接口org.springframework.web.servlet.HandlerMapping的实现返回一个包含org.springframework.web.servlet.HandlerExecutionChain类的实例。此实例包含可在控制器调用之前或之后调用的处理程序拦截器数组。你可以在Spring中有关于拦截器的文章中了解更多的信息。如果在所有定义的处理程序映射中找不到
HandlerExecutionChain，这意味着Spring无法将URL与对应的控制器进行匹配。这样的话会抛出一个错误。 现在系统进行拦截器预处理并调用由映射处理器找到的相应的controller(其实就是在找到的controller之前进行一波拦截处理)。在controller处理请求后，
DispatcherServlet开始拦截器的后置处理。在此步骤结束时，它从controller接收ModelAndView实例(整个过程其实就是request请求->进入interceptors->controller->从interceptors出来->ModelAndView接收)。DispatcherServlet现在将使用的该视图的名称发送到视图解析器。这个解析器将决定前台的展现内容。接着，它将此视图返回给DispatcherServlet，其实也就是一个“视图生成后可调用”的拦截器。
最后一个操作是视图的渲染并作为对客户端request请求的响应。
什么是DispatcherServlet？
通过上面讲到的前端控制器模式，我们可以很轻易的知道DispatcherServlet是基于Spring的Web应用程序的中心点。它需要传入请求，并在处理程序映射，拦截器，控制器和视图解析器的帮助下，生成对客户端的响应。所以，我们可以分析这个类的细节，并总结出一些核心要点。
下面是处理一个请求时DispatcherServlet执行的步骤：
1. 策略初始化
DispatcherServlet是一个位于org.springframework.web.servlet包中的类，并扩展了同一个包中的抽象类FrameworkServlet。它包含一些解析器的私有静态字段(用于本地化，视图，异常或上传文件)，映射处理器:handlerMapping和处理适配器:handlerAdapter(进入这个类的第一眼就能看到的)。DispatcherServlet非常重要的一个核心点就是是初始化策略的方法(protected void initStrategies（ApplicationContext context）)。在调用onRefresh方法时调用此方法。最后一次调用是在FrameworkServlet中通过initServletBean和initWebApplicationContext方法进行的(initServletBean方法中调用initWebApplicationContext，后者调用onRefresh(wac))。initServletBean通过所提供的这些策略生成我们所需要的应用程序上下文。其中每个策略都会产生一类在DispatcherServlet中用来处理传入请求的对象。
基于篇幅，有些代码就不给贴示了，请在相应版本的源码中自行对照查找，此处只给一部分源码:
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/**
* This implementation calls {@link #initStrategies}.
*/
@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
/**
* Initialize the strategy objects that this servlet uses.
* <p>May be overridden in subclasses in order to initialize further strategy objects.
*/
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
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需要注意的是，如果找的结果不存在，则捕获异常NoSuchBeanDefinitionException(下面两段代码的第一段)，并采用默认策略。如果在DispatcherServlet.properties文件中初始定义的默认策略不存在，则抛出BeanInitializationException异常(下面两段代码的第二段)。默认策略如下：
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/**
* Initialize the LocaleResolver used by this class.
* <p>If no bean is defined with the given name in the BeanFactory for this namespace,
* we default to AcceptHeaderLocaleResolver.
*/
private void initLocaleResolver(ApplicationContext context) {
try {
this.localeResolver = context.getBean(LOCALE_RESOLVER_BEAN_NAME, LocaleResolver.class);
if (logger.isDebugEnabled()) {
logger.debug("Using LocaleResolver [" + this.localeResolver + "]");
}
}
catch (NoSuchBeanDefinitionException ex) {
// We need to use the default.
this.localeResolver = getDefaultStrategy(context, LocaleResolver.class);
if (logger.isDebugEnabled()) {
logger.debug("Unable to locate LocaleResolver with name '" + LOCALE_RESOLVER_BEAN_NAME +"': using default [" + this.localeResolver + "]");
}
}
}
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抛出异常后调用getDefaultStrategy(因为容器里都是单例的存在，所以只需要判断基于这个接口的默认实现实例size为1即可，两个以上还能叫默认么，都有选择了):
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/**
* Return the default strategy object for the given strategy interface.
* The default implementation delegates to {@link #getDefaultStrategies},
* expecting a single object in the list.
* @param context the current WebApplicationContext
* @param strategyInterface the strategy interface
* @return the corresponding strategy object
* @see #getDefaultStrategies
*/
protected <T> T getDefaultStrategy(ApplicationContext context, Class<T> strategyInterface) {
List<T> strategies = getDefaultStrategies(context, strategyInterface);
if (strategies.size() != 1) {
throw new BeanInitializationException(
"DispatcherServlet needs exactly 1 strategy for interface [" + strategyInterface.getName() + "]");
}
return strategies.get(0);
}
/**
* Create a List of default strategy objects for the given strategy interface.
* The default implementation uses the "DispatcherServlet.properties" file (in the same
* package as the DispatcherServlet class) to determine the class names. It instantiates
* the strategy objects through the context's BeanFactory.
* @param context the current WebApplicationContext
* @param strategyInterface the strategy interface
* @return the List of corresponding strategy objects
*/
@SuppressWarnings("unchecked")
protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
String key = strategyInterface.getName();
String value = defaultStrategies.getProperty(key);
if (value != null) {
String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
List<T> strategies = new ArrayList<>(classNames.length);
for (String className : classNames) {
try {
Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
Object strategy = createDefaultStrategy(context, clazz);
strategies.add((T) strategy);
}
catch (ClassNotFoundException ex) {
throw new BeanInitializationException("Could not find DispatcherServlet's default strategy class [" + className +"] for interface [" + key + "]", ex);
}
catch (LinkageError err) {
throw new BeanInitializationException(
"Error loading DispatcherServlet's default strategy class [" + className +"] for interface [" + key + "]: problem with class file or dependent class", err);
}
}
return strategies;
}
else {
return new LinkedList<>();
}
}
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2.请求预处理
FrameworkServlet抽象类扩展了同一个包下的HttpServletBean，HttpServletBean扩展了javax.servlet.http.HttpServlet。点开这个类源码可以看到，HttpServlet是一个抽象类，其方法定义主要用来处理每种类型的HTTP请求：doGet（GET请求），doPost（POST），doPut（PUT），doDelete（DELETE），doTrace（TRACE），doHead（HEAD） ，doOptions（OPTIONS）。FrameworkServlet通过将每个传入的请求调度到processRequest(HttpServletRequest request，HttpServletResponse response)来覆盖它们。processRequest是一个protected和final的方法，它构造出LocaleContext和ServletRequestAttributes对象，两者都可以在initContextHolders(request, localeContext, requestAttributes)之后访问。所有这些操作的关键代码 请看：
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@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
...
/**
* 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);
}
}
...
private void initContextHolders(HttpServletRequest request,
@Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) {
if (localeContext != null) {
LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
}
if (requestAttributes != null) {
RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
}
if (logger.isTraceEnabled()) {
logger.trace("Bound request context to thread: " + request);
}
}
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3.请求处理
由上面所看到的，在processRequest的代码中，调用initContextHolders方法后，调用protected void doService(HttpServletRequest request，HttpServletResponse response)。doService将一些附加参数放入request（如Flash映射:request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap())，上下文信息:request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext())等）中，并调用protected void doDispatch(HttpServletRequest request，HttpServletResponse response)。
doDispatch方法最重要的部分是处理(handler)的检索。doDispatch调用getHandler()方法来分析处理后的请求并返回HandlerExecutionChain实例。此实例包含handler mapping 和`interceptors(拦截器)。DispatcherServlet做的另一件事是应用预处理程序拦截器（applyPreHandle()）。如果至少有一个返回false，则请求处理停止。否则，servlet使用与 handler adapter适配(其实理解成这也是个handler就对了)相应的handler mapping来生成视图对象。
doDispatch方法：
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/**
* 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) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.获取可处理request的Handler，适配器其实还 //是调用的相应的Handler,一样的功能，具体请参考本人的Spring设计模式中的适配器模式
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.此处就会调用我们写的controller来执行咯
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//视图解析
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);
}
//此处进行最后一步的视图渲染
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);
}
}
}
}
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4.视图解析
获取ModelAndView实例以查看呈现后，doDispatch方法调用private void applyDefaultViewName(HttpServletRequest request，ModelAndView mv)。默认视图名称根据定义的bean名称，即viewNameTranslator。默认情况下，它的实现是org.springframework.web.servlet.RequestToViewNameTranslator。这个默认实现只是简单的将URL转换为视图名称，例如(直接从RequestToViewNameTranslator获取):http:// localhost:8080/admin/index.html将生成视图admin / index。
代码如下:
下一步是调用后置拦截器(其实就是出拦截器)做的一些处理。
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/** RequestToViewNameTranslator used by this servlet */
@Nullable
private RequestToViewNameTranslator viewNameTranslator;
...
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context); //看下面注释
initViewResolvers(context);
initFlashMapManager(context);
}
...
/**
* Initialize the RequestToViewNameTranslator used by this servlet instance.
* <p>If no implementation is configured then we default to DefaultRequestToViewNameTranslator.
*/
private void initRequestToViewNameTranslator(ApplicationContext context) {
try {
this.viewNameTranslator =
context.getBean(REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME, RequestToViewNameTranslator.class);
if (logger.isDebugEnabled()) {
logger.debug("Using RequestToViewNameTranslator [" + this.viewNameTranslator + "]");
}
}
catch (NoSuchBeanDefinitionException ex) {
// We need to use the default.
this.viewNameTranslator = getDefaultStrategy(context, RequestToViewNameTranslator.class);
if (logger.isDebugEnabled()) {
logger.debug("Unable to locate RequestToViewNameTranslator with name '" +
REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME + "': using default [" + this.viewNameTranslator +"]");
}
}
}
....
/**
* Translate the supplied request into a default view name.
* @param request current HTTP servlet request
* @return the view name (or {@code null} if no default found)
* @throws Exception if view name translation failed
*/
@Nullable
protected String getDefaultViewName(HttpServletRequest request) throws Exception {
return (this.viewNameTranslator != null ? this.viewNameTranslator.getViewName(request) : null);
}
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org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator实现的org.springframework.web.servlet.RequestToViewNameTranslator接口，其内对上段代码中getDefaultViewName的实现为:
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public class DefaultRequestToViewNameTranslator implements RequestToViewNameTranslator {
private static final String SLASH = "/";
private String prefix = "";
private String suffix = "";
private String separator = SLASH;
private boolean stripLeadingSlash = true;
private boolean stripTrailingSlash = true;
private boolean stripExtension = true;
private UrlPathHelper urlPathHelper = new UrlPathHelper();
/**
* Set the prefix to prepend to generated view names.
* @param prefix the prefix to prepend to generated view names
*/
public void setPrefix(String prefix) {
this.prefix = (prefix != null ? prefix : "");
}
/**
* Set the suffix to append to generated view names.
* @param suffix the suffix to append to generated view names
*/
public void setSuffix(String suffix) {
this.suffix = (suffix != null ? suffix : "");
}
/**
* Set the value that will replace '{@code /}' as the separator
* in the view name. The default behavior simply leaves '{@code /}'
* as the separator.
*/
public void setSeparator(String separator) {
this.separator = separator;
}
...
/**
* Translates the request URI of the incoming {@link HttpServletRequest}
* into the view name based on the configured parameters.
* @see org.springframework.web.util.UrlPathHelper#getLookupPathForRequest
* @see #transformPath
*/
@Override
public String getViewName(HttpServletRequest request) {
String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
return (this.prefix + transformPath(lookupPath) + this.suffix);
}
/**
* Transform the request URI (in the context of the webapp) stripping
* slashes and extensions, and replacing the separator as required.
* @param lookupPath the lookup path for the current request,
* as determined by the UrlPathHelper
* @return the transformed path, with slashes and extensions stripped
* if desired
*/
protected String transformPath(String lookupPath) {
String path = lookupPath;
if (this.stripLeadingSlash && path.startsWith(SLASH)) {
path = path.substring(1);
}
if (this.stripTrailingSlash && path.endsWith(SLASH)) {
path = path.substring(0, path.length() - 1);
}
if (this.stripExtension) {
path = StringUtils.stripFilenameExtension(path);
}
if (!SLASH.equals(this.separator)) {
path = StringUtils.replace(path, SLASH, this.separator);
}
return path;
}
}
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5.处理调度请求 - 视图渲染
现在，servlet知道应该是哪个视图被渲染。它通过private void processDispatchResult(HttpServletRequest request，HttpServletResponse response，@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,@Nullable Exception exception)方法来进行最后一步操作 - 视图渲染。
首先，processDispatchResult检查它们是否有参数传递异常。有一些异常的话，它定义了一个新的视图，专门用来定位错误页面。如果没有任何异常，该方法将检查ModelAndView实例，如果它不为null，则调用render方法。
渲染方法protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception。跳进此方法内部，根据定义的视图策略，它会查找得到一个View类实例。它将负责显示响应。如果没有找到View，则会抛出一个ServletException异常。有的话，DispatcherServlet会调用其render方法来显示结果。
其实可以说成是后置拦截器(进入拦截器前置拦截处理->controller处理->出拦截器之前的此拦截器的后置处理)，也就是在请求处理的最后一个步骤中被调用。
下面是processDispatchResult和render(渲染)的相关代码:
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/**
* 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,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable 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()) {
//开始渲染
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);
}
}
...
/**
* Render the given ModelAndView.
* <p>This is the last stage in handling a request. It may involve resolving the view by name.
* @param mv the ModelAndView to render
* @param request current HTTP servlet request
* @param response current HTTP servlet response
* @throws ServletException if view is missing or cannot be resolved
* @throws Exception if there's a problem rendering the view
*/
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
// Determine locale for request and apply it to the response.
Locale locale =
(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
response.setLocale(locale);
View view;
String viewName = mv.getViewName();
if (viewName != null) {
// We need to resolve the view name.
view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
if (view == null) {
throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
"' in servlet with name '" + getServletName() + "'");
}
}
else {
// No need to lookup: the ModelAndView object contains the actual View object.
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +"View object in servlet with name '" + getServletName() + "'");
}
}
// Delegate to the View object for rendering.
if (logger.isDebugEnabled()) {
logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
}
try {
if (mv.getStatus() != null) {
response.setStatus(mv.getStatus().value());
}
view.render(mv.getModelInternal(), request, response);
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
getServletName() + "'", ex);
}
throw ex;
}
}
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/**
* 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);
}
}
/**
* Render the given ModelAndView.
* This is the last stage in handling a request. It may involve resolving the view by name.
* @param mv the ModelAndView to render
* @param request current HTTP servlet request
* @param response current HTTP servlet response
* @throws ServletException if view is missing or cannot be resolved
* @throws Exception if there's a problem rendering the view
*/
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
// Determine locale for request and apply it to the response.
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale);
View view;
if (mv.isReference()) {
// We need to resolve the view name.
view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
if (view == null) {
throw new ServletException(
"Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + getServletName() + "'");
}
}
else {
// No need to lookup: the ModelAndView object contains the actual View object.
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " + "View object in servlet with name '" + getServletName() + "'");
}
}
// Delegate to the View object for rendering.
if (logger.isDebugEnabled()) {
logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
}
try {
view.render(mv.getModelInternal(), request, response);
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'", ex);
}
throw ex;
}
}
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在这部分中，你需要记住的是我们定义了两个上下文：一个用于应用程序，另一个用于Web应用程序。他们有什么区别？应用程序上下文包含所有通用配置，比如service定义，数据库配置。Web应用程序上下文定义所有与Web相关的组件，比如controllers或视图解析器。
Custom DispatcherServlet
我们已经了解了DispatcherServlet的理论知识。通过文中的这些实用要点，我们可以编写自己的servlet来分派处理请求。同样的，我们也将按步进行，从捕获请求开始，以视图渲染结束。
通过上面的描述，为了捕获请求，我们需要覆盖doService方法：
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public class CustomDispatcherServlet extends FrameworkServlet {
private static final Logger LOGGER = LoggerFactory.getLogger(CustomDispatcherServlet.class);
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
LOGGER.debug("[CustomDispatcherServlet] I got the request !");
}
}
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这样，在我们的日志文件中，我们应该可以找到一条“[CustomDispatcherServlet]I got the request！”。接着，我们继续添加在DispatcherServlet中doDispatch方法所应该做的一些工作：
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@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
LOGGER.debug("[CustomDispatcherServlet] I got the request !");
try {
LOGGER.debug("[CustomDispatcherServlet] doService");
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
initContextHolders(request, localeContext, requestAttributes);
} catch (Exception e) {
e.printStackTrace();
}
}
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这个方法是做什么的？首先，它为构建一个Locale实例用来接收请求。第二步是初始化org.springframework.web.context.request.ServletRequestAttributes实例。它是RequestAttributes接口的实现，和本地化在同一级别。通过这个，我们可以访问servlet请求的对象和会话对象，而不必区分会话和全局会话。最后，我们调用初始化context holders的initContextHolders()方法，即从应用程序通过LocaleContextHolder和RequestContextHolder静态方法(分别为:getLocaleContext和getRequestAttributes)访问请求属性和区域设置的对象。
当请求被拦截，一些基本的设置就绪的时候。我们发现我们还没有执行链和处理器适配器。我们可以通过以下代码进行:
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private HandlerExecutionChain getHandlerExecutionChain(HttpServletRequest request) throws Exception {
for (HandlerMapping mapper : this.handlerMappings) {
HandlerExecutionChain executionChain = mapper.getHandler(request);
if (executionChain != null) {
return executionChain;
}
}
throw new Exception("Execution chain wasn't be found in provided handler mappings: "+this.handlerMappings);
}
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通过执行链，我们可以通过 handler adapter将处理当前请求。看以下代码:
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@Override
protected HandlerAdapter getHandlerAdapter(Object executionChain) throws ServletException {
for (HandlerAdapter adapter : this.handlerAdapters) {
LOGGER.debug("[CustomDispatcherServlet] "+adapter + " is instanceof HandlerMethod ? "+(adapter instanceof HandlerMethod));
if (adapter.supports(executionChain)) {
return adapter;
}
}
throw new ServletException("Handler adapter was not found from adapters list :"+this.handlerAdapters);
}
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只有应用程序上下文中定义的适配器（this.handlerAdapter）支持适配所生成的执行链（adapter.supports）才可以返回我们想要的适配器。最后，我们可以返回到我们的doService方法并操作它们来渲染视图：
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ModelAndView modelView = adapter.handle(request, response, executionChain.getHandler());
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale);
View view = null;
if (!modelView.isReference()) {
throw new UnsupportedOperationException("Only view models defined as references can be used in this servlet");
}
for (ViewResolver viewResolver : this.viewResolvers) {
view = viewResolver.resolveViewName(modelView.getViewName(), locale);
if (view != null) {
break;
}
}
if (view == null) {
throw new ServletException("Could not resolve view with name '" + modelView.getViewName() + "' in servlet with name '" + getServletName() + "'");
}
view.render(modelView.getModelMap(), request, response);
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我们的servlet中简化了渲染。实际上，我们仅处理ModelAndView的引用对象。这意味着ModelAndView是一个String的实例，用来表示要解析的视图模型，例如：我们定义好几个模板解析器(比如freemaker，Thymeleaf)，然后查看其配置。在这个检查之后，我们迭代当前视图解析器。能够生成View实例的第一个解析器被视为处理过的请求中使用的解析器。最后，我们检查视图是否正确生成。拿到view实例后，我们调用其render()方法来在屏幕中显示请求处理结果。
在这部分中，我们将描述和代码部分限制在最低限度。只是为了把Spring的整个过程给集中呈现以下，达到更好的理解，其实就是在Servlet中的service方法内做些对request和response的文章而已了。
本文介绍了Spring Web应用程序的中心点，一个调度器servlet。请记住，它是一个处理所有传入请求并将视图呈现给用户的类。在重写之前，你应该熟悉执行链，handler mapping 或handler adapter等概念。请记住，第一步要做的是定义在调度过程中我们要调用的所有元素。handler mapping 是将传入请求(也就是它的URL)映射到适当的controller。最后提到的元素，一个handler适配器，就是一个对象，它将通过其内包装的handler mapping将请求发送到controller。此调度产生的结果是ModelAndView类的一个实例，后面被用于生成和渲染视图。